2.
Do not expose the probe end (probe tip plus about 2 inches of
the probe body) to temperatures below -50
°
C (-58
°
F). The
remainder of the probe body should not be exposed to
temperatures below -40
°
C (-40
°
F).
3.
For liquid measurements, recommended applications range
from water, lubricants, and fuels to most solvents. Liquids as
shallow as 1/2 inch can be measured since the temperature
sensor is in the probe tip.
Warning
To avoid electrical shock, do not use this
instrument when voltages exceeding 60V dc or 30V
ac rms (42.4V peak ) are present. The probe tip is
electrically connected to the output terminals.
Caution
Long-term exposure of the probe to corrosive
environments will result in pitting and deterioration
of the aluminum probe tip.
Error Sources
When the probe tip is applied to a solid surface, it draws or sinks
heat from the surface. Therefore, if the measured surface has a
low mass (e.g., a transistor case), the indicated 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 is
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 1 approximates the effect of both
error sources on TO-3, TO-5, and TO-18 transistor cases.
RF signals applied to the 80T-150U probe tip can also cause errors
in temperature measurement. Figure 2 defines the rf signal limits
that can be tolerated without degrading measurement accuracy.
F1.eps
Figure 1. Initial Case Temperature Above Ambient vs
Meter Reading Above Ambient
F2.eps
Figure 2. Maximum Signal RF Limits (Vrms) at Probe Tip
Operation
Use the following procedure to operate the 80T-150U probe:
1.
Connect the banana plugs on the 80T-150U to the input
terminals of a high impedance DMM. Observe polarity.
2.
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
1/2-digit DMM is adequate. Ignore readings of less than 1
°
when a more sensitive DMM is used.
3.
Set the 80T-150U power switch to
°
C or
°
F, and energize the
DMM.
4.
Firmly touch the probe tip to the surface to be measured, or
expose it to a liquid or gas. The DMM will display the
temperature in degrees. Vary the probe angle and pressure
when measuring solid surface temperatures. The highest
stabilized reading will be the most accurate. (See the following
measuring technique.)
Caution
The force exerted on the probe tip should not
exceed 15 pounds.
Measuring Technique
Here are some suggestions for improving the accuracy of your
temperature measurements:
1.
When measuring higher than ambient temperatures, adjust
the connection between the probe and the surface until you
get the highest temperature reading.
2.
When measuring lower than ambient temperatures, adjust the
connection between the probe and the surface until you get
the lowest temperature reading.
3.
When measuring near ambient temperatures, make the
reading when the multimeter readout is most stable.
Theory Of Operation
The Model 80T-150U uses the negative temperature coefficient of
a semiconductor (P-N) junction to measure temperature. The PN
junction is thermally integrated into the probe tip and comprises
one leg of a bridge circuit as shown in the simplified circuit diagram
of Figure 3. One 9V battery is used to power both the bridge circuit
and operational amplifier U1. Since the bridge must be balanced to
provide 0
°
C and 0
°
F indications, separate range or temperature
scale resistors R7 and R6 are included in the bridge circuit. When
R6 and R2 are shorted by S1, the
°
C scale is selected and the
bridge is calibrated by R3 to null at 0
°
C. Conversely, when S1 is
open, the 0
°
F scale is selected, and the bridge is calibrated by R2
to null at 0
°
F. Deviations above and below 0
°
provide a bridge
output of approximately 2.45 mV/
°
C.
Operational amplifier U1 is used to measure the bridge output and
scale it to a 1 mV/degree signal. Since the
°
C and the
°
F scale are
sloped differently, the scale for U1 must be matched with the scale
selected for the bridge circuit. Shorting resistors R15 and R18
selects the
°
C scale. Conversely, when S1 is open, the
°
F scale is
selected. Resistor R4 calibrates both scales.
The output voltage used to drive the external voltmeter is taken
from the output of U1 (P2) and the reference side of the bridge
(P1). Since U1 is operating as an inverting amplifier, its output is
used as the low input to the voltmeter. This enables the voltmeter
to display an increase in temperature as an increase in voltage.
General Maintenance
Access Information
The battery and the calibration pots are located on the interior of
the temperature-to-voltage converter assembly. Access to these
locations is accomplished by removing the screw from the bottom
side of the assembly and removing the top of the plastic box.
