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ture 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 can
-
celed by using an AC bridge or a DC bridge with reversible current.
6.5 Measuring Current
Each SPRT has a specified drive current depending upon the particular sensor.
The recommended currents are in the following table.
Model Resistance Drive Current
5680
25
Ω
1.0 mA
5682
100
Ω
0.5 mA or 1.0 mA
5699
25
Ω
1.0 mA
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 Model 5680/5682/5699 measurements are made at 1 mA and 1.4 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.
RR
iR R
ii
01
1
2
21
2
2
1
2
=−
−
−
()
where:
R
0
= Zero current resistance
R
1
= Resistance measured at current i
1
R
2
= Resistance measured at current i
2
Equation 2: Calculation of R0
6.6 Cooling Rates at High Temperatures
The equilibrium concentration of point defects in the pure platinum wire in
-
crease exponentially to the increase in temperature. If a high cooling rate (i.e.,
removing a SPRT from a high temperature and cooling it to room temperature
in less than a minute) occurs above 500°C, some of the point defects in the
platinum wire become trapped in the crystalline structure causing a slight in
-
crease in resistance. This slight increase in resistance can be reversed by an
-
nealing the SPRT. To avoid this problem, slowly cool the SPRT at a rate of
roughly 150°C/hour above 500°C before removing to room temperature. The
5680/5682/5689 SPRT
User’s Guide
16
1.888.610.7664 sales@GlobalTestSupply.com
Fluke-Direct.com
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