Calibration Procedure; Calibration Points; Measuring The Set-Point Error - Fluke 7008 User Manual

Calibration bath

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In some instances the user may want to calibrate the bath to improve the

temperature set-point accuracy. Calibration is done by adjusting the controller

probe calibration constants D0 and DG so that the temperature of the bath as

measured with a standard thermometer agrees more closely with the bath set-

point. The thermometer used must be able to measure the bath fluid temperature

with higher accuracy than the desired accuracy of the bath. By using a good

thermometer and carefully following procedure the bath can be calibrated to an

accuracy of better than 0.1 °C over a range of 50 degrees.

11.1 Calibration Points

In calibrating the bath D0 and DG are adjusted to minimize the set-point error at

each of two different bath temperatures. Any two reasonably separated bath

temperatures may be used for the calibration however best results will be

obtained when using bath temperatures which are just within the most useful

operating range of the bath. The farther apart the calibration temperatures the

larger will be the calibrated temperature range but the calibration error will also

be greater over the range. If for instance 20 °C and 80 °C are chosen as the

calibration temperatures then the bath may achieve an accuracy of say ±0.2 °C

over the range 20 to 80 °C. Choosing 30 °C and 70 °C may allow the bath to

have a better accuracy of maybe ±0.05 °C over the range 30 to 70 °C but outside

that range the accuracy may be only ±0.5 °C.

11.2 Measuring the Set-point Error

The first step in the calibration procedure is to measure the temperature errors

(including sign) at the two calibration temperatures. First set the bath to the lower

set point which we will call t

minutes to stabilize at that temperature. Check the bath stability with the

thermometer. When both the bath and the thermometer have stabilized measure

the bath temperature with the thermometer and compute the temperature error

err

which is the actual bath temperature minus the set-point temperature. If for

example the bath is set for a lower set-point of t

measured temperature of 19.7 °C then the error is −0.3 °C.

Next, set the bath for the upper set-point t

temperature and compute the error err

bath was set for 80 °C and the thermometer measured 80.1 °C giving an error of

+0.1 °C.