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less of the selected system units. For the ratio to be displayed properly, the
R(TPW) value must be accurate. You may use the CAL TPW soft-key to cali-
brate this parameter (see Section 7.4.4.1).
7.4.3.2.8
Ratio
The RATIO option causes the measurement to be displayed as resistance ratio
rather than temperature. The ratio is the measured resistance divided by the ref-
erence resistance. There are no coefficients for this conversion option.
7.4.3.3
Reference
REFERENCE specifies which reference resistor to use with the probe. You can
select among the four internal reference resistors or you can select an external
resistor. You should choose the reference resistor closest to the resistance to be
measured (see Table 4). Using the wrong reference resistor can result in exces-
sive noise in the measurement and poor accuracy. For 25 ohm and 100 ohm
PRTs and SPRTs the 100 ohm internal reference resistor or a 100 ohm external
reference resistor is recommended.
TIP:
Program the data window to show the reference resistor so you can
easily see that it’s set correctly (see Section 7.5.3).
If you use an external standard resistor as the reference, be sure you have en-
tered its correct resistance with the SET EXT function in the SYSTEM - REF-
ERENCE menu (see Section 7.6.5.1). An external reference resistor must be
connected to Input 2 (see Section 5.1).
NOTE:
When using an external reference resistor, you must enter its pre-
cise value with the SET EXT function in order for the current and the mea-
sured resistance to be accurate.
7.4.3.4
Current
The driving current for the probe should be optimized to achieve the most accu-
rate measurement possible. If the current is too high there may be too much er-
ror in the measurement because of self-heating. If the current is too low noise,
drift in the measurements may be excessive and accuracy will be poorer. The
1575/1590 Thermometer Readout
User’s Guide
50
Input
Resistance
Internal Reference
0 to 1
1
1 to 10
10
10 to 1000
100
1 k to 500 k
10 k
Table 4
Reference Resistor Selection