Foil Technology Products for Accurate Measurements.

Micro-Measurements strain gages built with Advanced Sensors Technology are fundamental sensing devices that function as the building blocks of many other types of transducers—including pressure, load, displacement and torque sensors— which are used extensively in structural test and monitoring applications.

An example of such a transducer is a load cell that converts a mechanical force to an electrical output signal. In these designs, gages connected in a Wheatstone bridge, result in an accurate and rugged transducer that can operate in extreme environments. To achieve accuracy, additional resistive components are added to the Wheatstone bridge circuit, which correct for manufacturing tolerances and ambient temperature changes and self-heating effects.

The Wheatstone bridge is typically the circuit of choice for making precise strain gage based measurements. 

These circuits usually include:

(a) The strain gages for actively measuring the strains.

(b) Ultra-high precision foil resistors incorporated in the measuring instrument for completing the circuit, as shown below.

Bridge Completion Methods

A typical three-wire quarter-bridge strain-gage circuit requires three foil resistors for the bridge completion circuit; however, in some cases, the circuit must be completed externally to the instrument. The most common reason is when bridge completion resistors are not integral to the instrument, but accuracy considerations are also a factor. Under certain test conditions, external bridge completion can reduce the effects of noise pick-up in the lead wire system, or eliminate the effects of contact resistance.

Suitable elements for completion of the bridge outside the instrument in these cases include:

• Bonded Advanced Sensors Technology foil strain gages.

• Ultra-High Precision foil resistors.

• Bridge completion modules (BCM's) built on Bulk Metal Foil Technology from Vishay Foil Resistors (VFR).

Functional differences, along with differences in product configuration and installation techniques, provide both advantages and disadvantages for each type of external bridge completion.

Bonded Advanced Sensors Technology foil strain gages

Bonding strain gages to the test part (individually or with multiple grids on the same backing) is a good choice when large changes in test temperatures are expected, since this is the only completion method that will provide temperature compensation. Installation is relatively easy and inexpensive. Both active and completion gages must be of the same alloy, with the same S-T-C number. For highest accuracy, all should be from the same production batch of foil. Of course, the temperature of gages in adjacent arms of the bridge must always be identical for effective temperature compensation. Further, care must be taken to install the completion gages in a region that: (a) is free from applied strains, and (b) can undergo thermal expansion without any mechanical restraint.  Additionally, all the intra-bridge wire lengths must be equal to keep the bridge balanced and to cancel the thermal resistance change in the wire.

"Compensation" ultra-high precision resistors built with Bulk Metal Foil Technology are an alternative to Micro-Measurements strain gages for bridge completion when their main use is to correct for bridge unbalance errors that result when the transducer is used over a widely changing temperature range.

Temperature sensing resistors, when added to the circuit, correct the output sensitivity with temperature, correcting for not only the variation in gage factor of the strain gage but also the change in modulus of elasticity of the load cell element. They are particularly useful when nonstandard bridge resistances are required, and when higher power levels must be dissipated within the bridge circuit. Vishay Precision Group (VPG) designs and manufactures foil resistors so they are very insensitive to thermal changes and, therefore, are not for use as a temperature-compensating resistor. Unlike strain gages, foil resistors are mechanically decoupled from the test part. Typically, a bonded terminal is installed on a test part from which the resistor leads soldered to the terminals. As with strain gages, care must be taken to protect both the leads and solder joints from the test environment, making installation somewhat demanding.

Bridge completion modules (BCM's) – Vishay Foil Technology (Z1-Foil)

Since many general-purpose instruments do not provide internal bridge completion capability, the MRF-Series of BCMs effectively completes a full Wheatstone bridge for accurate and stable measurements when testing specimens structured as a ¼ -bridge (single gage) or ½ -bridge (dual gages).

The accuracy of the strain measurement depends on the quality of the strain gage used, but also on the bridge completion components. Bridge completion circuits installed close to the strain gage made from foil resistors optimize the measurement accuracy. Micro-Measurements offers a wide selection of qualified and dedicated BCM for strain gage sensors that cover the spectrum of values.  The MRF-Series are specially fabricated using Vishay foil resistor technology to meet the highest performance standards under a wide range of environmental conditions.  These MRF modules are packaged to ensure reliable and repeatable accurate results as well as easy installation.

The MRF-Series modules are available in a wide range of configurations that will allow for bridge completion for 120, 350, 1000, and even 5000-ohm quarter-bridge, and half bridge circuits.

These versatile modules also enables a user to make ¼-bridge and ½-bridge inputs to instrumentation that may not have internal Wheatstone bridge completion built-in.  They are especially useful when completing the Wheatstone bridge near the strain gage installation is preferred.

Applications of Foil Technology Resistors in the Measuring Instrument.

Even though strain gages are very common, acquiring reliable data from them can be a challenge. Several factors may affect the measurement performance of a strain gage: the signal conditioning, the construction and location of the Wheatstone bridge (the most common bridge type used to measure resistance), inductance and capacitance, the precision resistors used in the circuit, and the excitation source.

Vishay Precision Group (VPG) Ultra-High Precision foil resistors have three - basic uses in standard Micro-Measurements strain gage applications:

• Shunt calibration of strain-measuring instruments
• Bridge completion.
• Stable low TCR gain resistors in the signal conditioning stage

In shunt calibration, a fixed ultra-high precision resistor is temporarily shunted across one of the bridge arms to produce a known and controlled resistance change in the bridge circuit. The resulting instrument indication (bridge output) is compared to the calculated strain corresponding to the resistance change and used as the calibration factor for the measuring channel.

In bridge-completion, a precision resistor is used in the adjacent arm of the Wheatstone bridge and two additional precision resistors for the remaining half bridge completes the external half-Wheatstone bridge circuit when a single strain gage is connected in a quarter-Wheatstone bridge arrangement.

In the input signal conditioning circuit the Foil resistors are used as gain resistors in the input amplifier circuit. The low temperature coefficient of resistance (TCR) and high stability of the Foil resistors result in low temperature drift and excellent performance of the instrument over wide temperature range.

In each of these applications, the accuracy of the strain measurement is affected, directly or indirectly, by the accuracy and stability of the precision foil resistors used in the circuit. That’s why only the highest-precision, highest-stability resistors should be used; the choice is basically between Vishay Foil Resistors Z Technology and Z1 Technology Bulk Metal® Foil resistors.

When choosing a precision resistor for shunt calibration, the following features are highly desirable:

• Fast field calibration of the pressure transducer and load cell
• Ease of use
• Temperature coefficient of resistance (TCR)
• Power coefficient “∆R due to self-heating”
• Low thermal EMF and fast thermal stabilization
• High stability
• High immunity to Electrostatic discharge (ESD)

Figure 1.  Basic Wheatstone Bridge Circuit Diagram

Shunt Calibration

Shunt calibration of a Wheatstone bridge strain gage circuit  is a common and convenient method of setting span (scaling the instrument to read in appropriate engineering units) of a DAQ system or signal conditioning amplifier measuring the output from a strain gage or strain gage based transducer. A fixed precision resistor such as the Bulk Metal Foil leaded or surface-mount resistor switcher, or “shunted,” across one leg of the Wheatstone bridge simulates a calculated strain or load level. Gain adjustments then assure proper instrument scaling.  This does not amount to a complete calibration, since no mechanical force is actually applied. Instead, the shunt calibration provides a simulation of the mechanical input to a strain gage or transducer by unbalancing the bridge and providing with the subsequent gain adjustment correcting for lead wire resistance, voltage drop and other factors affecting span accuracy. 

This approach works best when done using a high-precision resistor with a tight tolerance (such as 0.001% to 0.01% VHP100, which is oil-filled and hermitically sealed), a known resistance, low sensitivity to temperature (especially power TCR), and low thermal EMF. The output in millivolts (mV) can be compared to what would be expected should actual force be applied. Again, this difference in output signal is corrected by a simple gain adjustment.

Shunt calibration is the standard method used throughout the industry as means of routine verification of a signal conditioner and transducer between calibrations of known, applied, traceable, mechanical, input values. It is important to remember that the shunt resistor can simulate either a tension or compression input in the Wheatstone bridge. Thermal EMF and TCR errors can affect the process and should be minimized by choosing a proper resistor. The shunt 

calibration can be applied conveniently and at any moment and most important just before starting the test process.  

Consequently, strain-gage and transducer manufacturers supply shunt calibration data, along with a shunt calibration precision resistor, as a standard feature. Of course, regular physical calibration is recommended as well to ensure the accuracy, stability, reliability and linearity of the instrument itself.

Figure 2. Shunt-calibration resistors are chosen to accurately simulate resistance change in a strain gage subjected to specified levels of compressive strain. Strain indicators generally will produce a linear output with a fully active half-bridge or full-bridge input circuit, and will be slightly in error when a single active arm is used. The same nonlinearity occurs whether the gage is actually strained in compression or simulated by shunting the gage with the corresponding calibration resistor

Strain gage installations in special projects such as naval vessels, aircrafts or other industrial applications usually will require long cable lengths between the strain gage and the measuring instrumentations due to the physical size of test article. The resistance of the signal cable will create a signal attenuation shift (voltage drop) during the acquisition process of strain data. Shunting across the active strain gage will allow for correct scaling and correct for lead wire resistance (voltage drop); however, since strain gages are often unreachable after installation, the shunt must be applied to the adjacent arm of the Wheatstone bridge whether it be near the gage or at the instrumentation. Shunting other portions of the bridge can result in errors and will not correct for lead wire resistance.

Example of Applications:

• Bridge Completion
  • Common values include 120Ω, 350Ω, 1KΩ
  • While shunt calibration is recommended for low value strain gages, in the case of 5K gages a low value series connected precision resistor can be used for calibration
  • To maintain long term precision of the measurements, the ratio arms of the bridge should also consist of high precision foil resistors
• Signal Conditioning
  • Classical amplifier “normal values”: 1 kΩ, 5 kΩ, 10 kΩ, 100 kΩ
• Calibration Shunts
  • High values: 75 kΩ and up
  • Low power, standard conditions
• A high-value Bulk Metal Foil resistor (typically 50K to 200K) may be placed in parallel across a strain gage or one leg of the Wheatstone bridge to produce a predictable mV offset shift, which can be measured and used to calibrate a load cell instead of a more inconvenient weighing standard. This resistor should be stable over a wide variety of temperatures, a long period of time, and various humidity conditions.