Fluke 5686, User Manual

Glass Capsule Standard Platinum Resistance Thermometer

Operation

7

Comparison Calibration of Other Instruments

The uniformity and stability of the bath and the degree of accuracy required determine the number 

of temperature measurements necessary. However, to follow “good” practice procedures, always 

measure the triple point of water resistance (R

tpw

) after each temperature measurement. This 

provides the most accurate measurement of the resistance ratio:

W(o) 

= 

R(o)

R

opw

Measuring Current

The recommended current for the 5686 is 1 mA for most situations. The resistance sensitivity 

with temperature becomes lower at lower temperatures. To compensate for the loss of sensitivity, 

measuring currents below 24.5 K are generally increased from 1 mA usually used above that 

temperature, up to about 5 mA at 13.8 K.

Errors caused by self-heating of the element need to be minimized. Allowing sufficient time for 

the SPRT to stabilize and the heat to be dispersed into the surrounding medium will provide the 

most accurate results. 

The Fluke testing or calibration certificates provided with the SPRT represent data that has been 

extrapolated. For example, on Model 5686 measurements are made at 1 mA and 1.41 mA. These 

measurements are then extrapolated to zero current. This can be done graphically by plotting 

i² vs R and extrapolating to zero power or by using the following equation:

R

0

=R

1

 - 

i

1

2

(R

2

 - R

1

)

i

2

2 

- 

i

1

2

Where:
R

0

 = Zero current resistance

R

1

 = Resistance measured at current i

1

R

2

 = Resistance measured at current i

2

The exact immersion depth required can be determined by performing a gradient test taking 

measurements approximately every 1/2 inch (1.27 cm) until there is a significant difference in 

readings. Allow the thermometer to stabilize at each new depth. Plot the results to see the stem 

effect.

Thermal EMF

Two factors contribute to thermal EMF, chemical consistency and physical consistency. Variations 

in chemical structure due to impurities and discrepancies in crystal structure can contribute 

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

construction of the SPRT.
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 temperature between two connections. If 

the two connections are the same temperature, there will be little or no thermal EMF effects. 

However, if there is a substantial temperature difference between connections, the thermal EMF 

effects will 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 canceled by using an ac 

bridge or a dc bridge with reversible current.