Temperature Measurement using RTDs

Integrated Scientific Resources uses Resistance Temperature Detectors (RTDs) when final testing silicon strain gages. The temperature is obtained using the IEC 571 standard temperature versus RTD resistance for PT100 RTD.  The temperature is obtained as follows:

• The resistance of the RTD is measured using a moving average filter;
• The relative resistance is calculated by dividing the measured resistance by 100 for 100 ohm RTD or 1000 for 1000 ohms RTD;
• The temperatures values of relative resistances from IEC 571 lookup table;
• The temperature is estimated by linear interpolation of the two values above and below the measured average relative value to obtain the value at the measured resistance.

Calibrating the Reference, Zero Centigrade, Resistance

General Description

The average resistance is measured at ambient. The assumption is that ‘Calibration’ is achieved by adjusting the value of the RTD resistance at zero degrees from the standard of 100 or 1000 ohms to a new value as determined by a correction formula.

Specific Procedure

The temperature of the RTD is estimated based on the measured resistance relative to the resistance of the RTD at zero degrees centigrade.  The ‘Calibration‘ procedure establishes a correction for the RTDs by adjusting the reference resistance of the RTDs based on the population average of the RTDs used for calibration so that all RTDs have the same relative resistance value. RTD calibration is accomplished by estimating the RTD resistance at zero degrees centigrade temperature.  Once corrected in this manner, the temperature of all RTDs measured at some arbitrary ambient condition will equal the average temperature of these RTDs lookup values using the standard RTD resistance.

More specifically assume the RTD standard zero resistance Ro (e.g., 100 ohms):

1. Measure the resistance of the RTDs set (in practice, a set of 32 RTDs were used);
2. Alert the operator to replace any RTD that is outside an outlier fence.
3. Using only non-outlier RTDs, calculate the average RTD resistance of the 32 RTD Resistances Ri, i = 1 to 32, as Ra.
4. The ‘calibration’ procedure assumes that Ra is the correct resistance at the measured temperature.
5. The ‘calibrated’ reference resistance of each of the RTDs at zero temperature is Ri’= Ro * Ri / Ra.

The ‘calibrated’ resistance Ri’ is then used to lookup for the temperature measured for each RTD.

Validation

For example, and as a validation, if RTD x measured resistance is again, Ri, the corrected resistance ratio, which is used for looking up the temperature will be:  Ki = Ri/Ri’ = Ei/(Ro*Ri/Ra) = Ra/Ro. The lookup will then return Ta, which is the correct temperature for RTDi with a resistance of Ri at Ta.