Theory Of Operation - Fluke 80T-150 Instruction Manual

80T-150
TEMPERATURE-
TO-VOLTAGE
CONVERTER
POWER SWITCH Va
ON/OFF
BANANA PLUGS
(0.75 " SPACING)
PROBE TIP
ЕЕ]
PROBE CABLE PROBE BODY
Figure 1. 80T-150 Features
Error Sources
When the probe пр 15 applied to a solid surface it draws
or sinks heat from the surface. Therefore, if the measured
surface has a low mass (е.р., a transistor case), the in-
dicated temperature may be lower than the actual
temperature.
Similarly, a steady-state error or gradient exists between
the measured surface and the sensing device in the probe
tip. This 15 due to the flow of heat from the measurement
surface to the probe body. The effect of the steady-state
error increases as the differential between ambient and
surface temperature increases.
To determine the actual surface temperature of a device,
both the heat-sinking and steady-state errors must be
considered. The correction curve given in Figure 2
approximates the effect of both error sources on TO-3,
TO-5 and TO-18 transistor cases.
son
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DEGREES ABOVE AMBIENT (°C)
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INITIAL SURFACE TEMPERATURE IN
INDICATED SURFACE TEMPERATURE
DEGREES ABOVE AMBIENT
Figure 2. Initial Case Temperature Above Ambient vs.
Meter Reading Above Ambient
RF signals applied to the 80T-150 probe tip can also
cause errors in temperature measurement. Figure 3
defines the rf signal limits that can be tolerated without
degrading measurement accuracy.
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Figure 3. Maximum Signal RF Limits (Vrms) at Probe Tip
OPERATION
Use the following procedure to operate the 80T-150
probe:
a. Connect the banana plugs on the 80T-150 to the
input terminals of a high impedance voltmeter.
Observe polarity.
b. Select a dc voltage range that will provide at least 1
mV resolution (1 mV/degree), and a full scale
readout that will encompass the expected
temperature. The 2V range of a 3% digit DVM is
adequate. Ignore readings of less than 1° when a
more sensitive DVM is used.
с: Set the 80T-150 power switch to ON and energize
the voltmeter.
d. Firmly touch the probe tip to the surface to Бе
measured, or expose it to a liquid or gas. The volt-
meter will display the temperature in degrees. Vary
probe angle and pressure when measuring solid
surface temperatures; the highest stabilized reading
will be the most accurate.
CAUTION
The force exerted on the probe tip should not
exceed 20 pounds.
THEORY OF OPERATION
The Model 80T-150 utilizes the negative temperature
coefficient of a semiconductor (P-N) junction to measure
temperature. The PN junction is thermally integrated