Fluke 5623A 6 Operation

6 Operation

Forb est results, be familiar with the operation of the heat source and the

read-out instrument.Be sure to follow the manufacturer’s instructions for the

read-out instrumentand the heat source.

Stem effectcan cause measurement errors for any thermometer not immersed

in the fluid at least 114 mm (4.5 inches). This error is due to heat lost or gained

by the sensing elementthrough the thermometer stem. In addition, heat losses

occur due toradiation losses from the sensing element to the housing.

The immersion depth for standardsis dependent on several factors including

accuracyrequirements and type of liquid. Therefore, we recommend a 114 mm

(4.5 inches) minimum immersion depth. However,remember the handle limita-

tions.The handle is designed to be immersed. The temperature limits of the

handleare –200°C to 156°C. Temperatures outside these limits can damage the

handleand the probe. Convectionof heat from the heat source must be kept

within the handle limits.

The exactimmersion depth required can be determined by performing a gradi-

ent test taking measurements approximately every1.27 cm (.5 inches) until

there is a significant differencein read ings. Allow the thermometer to stabilize

at each newdepth. Plot the results to see the stem effect.

Twofactors contribute to thermal EMF, chemical consistencyand physical con-

sistency.Variations in chemical structure due to impurities can contribute to

thermal EMF.Also discrepancies in crystal structure can contribute to thermal

EMF.These factors are minimized by annealing the full length of wire before

constructionof the PRT.

Likewise,connection to extension lead wires and readout instruments can be a

source of thermal EMF.The thermal EMF is caused by a difference in tempera-

ture between twoconnections. If the two connections are the same temperature,

there will be little or no thermal EMF effects. However,if there is a substantial

temperature differencebetween connections, the thermal EMF effectswill be

significant.Therefore, cover or insulate any exposed bridge or galvanometer

terminals to lessen the source of error.The effects of thermal EMF can be can-

celed by using anAC bridge or a DC bridge with reversible current.

5623A 13

6Operation