E018 - High Temperature Specification Loadcells
Loadcells have output spans that change with temperature. This causes significant measurement errors particularly if the loadcell is required to operate over a wide temperature range. The currently used method for compensating for this error in strain gauged loadcells is to use nickel or balco foil resistors in series with the sensor excitation connections. The method is limited in accuracy by the non-linearity of the resistance change of the foil resistors with temperature. This non-linearity increases at temperatures above 100°C (212°F). The foil resistors are not suitable for use at temperatures above 200°C (392°F).
The new invention compensates for loadcell positive span changes with temperature using passive temperature sensitive elements in the bridge circuit. The temperature coefficient of the temperature sensitive elements is adjusted to be the inverse of the sensor's uncompensated span change with temperature by selecting the value of a single low temperature coefficient resistor that is mounted externally. Accurate linear compensation can be achieved over a temperature range of 0 to 250°C (32 to 482°F). The compensation works to -50°C (-58°F) but we are not currently able to calibrate at these temperatures. The method is inherently more linear than currently used methods that employ nickel or balco resistors in series with the sensor excitation connections.
Points To Note
- Multiple sensor outputs cannot be connected in parallel.
- Output rationalising can still be carried out using resistors in series with the sensor supply. The rationalising resistors need to be mounted in an area where the ambient temperature remains below 120°C (248°F).
- The sensor output needs to be read using a meter or amplifier with an input impedance that is high relative to the value of the terminating resistor. One megohm or greater is usually adequate.
- The final compensated span drift may change its sign at some point in the temperature range.
- An external terminating resistor is required for modulus compensation trimming. This should be fitted across the output signal wires in stable ambient conditions, below 130°C (266°F).
- Output and bridge resistance values may vary from data sheet values.
- We offer a 1 year warranty on all high temperature specification loadcells.
- Full modulus compensation by testing and trimming may be applied to all loadcells covered by a quantity order, but a percentage sample may be economical.
- Our own passive modulus compensation technology was patented in July 2004. (Patent No: GB2386196)
- Above 150°C high temperature rated design is limited to steel body loadcells and is not suitable for aluminium alloy based designs.
- By limiting the temperature change bandwidth of specific interest anywhere within the maximum temperature range we can improve the accuracy of modulus compensation.
- High Temperature rated loadcells are best suited to dynamic force measurement as the static load creep performance may be detrimental to overall accuracy criteria if timed measurement period techniques are not practical. See Engineering Application Sheet E013.
- Zero drift with temperature changes as low as 0.5µV/°C (0.28µV/°F) can be available to aid zero referenced force measurements.
- All loadcell temperature change related performance parameters require the whole loadcell body to experience the same temperature change without temperature differentials across the structure. If an application has a non-uniform temperature distribution our engineering department needs to be consulted so that drift effects can be reduced by loadcell structural design features.