Meet the sensors resistance movement
When building high precision transducers you will likely need compensation resistors to reduce/eliminate undesirable output. These resistors can be classified by function and TCR (temperature coefficient of resistance) into four basic categories.
- Zero Set
- Zero TC
- Span TC
- Span Set
Zero Set, low TCR (temperature coefficient of resistance) materials - Constantan, Karma, Manganin These resistors are added into the Wheatstone bridge circuit in order to achieve resistive balance of the circuit. This compensation should be accomplished prior to Zero TC compensation below, as an unbalanced bridge can influence Zero TC adjustment. Three types of resistor adjustment are available: The E01 bondable resistor is adjusted in resistance by selectively cutting foil shorting bars (increasing resistance). The H21 series of resistor is adjusted in resistance by rubbing one of the foil loops until zero balance is achieved. The Manganin wire is a low TC resistance material that is cut to the required length (resistance).
Zero TC (Zero Shift w/Temperature) – high TCR materials Balco, Copper These resistors are similar to the zero balance resistors but they correct changes in zero balance as a function of temperature. Actual transducer test data is required to calculate the needed resistance to be added. Adjustment to the required value is accomplished in the same manner as zero balance above.
Span TC (span shift w/temperature) – High TCR materials Nickel, Balco, Copper. Uncompensated transducers will exhibit a change in measurement sensitivity as a function of temperature. To correct this error a temperature sensitive resistor is typically added to the bridge excitation lead(s). Under constant voltage excitation (normal) this resistor acts as a voltage dropping resistor that varies with temperature. Adjusting the bridge voltage in this manner can result in a transducer that has very low temperature effects on sensitivity.
Span Set Low TCR materials – Constantan, Karma, Manganin. The final transducer adjustment is often to set the transducer sensitivity to a convenient “standard” level. These levels could be 1.00 mv/v, 2.00 mv/v or even 3.00 mv/v. Such precise sensitivities cannot be obtained by simply bonding gages to a spring element because machining tolerances and gage placement accuracy will influence mv/v sensitivity. It is typical to design a bit higher sensitivity that needed and then “trim it down” to the required level. This trimming is accomplished by adding a resistor to the power lead like span TC above except this resistor is not temperature sensitive.
The necessary calculations for determining all of the required resistances can be found in the Micro-Measurements’ publication Strain Gage Based Transducers (SGBT) Section 5 (Available upon request in paper or digital form) or in TransCalc (a program designed to determine the required resistances used in transducer compensation along with output predictions).
Our selection of compensating bondable resistors and resistive wire can be found at the following link.