Autozero; Low Output Impedance - Ectron 1140A Instruction Manual

Thermocouple simulator-calibrator

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Note that the sense of the offset value can be thought of as a correction to the actual

thermocouple emf. Thus, if the calibrated thermocouple's output was less than the standard emf

for that type, the correction applied would need to be positive. On the Model 1140A, a positive

value would be entered in the THERMOCOUPLE OFFSET menu.

Autozero

Available for both thermocouple and linear voltage operation when using the Model 1140A in the

meter mode, an autozero can be commanded by local or remote control. When an autozero is

commanded, the readout is forced to a zero reading and further changes in signal are shown as

changes around the zero reading. Autozero is noted in the display as AZ and is located in the

upper middle of the display.

For example, if monitoring a temperature chamber whose temperature was 1600°C, the user

could command an autozero. Immediately the instrument reads zero and thereafter follows the

temperature variation of the chamber about zero with a high degree of accuracy and resolution.

The readout signals are coupled to the user's interface for data logging or computation as

desired.

Unlike many dc or thermocouple calibrators, the Model 1140A offers very low output impedance

(0.05 Ω maximum) in the source mode at all output levels. This very helpful characteristic

provides some important advantages for general operation.

A precision dc source can have loading problems even with high-impedance loads. One very

popular high-precision calibrator has an output resistance of 220 Ω for all voltages under

200 mV. With a 1 MΩ load the error at 100 mV is 20 µV or 220 ppm, well over its claimed

accuracy at this voltage. In contrast, using the Model 1140A the error would be under 0.5 ppm.

Another advantage is that several loads can be simultaneously connected to the output of the

Model 1140A when operating as a source. For instance, when operating as a thermocouple

simulator, usually several loads can be paralleled with no problem. This instrument allows this

practice but most other simulators or precision dc sources do not.

Each thermocouple type has its own offset. If the

thermocouple type is changed, for example, from

Type E to Type J, the offset that was set for Type E

will not affect Type J measurements.

Until an autozero is canceled both the display and

the interface data will reflect the change of the

autozero.

NOTE

5-5

Applications

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