Fluke 9140 7.5 Secondary Menu

sistor. The calibrator measures the voltage at the red terminal and interprets

+5V as open and 0V as closed.

This section describes a possible application for the temperature hold feature

and how the instrument is set up and operated.

Suppose you have a thermalswitch which is supposed to open at about 75°C

and close at about 50°C and you want to test the switch to see howaccurate and

repeatable it is. Youcan use the temperature hold feature and the scan function

to test the switch. Measurements can be made by observing thedisplay or, pref-

erably,by collecting data using a computer connected to the RS-232 port. To

set up the test do the following steps.

1. Connect the switch wires to the terminals on the front of the dry-well and

place the switch in the well.

2. Enable set-point scanning by setting the scan to “ON” in the primary menu

(see section 7.3.1).

3. Set the scan rate to a low value, say 1.0°C/min. (see section 7.3.2). If the

scan rate is too high you may lose accuracy because of transient temperature

gradients. If the scan rate is too low the duration of the test may be longer than

is necessary.You may need to experiment to find the best scan rate.

4. Set the first program set-point to a valuebelow the expected lower switch

temperature, say 40°C, in the program menu.

5. Set the second program set-point to a value abovethe expected upper switch

temperature, say 90°C.

6. Set the program soak time to allow enough time to collecta number ofdata

points, say 2 minutes.

7. Collect data on a computer connected to the RS-232 port.Refer to Section 8

for instructions on configuring the RS-232 communications interface.

7.5 Secondary Menu

Functions which are used less often are accessed within the secondary menu.

The secondary menu is accessed by pressing “SET” and “EXIT” simulta-

neously and then releasing. The first function in thesecondary menu is the

heater power display.(See Figure 4.)

The temperature controller controls the temperature of the well by pulsing the

heater on and off. The total power being applied to the heater isdetermined by

the duty cycle or the ratio of heater on time to the pulse cycletime. By knowing

the amount of heating the user can tell if the calibratoris heating up to the

set-point, cooling down, or controlling at a constant temperature. Monitoring

the percent heater power will let the user know how stable the well temperature

9140 Dry-well Calibrator

User’s Guide