Strain and Stress Measurement

Course overview

This course provides a thorough introduction to strain and stress measurement using strain-gage technology with DewesoftX. You’ll start by covering the basics: strain as deformation per original length, stress as force per area, modulus of elasticity, and common measurement scales in the microstrain range.

You’ll then learn the theory and setup of strain-gage circuits: how foil sensors change resistance under load, and how Wheatstone bridges detect minute voltage changes related to strain. The course walks you through selecting gage types (e.g., 120Ω or 350Ω), mounting methods, and bridge configurations (quarter, half, full) optimized for your measurement goals.

A key module covers hardware setup in DewesoftX, including wiring to SIRIUS-STG amplifiers, enabling bridge completion modes, and performing automatic Zero/Balancing, Short, and Shunt tests for signal integrity. The training also delves into shunt calibration and lead-wire compensation, ensuring accuracy by injecting known resistance changes and compensating for cable effects.

Advanced lessons introduce strain rosette calculations: combining multiple strain gages to compute principal strain/stress values, maximum shear stress, and von Mises stress using linear-elastic material theory. These are essential for fully characterizing complex load states in materials.

By course end, you’ll be adept at setting up, calibrating, and analyzing strain and stress measurements in DewesoftX—ready for applications like structural testing, material characterization, torque sensing, and load monitoring.

Strain and stress occur when external forces are applied to an object at rest.

Strain

Strain is defined as the amount of deformation an object undergoes relative to its original size and shape. It is expressed as the ratio of the change in length to the original length.

The term strain is most often used to describe the elongation of a section.

Strain occurs as a result of an applied force.

Strain is a dimensionless quantity and is usually expressed as a percentage. For steel, typical strain values are less than 2 mm/m, and they are often expressed in microstrain units. One microstrain corresponds to a deformation of one part per million.

Stress

Stress is defined as the applied force per unit area.

Stress usually occurs as a result of an applied force but can also arise from internal forces within a material or within a larger system.

For example, imagine a wire anchored at the top and hanging freely. If we apply weights to the end of the wire, a downward force is exerted, pulling it down. In the illustration below, A represents the original cross-sectional area of the wire, and L represents the original length. In this case, the material (the wire) experiences stress, specifically axial stress.

The units of stress are the same as those of pressure, since pressure is a special case of stress. However, stress is a more complex quantity than pressure because it varies with direction and depends on the surface it acts upon.

Stress (σ) can be calculated using the formula:

σ = ε × E

where ε is strain and E is Young's modulus.

Force

Therefore

Young's modulus, also known as the tensile modulus or elastic modulus, is a measure of the stiffness of an elastic material and is used to characterize materials.

It is defined as the ratio of stress (force per unit area) along an axis to strain (deformation relative to the initial length) along that same axis, within the range where Hooke's law is valid.

A material with a very high Young's modulus is considered rigid.

Young's modulus [E] can be calculated by dividing the tensile stress by the extensional strain in the elastic (initial, linear) portion of the stress-strain curve:

Where

• E is Young's modulus (modulus of elasticity).

• F is the force exerted on an object under tension.

• A₀ is the original cross-sectional area through which the force is applied.

• ΔL is the change in length of the object.

• L₀ is the original length of the object.

According to the International System of Units (SI), the unit of Young's modulus is the Pascal (Pa = N/m² = kg/ms²). In practice, megapascals (MPa = N/mm²) or gigapascals (GPa = kN/mm²) are commonly used.

In the United States customary system, Young's modulus is expressed in pounds per square inch (psi).

Measuring modulus of elasticity

The modulus of elasticity and yield stress are two commonly measured material properties that can be determined by performing tensile tests with a mechanical testing system.

In this procedure, the test specimen is clamped between two grips. The bottom grip holds the specimen securely, while the top grip moves upward at a controlled displacement rate.

The testing system records both the applied force and the corresponding displacement of the grips. Engineers then measure the specimen’s original cross-sectional area and the initial length between grips. Using this data, stress is calculated from the applied force, while strain is calculated from the displacement.

Finally, these results are combined to create a stress-strain diagram, as illustrated in the image below.

1. Normal stress

We distinguish between two types of normal stress: tensile stress and compressive stress. Tensile stresses are considered positive, while compressive stresses are considered negative.

Normal stresses occur when tensile or compressive forces act in opposition to one another.

1.1 Tension

In the figure below, we can see a tensile load applied to a rectangular solid. The response of the solid to tensile loads strongly depends on the tensile stiffness and strength properties of the reinforcement fibers, as these values are significantly higher than those of the resin system alone.

1.2 Compression

The figure below shows a composite material under a compressive load. In this case, the adhesive and stiffness properties of the resin system are crucial, since the resin’s role is to hold the fibers as straight columns and prevent them from buckling.

2. Shear stress

The figure below shows a composite material subjected to a shear load. This type of load attempts to slide adjacent layers of fibers over one another. Under shear loads, the resin plays a critical role by transferring stresses across the composite. For the material to perform well under such conditions, the resin must not only demonstrate strong mechanical properties but also exhibit high adhesion to the reinforcement fibers. The interlaminar shear strength (ILSS) of a composite is often used to characterize this property in a multi-layered composite (laminate).

Strain gage

A strain gage is a sensor whose resistance varies with the applied force and is commonly used for load, weight, and force detection. It is essentially a foil resistor, where the resistance is proportional to its length and inversely proportional to the cross-sectional area. The gage consists of a small-diameter wire attached to a backing material (usually plastic). The wire is looped back and forth multiple times to create a longer effective length. The longer the wire, the greater the resistance, and the larger the change in resistance.

However, this change in resistance is very small, so a highly sensitive amplifier and precise measurement principle are required to detect such minimal differences. The strain gage is one of the most important tools in electrical measurement techniques used for mechanical quantity measurements. Strain gages bonded to a larger structure under stress are referred to as bonded gages.

Typical strain gages have an unstressed resistance ranging from 120 Ω to 350 kΩ and are usually smaller than a postage stamp. Within the full force range of the gage, the resistance may change only by a fraction of a percent, given the limits imposed by the elastic properties of both the gage material and the test specimen. Forces large enough to induce greater resistance changes would permanently deform either the test specimen or the gage conductors, thereby rendering the gage unusable as a measurement device.

Therefore, to use a strain gage effectively, extremely small resistance changes must be measured with high accuracy. The ideal strain gage would change resistance solely due to the deformation of the surface to which it is attached. In practice, however, many factors influence the detected resistance, such as temperature, material properties, the adhesive bonding the gage to the surface, and the stability of the metal.

Gage factor (GF or k)

If a wire is held under tension, it becomes slightly longer and its cross-sectional area is reduced. This change alters its resistance (R) in proportion to the strain sensitivity (S) of the wire. When strain is applied, the strain sensitivity—also called the gage factor (GF)—is defined as:

General examples of strain gages:

The gage factor itself is not dependent on temperature. However, it is important to note that it relates resistance to strain only in the absence of temperature effects. The ideal strain gage would exhibit changes in resistance solely due to surface deformations of the material to which it is attached.

1. Selection based on gage length:

First, let's explain what a gage length is.

Gage length is the distance along the specimen over which extension calculations are made. It is sometimes defined as the distance between the grips.

It typically ranges from 0.2 mm to 100 mm, but a length of 3 mm to 6 mm is generally recommended for common applications.

• Select a shorter gage (< 3 mm) if mounting space is limited, if a localized strain gradient needs to be measured (e.g., on a fillet, hole, or notch with a small diameter < 25 mm), or if high accuracy is not critical.

• Select a longer gage (> 6 mm) if installation speed is important, as longer gages are easier to mount. A longer gage is also recommended if heat dissipation is a concern (since it is less sensitive to temperature effects), if the measured object has non-homogeneous material properties (such as concrete), or if cost is a factor. Gages with lengths between 5.0 and 12.5 mm are usually less expensive than other sizes.

2. Selection based on gage resistance:

The electrical resistance of a strain gage is directly related to its sensitivity. Generally, the higher the resistance, the greater the sensitivity.

• Select a higher resistance gage (350 Ω or 1000 Ω) if you require higher sensitivity or compatibility with existing instrumentation.

• Select a lower resistance gage (120 Ω) if fatigue loading is an issue. Lower resistance wires have a larger diameter, making them more fatigue-resistant. They are also a cost-effective option, as 120 Ω gages are usually less expensive than 350 Ω gages.

3. Selection based on gage pattern:

Before we discuss gage patterns, it is important to explain strain rosettes.

Strain rosette

A single strain gage can only measure strain in one direction. To overcome this limitation, we use a strain gage rosette. This is an arrangement of two or more closely positioned gage grids, each oriented differently to measure normal strains along various directions in the underlying surface of the test part.

Rosettes serve an important function in experimental stress analysis. In many cases, such as a general biaxial stress state with unknown principal directions, three independent strain measurements (in different directions) are required to determine the principal strains and stresses. Even when the principal directions are known in advance, at least two independent strain measurements are necessary to calculate the principal strains and stresses accurately.

Gage pattern refers to both the number of grids and their layout.

• Select a uniaxial strain gage if you only need to measure strain in one direction, or if budget is a concern. Two or three single uniaxial strain gages are usually less expensive than a biaxial or tri-element strain gage.

Select a biaxial strain rosette (0°–90° Tee rosette) if you need to measure principal stress and the principal axes are already known.

Select a three-element strain rosette (0°–45°–90° rectangular rosette or 0°–60°–120° delta rosette) if you need to measure principal stresses and the principal axes are unknown.

We distinguish between two different layouts in multi-axial strain rosettes: planar and stacked.

Select a planar layout strain rosette if you need better heat dissipation or if high accuracy and stability are critical. In this layout, each gage is positioned closer to the measuring surface, with no interference between them.

Select a stacked layout strain rosette if the strain gradient is large or when mounting space is limited. A stacked layout measures strain at the same point, making it suitable for confined installations.

Mounting a strain gage is not a difficult task if the recommended procedure is followed. However, strain gages are fragile and can be easily damaged.

To mount a strain gage, you will need the following tools and materials: an illuminated magnifier, electrical-grade solder, rosin soldering flux, epoxy adhesive, cyanoacrylate adhesive, lacquer thinner, acetone or alcohol, masking tape, toothpicks, tweezers, an awl, a ruler, fine-gage tinned-copper lead wires, and an ohmmeter.

For further guidance, refer to the YouTube video below, which demonstrates the installation process of a strain gage. The video was created by the Mechatronics Department at the University of Jordan.

A short story about strain gage connections fits perfectly in this context.

Most people are familiar with Murphy's Law, which states: “Whatever can go wrong, will go wrong.” While the law is widely known, what is less commonly known is that it originated from strain gage measurements. The "inventor" of this law, Capt. Ed Murphy, developed a strain gage measurement system for a g-force testing setup at Edwards Air Force Base. The goal was to determine the maximum g-force the human body could withstand. (As a side note, a real human was used in the test, and the maximum force reached was 40 g.)

The result of the first measurement was zero—because the strain gages had been connected in such a way that they canceled each other out. Capt. Ed Murphy blamed his assistant, who had wired the gages incorrectly. Even more interesting, Murphy had declined the offer to verify the system before performing the test.

The moral of the story is clear: Connect – Calibrate – Verify – Measure.
Had Capt. Ed Murphy followed this procedure, Murphy’s Law might never have been coined—at least not on that occasion.

Before continuing our discussion about strain gages, we must first understand the Wheatstone bridge circuit. This circuit, used for measuring electrical resistance, was popularized by Sir Charles Wheatstone in 1843.

The Wheatstone bridge is essentially two simple series–parallel arrangements of resistors connected between a voltage supply terminal and ground. When the two parallel resistor legs are balanced, the bridge produces zero voltage difference.

It consists of two input terminals and two output terminals, with four resistors configured in a diamond shape. The diagram below shows a typical representation of the Wheatstone bridge.

Because it is highly sensitive to small changes in resistance, the Wheatstone bridge is particularly suitable for use with strain gages.

The example below demonstrates how strain gages utilize the Wheatstone bridge circuit and how they can be bonded to a test specimen.

When no force is applied to the object, both strain gages have equal resistance, and the bridge circuit remains balanced. However, when a downward force is applied, the object bends, stretching strain gage #1 while simultaneously compressing strain gage #2. This causes the bridge to become unbalanced, resulting in a voltage difference. This effect is clearly illustrated in the second picture.

There are several configurations for basic measurements. First, we need to understand the special properties of materials. When a material is stretched, it usually becomes thinner in the other two directions. The ratio of transverse strain to longitudinal strain is called Poisson's ratio [ν]. When strain gages are positioned 90° apart, it becomes especially important to account for this ratio and include it in the calculations. For steel, Poisson’s ratio ranges from 0.27 to 0.31 (typically 0.3 is used), while for aluminum it is approximately 0.33.

The following table shows several basic strain gage configurations. These are generally divided into quarter-bridge, half-bridge, and full-bridge circuits.

A strain gage Wheatstone bridge can be configured as a quarter, half, or full bridge depending on the measurement purpose.

Quarter bridge system

In a quarter-bridge system, a single strain gage is connected to one arm of the Wheatstone bridge, while fixed resistors occupy the remaining three arms. This system is simple to configure, which makes it widely used for general stress and strain measurements.

However, the quarter-bridge two-wire system (shown in the first figure) is significantly affected by the resistance of the lead wires. Therefore, if large temperature variations are expected, or if the lead wire length is considerable, the quarter-bridge three-wire system (shown in the second figure) should be used instead.

Half-bridge system

The half-bridge system is used to eliminate strain components other than the target strain, depending on the measurement purpose. In this configuration, strain gages are connected to adjacent or opposite arms of the Wheatstone bridge, while a fixed resistor is placed on the remaining arm.

This setup allows two different configurations:

• Active-Dummy Method: One strain gage functions as an active gage, while the second acts as a dummy gage for temperature compensation.

• Active-Active Method: Both gages serve as active gages, enhancing sensitivity and accuracy.

Full bridge system

In the full-bridge system, strain gages are connected to all four arms of the Wheatstone bridge. This configuration provides the highest output signal from strain gage transducers, offers excellent temperature compensation, and effectively eliminates unwanted strain components, leaving only the target strain.

4 - wire circuit

The 4-wire circuit is used for strain gage measurements with short lead wires. Its limitation is that the supply voltage is only accurate at the amplifier connector.

6 - wire circuit

The 6-wire circuit is recommended when long lead wires are required to connect the sensor. In this configuration, the sense wires are connected to the excitation at the sensor side. This allows the amplifier to "sense" or measure the actual excitation voltage at the sensor and adjust it accordingly, ensuring that the correct voltage is applied at the sensor. This significantly improves the amplitude accuracy of the measurement.

4-wire4 - wire circuit

The 4-wire circuit is used for strain gage measurements with short lead wires. Its limitation is that the supply voltage is only accurate at the amplifier connector.

6-wire circuit

The 6-wire circuit is recommended when long wires are used to connect the sensor. In this setup, the sense wires are connected to the excitation at the sensor side. This allows the amplifier to "sense" or measure the actual excitation voltage at the sensor and adjust it accordingly, ensuring the correct voltage is applied. As a result, the amplitude accuracy of the measurement is significantly improved.

For this experiment, we will connect the tuning fork to a Sirius device. The tuning fork has a single gage attached.

We will be measuring two different physical quantities: strain and stress.

Quarter bridge setupž

Let’s take a look at a quarter-bridge setup in Dewesoft using the Sirius STG.

A single strain gage is attached to the tuning fork, and we are measuring strain.

The picture below shows the specifications of a strain gage. The resistance of the strain gage is 120 Ω, and the gage factor (k) is 2.07.

When selecting strain gages, the most common options are 120 Ω or 350 Ω.

• 120 Ω gages consume less power and generate less heat.

• 350 Ω gages provide larger signals, making them more suitable for use with longer cables.

1. Measurement of strain

For our measurement, we will use a 350 Ω strain gage.
The channel setup looks as follows:

First, we must set the input type to quarter-bridge with 350 Ω. In this configuration, the single external gage is the active measuring element, while the other three gages are internal precision resistors. At this stage, the input scaling is expressed in mV/V.

Next, we can freely select the excitation voltage. A higher excitation voltage increases the signal level and therefore improves the signal-to-noise ratio. However, it also causes greater gage self-heating and higher power consumption. Since every strain gage has a specific excitation voltage limit, always check the manufacturer’s specifications before applying voltage to avoid damaging the sensor.

We then select the physical quantity <Strain>. Once selected, the scaling changes from mV/V to µm/m.

The following step is to enter the gage factor (k) and bridge factor from the specification. Since this is only a quarter-bridge configuration, the bridge factor (bf) should be set to 1.

The default equation for scaling to relative deformation (Ɛ) is:

Consideration about strain scaling:

The equation used for strain scaling is based on the assumption that the resistance variation ΔR₁ is much smaller than the resistance R₁ itself, which is always true for metal strain gages.

For higher strain values, the relationship between the reading in mV/V and the strain ε is not linear. Assuming a bridge factor (bf) of 1, the equation for the quarter-bridge is defined as follows:

Let’s compare the results of both equations using a commonly applied shunt calibration with a bridge resistance (R₍Bridge₎) of 350 Ω and a shunt resistance (R₍Shunt₎) of 175 kΩ, where the bridge amplifier outputs 0.4995 mV/V.

The difference in this case of -0.1% can generally be expressed as:

As an option, Dewesoft can also use this equation to calculate strain for ¼-bridges based on the same principle.

If this option is selected, the formula used will also be displayed in the Info field of the amplifier.

2. Measurement of stress

Now, let's create another channel setup. This time, we will be measuring stress, and here is how the setup looks:

When measuring stress, we configure everything the same way as when measuring strain, except for the Physical Quantity in the General Settings and the options in the lower-right corner.

Depending on the bridge mode selected in the General Amplifier Settings, we can then choose the graphically presented bridge configuration and the material, ensuring that the correct Young's modulus is applied.

This way, another potential source of error is eliminated.

Let’s take a look at the recorder. If a static force is applied to the tuning fork, we can observe a change in the signal offset. We might also try striking the tuning fork so that it produces a sound, reflecting its natural frequency (the conventional way it is used). This appears as a high-frequency vibration with decreasing amplitude due to air friction and internal friction within the fork.

When viewing the FFT screen (set to a logarithmic scale to display all amplitudes more clearly), we can see a distinct peak at approximately 440 Hz. By placing a cursor on the peak in the FFT—simply by clicking on it—we find the frequency to be 439.5 Hz. It is not exactly 440 Hz because the tuning fork used here is likely worn after many years of use, and also because the FFT has a certain line resolution.

The line resolution depends on both the sampling rate and the number of lines chosen for the FFT. If we want a faster response from the FFT, we would select fewer lines, but this comes at the cost of lower frequency resolution. Conversely, if we want to measure the frequency more precisely, we need to use a higher line resolution. This principle is described in detail in the reference guide, but a simple rule of thumb is as follows:

• If it takes 1 second to acquire the data for the FFT calculation, the resulting FFT will have a 1 Hz line resolution.

• If we acquire data for 2 seconds, the line resolution improves to 0.5 Hz.

This is also a perfect example to demonstrate how filters are used in Dewesoft. The signal clearly consists of two components: an offset (static load) and a dynamic ringing at approximately 440 Hz.

To separate these two components from the original waveform, we need to apply two filters—one low-pass filter and one high-pass filter. Both filters can be added in the Math section.

1. First, we set the input channel (in this case, the tuning fork). Then, we apply a 6th-order low-pass filter with a cutoff frequency (Fc2) of 200 Hz. This allows all signals below 200 Hz to pass while attenuating all frequencies above that threshold.

The second filter is a 6th-order high-pass filter with the same cutoff frequency.

If we display these two filters on the recorder, we can see that the signal is clearly decomposed into the static load and the dynamic ringing. This technique can be used to remove unwanted parts of the signal or to extract desired frequency components.

At this point, it is worth noting that IIR filters are preferred when higher calculation speeds and sharper cutoff rates are required. On the other hand, FIR filters can be used when avoiding phase shifts is important. More details can be found in the Filter Comparison section of the user's manual.

For this experiment, we will connect an off-the-shelf load cell with a full-bridge connection to a Sirius device.

Dog bone specimen for measuring force

To help engineers select the right material for their applications, many materials have been tested repeatedly, and their properties have been published in material handbooks. Beyond that, engineers have also established a set of standards to ensure that testing is conducted consistently, regardless of the material being tested.

At this point, it is important to explain why the specimen is shaped like a dog bone with widened ends. The geometry of the specimen is crucial. A simple rectangular specimen would create higher stress concentrations at the grips. To prevent this issue, engineers developed the special dog bone shape. This design is wider at the top and bottom where the grips hold the specimen, while the cross-sectional area is narrower in the center.

Why is this important? The smaller cross-sectional area ensures that stress is concentrated in the middle of the specimen, eliminating the unwanted influence of the grips. By using this shape, engineers can be confident that the stress measured by the testing system accurately reflects the actual stress experienced by the material.

Example 1

Stress-Strain Diagram with Characteristic Values for Steel (Young's Modulus [E] = 210 MPa)

The image below shows an excellent example of a dog bone-shaped specimen and a typical stress-strain diagram for a ductile, elastic material such as steel. Engineers can extract valuable information from this diagram to better understand material behavior, including its modulus of elasticity and yield stress.

• Elastic range: Defined by the linear portion of the stress-strain curve.

• Plastic range: The portion of the diagram to the right of the elastic region, representing permanent deformation. Once the material enters this region, it begins to deform permanently.

• Yield stress: The minimum stress that causes permanent deformation.

• Ultimate tensile stress: The maximum stress a material can withstand. This corresponds to the peak point on the diagram. At this stage, necking begins, and the material progresses toward ultimate failure.

• Necking: A localized reduction in the cross-sectional area of the specimen.

• Fracture stress: The stress at which the material ultimately fails. This is the final stress state experienced before fracture.

By examining this diagram, engineers can clearly identify and record the key mechanical properties that define how a material responds under load.

For the full-bridge sensor, we will use a "homemade" force sensor. The sensor is built from two XY strain gages: one oriented in the principal direction and the other in the transverse direction. This design provides temperature compensation and cancels out bending effects.

Let's perform a step-by-step calibration of this configuration. First, change the input type to Full Bridge and perform a sensor balance. If zeroing the bridge is unsuccessful, select the highest possible range, perform zeroing, then switch back to the desired range and perform zeroing again.

Next, set the Low-pass filter. Since this example involves mostly static measurements in the lower region of the probe, the results can be improved by applying a very low low-pass filter. In this case, it is set to 100 Hz.

Next, set the Strain Scaling to Used, and assign a k-factor of 2 to the gages and a Bridge Factor of 2.6. The value of 2 is taken from the strain gage datasheet, while the value of 2.6 is obtained from the table of bridge configurations. At this point, the input values represent strain in µm/m.

The final step is to apply Scaling by Function, since the force should be measured in kN. To achieve this, we perform calculations using the following equations:

Therefore

Given that the elastic modulus (Young's modulus) of steel is 210,000 N/mm² and the cross-sectional area of the sensor is 139 mm², we obtain:

The factor 1E6 is included because strain is measured in µm/m, and a scaling factor is required for this unit.

Now the real data is scaled in N. The final step is to select the appropriate measurement range based on the input. In this case, the lowest range is more than sufficient for the measurement.

The Short and Shunt buttons are used to check the strain gage. The Short function shorts the input pins to measure the bridge offset. The Shunt function (also used in the shunt calibration routine) verifies that the bridge is responding correctly, ensuring that the connections are working.

For this Pro training course, we organized a short in-house competition to see who could break the load cell. The measurement range of this load cell is 30 kN (approximately 3 tons), so it was quite reasonable to expect that none of the competitors would succeed.

To make the measurement more fun, the load was pulled by our students—and even one of the girls joined in! We also connected a USB camera to record a video of the attempt.

Now, let’s take a look at the results.

The boys did a great job, pulling 466 N and 397 N, which corresponds to 46.6 kg and 39.7 kg, respectively.

In the picture below, you can see that the girl can beat everyone—either because she is a great athlete or simply because she knows how to use math in Dewesoft. 🙂

In electronics, signal conditioning refers to manipulating an analog signal so that it meets the requirements of the next stage for further processing. Signal conditioning is necessary before a data acquisition device can effectively and accurately measure the signal.

Signal conditioning can involve amplification, filtering, converting, range matching, isolation, and other processes required to make the sensor output suitable for further processing.

Filtering is the most common signal conditioning function, as not all parts of a signal’s frequency spectrum contain useful data. A common example is the 60 Hz AC power line noise present in most environments, which produces interference if amplified.

Signal amplification serves two important purposes: it increases the resolution of the input signal and improves its signal-to-noise ratio. For example, the output of an electronic temperature sensor—typically in the millivolt range—is too low for an analog-to-digital converter (ADC) to process directly. In such cases, the signal must be amplified to a level suitable for the ADC.

Signal isolation allows the signal to pass from the source to the measurement device without a direct physical connection. This helps prevent disturbances from external sources and protects expensive measurement equipment from potential damage.

Bridge balancing

Bridge balancing is a function of bridge amplifiers used to eliminate the offset of a bridge sensor. Mathematically, this means removing the initial sensor offset on the amplifier side. For the demonstration, we connect a quarter-bridge strain sensor.

On the scope screen, the unscaled value may not be exactly 0 mV/V. This happens because the strain gage does not measure exactly 350 Ω, which is normal due to manufacturing tolerances.

To correct this, click “Balance sensor” to zero the bridge.

The unbalance will be measured and displayed. The amplifier will automatically select the correct setting to achieve a full-scale range.

At this point, the input signal is 0 mV/V.

Balancing is normally performed just before starting a measurement. If balancing needs to be applied to multiple channels, this can be done using the Group Operations function (described at the end of this topic).

Short on

When sensor balance is applied, the value of the unbalance can always be checked by using the Short function. When the SHORT ON operation is activated, pins 2 and 7 (the amplifier’s input pins) are internally shorted.

To perform this, simply click “Short on.”

The resulting value will display the sensor unbalance.

Please note that the sensor balance can be removed by clicking the Reset button. This action clears the sensor offset and returns the amplifier to its initial state.

After resetting, the screen will once again show the sensor’s unbalance, which in this example is 0.9 mV/V.

"Balance" and "Reset" are therefore opposite operations.

Zero

There is also a function called "Zero", which is similar to "Balance sensor". Let’s examine the difference below:

Imagine we have a force transducer with a full-bridge strain output that measures weight in our experiment.

• In the first case, we measure the unbalance of the bridge sensor, e.g., 35 N.

• After performing a "Balance sensor", the output is reset to 0 N.

• Next, a vehicle is placed on the testbed, and we measure its weight: 12,000 N.

Since in our measurement only the change in weight is of interest, we cancel out the fixed offset by using the Zero function.

Click the "Zero" button in Channel Setup (it can be reset with a right-click). The output is now zero again. Note that this is a pure software subtraction. If the range is set to "Automatic", it will automatically adapt to -52,000 … +28,000.

The range can be set to "Automatic" in the Channel Setup window of the appropriate channel (right-click).

Now all offsets are canceled, and we can begin the measurement. This function can also be accessed in Measure mode (but not while storing).

We use shunt calibration for several purposes:

• To check if the amplifier is working properly (excitation and value readout).

• To verify that the strain gage is connected and functioning correctly.

• To compensate for the length of the lead wires.

The SHUNT ON operation is used to check whether the connected strain gage is functioning properly. From the wiring schematic in Dewesoft, you can see that the amplifier already includes an integrated shunt resistor.

The idea behind this operation is to “shunt” or connect a resistor of a known parallel value to one resistor of the bridge, thereby creating a known and calculable unbalance.

With Shunt Calibration, we can automatically compare the measured value against a predefined reference (from the sensor database or TEDS). For the actual measurement, this internal shunt resistor is, of course, disconnected again.

Now, let’s look at the theory of strain calculation:
The parallel connection of a 175 kΩ resistor would result in the following change:

The result of applying the shunt will produce a reading at the strain gage amplifier of:

To scale from mV/V to strain, we first look at the basic formula for Wheatstone bridges:

Since the shunt is applied only to R1, we do not need to consider R2–R4, and we substitute accordingly.

With the assumption that the bridge factor (bf) = 1, we arrive at the following equation:

We calculate the strain from the amplifier reading using a gage factor of 2 with:

Now let’s validate the theory with real measurements by first checking the mV/V reading.
We begin by zeroing the sensor using “Balance Sensor” (1), and then perform “Shunt On” (2) to obtain the reading.

The output value comes very close to the expected value (0.4995 mV/V).

Next, let’s check the exact deviation of the strain result.

We change the physical quantity to “Strain” and define the target value.

After completing all the preparations, we perform the Shunt Cal Check.

The result of our shunt calibration check looks very promising in this case (0.2%). This means that the strain gage is functioning properly.

Notes about SHUNT CAL CHECK 

• During the process, the sensor unbalance is automatically and temporarily compensated in the background to obtain the real signal deviation caused by the applied shunt.

  ⚠️ Important: The active strain gage must not experience any load change (different strain) during the shunt calibration measurement. Any additional sensor signal would distort the result.

Shunt cal auto target:

If the shunt calibration target is not known or defined in the sensor database, the system can automatically calculate the theoretical result based on the known strain gage and shunt resistor values.

For example, let’s consider a 350 Ω strain gage resistor and a 59.88 kΩ shunt resistor.

We can see that the values in Dewesoft correspond to the formula above.

• The value of R_Shunt is automatically taken from the selected shunt resistor of the amplifier.

  For a Quarter-bridge, R is taken directly from the amplifier settings. For Full- and Half-bridge configurations, the value must be entered manually. This entered value does not affect the measured signal but is only used to calculate the target.

  The shunt calibration target is calculated as the scaled sensor value (e.g., µm/m, N, etc.).

  Depending on the amplifier type, we offer different shunt connection options. The example below shows a connection to the shunt against Sns-, which results in a negative signal and, consequently, a negative shunt calibration target result.

Use custom shunt resistor: 
Many different values of shunt resistors are used by different suppliers.

• 59.88 kΩ and 175 kΩ are used to obtain 1000 µm/m at 120 Ω or 350 Ω (as in the SIRIUS-STG type).

• 100 kΩ is used in most Dewesoft strain gage amplifiers.

• 320 kΩ, 160 kΩ, 80 kΩ, 55 kΩ, and many others can also be found on the market.

Of course, no strain gage amplifier on the market can include all these different shunt values.
But how can we check the shunt calibration target if the built-in shunt resistor of the amplifier does not match the sensor data?

Let’s assume a 350 Ω strain gage, where the shunt calibration target T_SNS of 0.5 mV/V is defined with a shunt resistor R_SNS = 175 kΩ.
We want to verify this result with our SIRIUSi-HD-STGS module, which has a built-in shunt resistor R_AMP = 100 kΩ.

In this case, we simply need to enable the custom shunt option and define the given value T_SNS, which was originally defined for 175 kΩ.

With the <Shunt Cal Check>, we have all the information needed to calculate the deviation of the sensor’s given result using the formulas below.

The nominal target result for the amplifier, T_AmpNorm, is:

The final indicated result is:

Thus, regardless of which shunt value is defined in the sensor, we recalculate the result according to the selected shunt value of the amplifier.

In some cases, strain gages are mounted at a distance from the measuring equipment. This distance increases the likelihood of errors due to temperature changes and lead-wire desensitization. As a result, the resistance of the lead wires changes. In a two-wire installation, as shown in the figure below, the two leads are connected in series with the strain gage, and any change in lead-wire resistance cannot be distinguished from changes in the resistance of the strain gage itself.

However, this issue can be corrected. By adding a third wire, as illustrated in the next figure, the effect of lead-wire resistance can be compensated.

In this configuration, the third wire acts as a sense lead, through which no current flows. This wiring method for strain gages cancels out part of the errors introduced by extension wires. From a theoretical standpoint, if the lead wires running to the sensor have the same nominal resistance, temperature coefficient, and temperature, full compensation is achieved. In practice, however, wires typically have a tolerance of about 10%, meaning that a three-wire installation cannot completely eliminate two-wire errors. Nonetheless, it significantly reduces them. If the lead-wire resistance does not exceed the gage resistance and remains small in comparison, the error is negligible. However, if the lead-wire resistance exceeds 0.1% of the nominal gage resistance, the error becomes significant. For this reason, in industrial applications, lead-wire lengths should be minimized or avoided altogether by placing the transmitter directly at the sensor.

Lead wire compensation

As explained above, lead-wire resistance reduces the sensitivity of strain gage signals due to the voltage drop. This drop lowers the voltage level across the active strain gage, thereby decreasing sensitivity. The example below illustrates a reduction of 0.1 V, which results in a noticeable loss of sensitivity.

This reduced sensitivity is also observed when performing a shunt calibration. Therefore, we use shunt calibration to calculate the lead-wire resistance (R_Leadwire) based on the supply voltage (V_S), bridge resistance (R_Bridge), and shunt resistance (R_Shunt).

Using the bridge resistance (R_Bridge) and the lead-wire resistance (R_Leadwire), the compensation factor (Corr_Leadwire) is calculated.

With this factor, the raw data (Data_Raw) from the strain gage amplifier is corrected to (Data_Corr). The corrected data is then used for further processing inside the Dewesoft X software package, such as sensor scaling, data storage, and visualization.

By pressing <Compensate> in the channel setup screen, all the above steps are performed automatically in the background:

• Shunt calibration is initialized to determine V_S.

• R_Leadwire and Corr_Leadwire are calculated and applied for further data processing.

• These values are stored together with the complete DewesoftX setup file.

Important note: The active strain gage must not experience any load change (different strain) during the shunt calibration measurement. Any additional sensor signal will falsify the result.

The figure below shows the result for a 120 Ω strain gage connected with a 100 m cable of 0.25 mm² to our amplifier. Without correction, the gain error would be as high as 5.6%, which would significantly affect the measurement results.

After lead-wire compensation, no additional sensor balance is required. The indicated sensor unbalance (1 mV/V) is correctly displayed by rescaling with the equation above (Data_Corr).

Before compensation, this same sensor unbalance was displayed as 0.944 mV/V, which incorrectly suggested an offset.

Lead-wire compensation can also be performed in a Half-Bridge 3-wire or Full-Bridge 4-wire connection, where Exc and Sens are connected directly to the amplifier.

For different bridge and shunt configurations, the formulas for calculating R_Leadwire and Corr_Leadwire vary slightly, but the underlying principle remains the same. In Full-Bridge 6-wire, Half-Bridge 5-wire, or Quarter-Bridge 4-wire configurations, lead-wire compensation is performed automatically by the amplifier. Therefore, no additional compensation settings are required.

We have several group operations (Channel actions), which can be found in the Channel setup.

By pressing the three-dot button, we can access additional Channel actions. The number of visible buttons depends on the functionality of the installed amplifier type. Dedicated functions for a strain gauge amplifier are only visible if such an amplifier is installed.

Lead wire compensation

To activate the Lead Wire Compensation function for all channels, press this button.

This allows us to perform lead wire compensation for all installed channels in the system at once.

Shunt cal check

To get a complete overview of the Shunt-Cal check results in the channel grid, we need to enable “Sensor check target,” “Sensor check result,” “Sensor check error,” and “Group.”

After performing the Shunt-Cal check, the complete results are displayed in the channel table.

Multi group actions

We can also perform group operations with different groups. This is done by assigning one channel to “Group 1” and another to “Group 2.”

You can then apply channel operations on a group-by-group basis.

Example:

We will demonstrate the group operations with one short measurement.

For this experiment, we will use the Sirius instrument. The measurement begins by connecting two tuning forks (quarter bridge, 3-wire, 350 ohms). The tuning forks can be connected to Sirius via STG adapters.

Unfortunately, the Sirius instrument we are using does not have two STG channels, so we will connect one tuning fork via STG and the second one via an STG-to-MULTI converter. We already know how to configure a 3-wire, 350-ohm quarter bridge connected via STG.

This is the setup:

The settings differ slightly when using the STG-to-MULTI converter.

Let’s take a closer look.

We can select Bridge Measurement and Bridge Mode. However, when we try to choose the internal 175 kΩ bridge shunt value, this option is not available. At this point, we should explain one of Dewesoft’s great features: the software automatically recalculates all values, regardless of which bridge shunt you select. This means that Dewesoft ensures proper calculations even if different resistor values are used.

In the General Information tab, a 59.88 kΩ resistor is shown. If we enable the Use custom shunt resistor option, we can enter the correct value of the resistor being used.

When entering a resistor value, the field turns yellow. We confirm the entry by pressing Enter.

The software then automatically recalculates the values according to the specified bridge shunt and chosen resistance values. An asterisk (*) after the result indicates that the check was performed with a different resistor value than the one physically available.

We can also perform group operations in Measure Mode, so let’s go there first. In this mode, we will see two new buttons: Zero and Amplifier.

If the Zero button is not visible, sensors from the sensor database or TEDS sensors are most likely being used, and changing the offset is not allowed. Check the sensor settings in Channel Setup or the Sensor Editor.

If the Amplifier button is not visible, no amplifiers have been set to “Used” in the channel setup.

Balance sensor

The Balance Sensor function can be activated in measurement mode only if storing is not active. Depending on the group settings in the setup screen, either all channels can be balanced at once or only the selected groups.

Shunt/Short at the beginning and end of the measurement

At the end of a measurement, you may want to check whether the strain gauge and the amplifier are still functioning correctly. You may also want to verify if the bridge has drifted over time due to temperature changes or other effects.

Start the measurement and click the Store button. The Zero button will disappear because zeroing also changes the channel min/max limits, which is not allowed during measurement. Additionally, balancing the bridge is no longer possible at this stage.

Perform a “Short on for 1s”, wait briefly, and then press “Shunt on for 1s.”

At the end of the measurement, while still storing, perform a “Short on for 1s” followed by a “Shunt on for 1s.” Then stop the measurement and switch to Analyse Mode. Activate the cursors in the Properties of the recorder instrument (on the left side).

Move the white Cursor I to the Short position at the start, and Cursor II to the Short position at the end (indicated by grey arrows). You can also lock the cursors to prevent losing their position when zooming in and out of a longer measurement.

The Delta value will be displayed on the right side. In our case, it is 0.0 — measurement OK.