Fluke 1594A, User Manual

Page: 34 / 36

1594A/1595ASuper-Thermometer
RatioMeasurement
28
Table 12 Recommended Reference Resistors and Sensing Current
Probe Type Reference Resistance Sensing Current
25
W
SPRT 25
W
1.0 mA
100
W
PRT or RTD 100
W 1.0 mA
2.5
W
SPRT 10
W 5.0 mA
0.25
W
SPRT 1
W 14.14 mA
1000
W
PRT or RTD 10 k
W
0.05 mA
Thermistors, 2 k
W
to 10 k
W
10 k
W 0.01 mA
6.3 Ratio Measurement
The ratio measurement is the fundamental measurement from which the resistance and temperature measure-
ments are derived. It is simply the ratio (Rx/Rs) between an unknown resistance (Rx) and a reference resistor
(Rs). To configure the Super-Thermometer for a ratio measurement, follow the steps listed in Section 6.2,
Temperature measurement, on page 27 except select the RATIO option in step 6.d.
6.4 Using an External Resistor
The Super-Thermometer is equipped with a set of internal resistors. If needed, up to four external resistors can
be connected using Rs1 and Rs2 inputs on the back panel and Ch2 and Ch4 inputs on the front panel. Use the
following instructions for connecting and measuring with an external resistor:
1. Suspend measurement by going to the Measurement Screen and pressing the STOP MEASUREMENT
(F1) function key.
2. Connect a reference resistor using one of the Rs inputs. Ensure the current and potential connections are
correct.
3. Go to the Add Resistor screen by using the following steps:
a. Press the SETUP key to open the Channel Setup screen.
b. Select the CHANNEL SETTINGS (F2) function key.
c. Select the SETUP RS (F1) function key.
d. Select the DEFINE RESISTOR (F2) function key.
e. Select the ADD RESISTOR (F1) function key.
4. Enter a unique ID in the ID field. Pressing the ENTER key will open the Alpha-Numeric Interface for
text entry.
5. Enter the resistance value of the resistor in the RESISTANCE VALUE field.
CAUTION:
The Resistance Value field is used in measurement for the calculation of the Rx resistance.
Entering this value incorrectly will cause measurement error.
6. Set the maximum measurement current allowed for the reference resistor.
7. If necessary, enter the calibration date and due date for the external reference resistor. The Super-
Thermometer will use the due date to warn the user when the resistor is due for calibration.
8. Select the SAVE (F1) function key to save the definition.
9. Press the EXIT key to go up one menu to the Set Up Rs screen. Use the Up/Down arrow keys to
highlight the Rs input to which the external resistor is connected.
10. Select the ASSIGN RESISTOR (F1) function key to open the list of defined reference resistors. Use the
Up/Down arrow keys to highlight the resistor’s definition. Press the ENTER key to assign the resistor to
the selected Rs input.
11. Now, when selecting the reference resistor in Section 6.2, Temperature measurement, on page 27, step
6.a, the newly entered external resistor will appear as an option.