Load cells maintain accuracy when the heat is on

Edited by the Engineeringtalk editorial team May 21, 2002

Novatech can now offer load cells that maintain their high accuracy figures right up to 250C, opening up a whole new arena of possible applications.

Novatech can now offer load cells that maintain their high accuracy figures right up to 250C.

This hitherto unknown area of load cell performance opens up a whole new arena of possible applications where a higher than normal temperature is involved.

All load cells have output spans that change with temperature.

This causes significant measurement errors particularly if the load cell is required to operate over a wide temperature range.

The currently used method for compensating for this error in strain gauged load cells is to use nickel or Balco foil resistors in series with the sensor excitation connections.

The method is limited in accuracy by the nonlinearity of the resistance change of the foil resistors with temperature.

This nonlinearity increases significantly above 100C and foil resistors are simply not suitable for use at temperatures above 200C.

Novatech's new patented invention compensates for load cell output span changes with temperature by 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.

Very high accuracy linear compensation can be achieved over a temperature range of -50 to +250C.

This method is inherently more linear than the current "traditional" methods that employ nickel or Balco resistors in series with the sensor connections, giving a performance specification several times better on span (typically less than 0.0025% output/degC) than most other load cells currently available for high temperature use.

With additional trimming over a range of temperatures of particular interest we would expect to achieve better than 0.001% output/degC.

Novatech has an international patent application pending for this novel approach to high temperature modulus compensation, currently available on 70% of Novatech products.

Alstom Energy established a need for a particular large generator rotor to operate at temperatures up to 180C.

Part of an exhaustive development project included creep testing copper conductors under simulated loads up to 200kN (20 tonnes) and temperatures up to 200C.

An existing load frame encompassing an oven chamber formed the base apparatus for test of 2000 hour duration.

Novatech's modulus compensation technique and tightened zero drift specification provided the necessary precision to meet Alstom's uncertainty requirement of less than 1% applied load.

In addition to this, the physical profile of the load cell had to fit into a restricted height and spread the load over a specified area to earth.

The load cell took the form of a double ended shear beam.

Performance obtained was: span drift 0.002%/degC; zero drift 0.3uV/degC; and repeatability of 0.02%.

(This was Engineeringtalk's Top Story on 20 May 2002).