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3.0 PROBES
Probes compatible with the 392 HEAT-Prober are constructed utilizing a wire
wound Platinum RTD sensor with an R0 of 100 Ohms wired in a 3-wire con-
figuration to compensate for lead length effects. To provide probe inter-
changeability at 0 degrees C, a balancing resistor is added to the sensor cir-
cuit and trimmed for a total resistance of 100.94 Ohms at 0°C. (See Figure
3-2)
The High Performance HEAT-Prober, Model 392HP, utilizes probes similar to
the 392 but incorporating an additional calibration resistor that provides
probe interchangeability at all temperatures.
3.1 Interchangeability
Probes are individually balanced to 100.94 Ohms at 0°C to compensate for
variations in different probe configurations. This guarantees probe inter-
changeability at 0°C. In addition, each probe is tested at a minimum of one
higher temperature. This calibration data is recorded on a calibration tag
attached to the probe. (See Figure 3-1)
Probe Interchangeability (Immersion)
Ice Point
±0.3°C (±0.5°F)—(392HP ±0.2°F)
100°C
±0.6°C (+1.0°F)
200°C
±1.1°C (±2.0°F)
Surface probes are tested on surface calibration plates especially built to
replicate the end-use. The variance is somewhat larger as follows:
100°C
±1.1°C (±2.0°F)
200°C
±2.2°C (±4.0°F)
Figure 3-2. Probe Circuit NOTE: Platinum sensor element manufacturers
quote 0.1% tolerance at 0°C, or 0.1 ohms on a 100 ohm sensor. This results
in 0.3°C variance. The tolerance at higher temperatures is greater than 0.1%
in accordance with the following formula as specified by DIN 43760:
Tolerance = ±.3°C + .0054t (°C)
Thus at 200°C, the expected sensor error could be 0.3°C + .0054 (200) =
±1.38°C.
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3.2 Temperature Sensors in Use
An understanding of heat-transfer is needed for accurate field use of temper-
ature sensors. The sensor measures its own temperature, therefore, to meas-
ure a body temperature it requires good contact with that body, or in the case
of liquids or gases good immersion and mixing.
A portable probe temperature sensor must have the following characteristics:
(a) a low thermal mass (product of its specific heat and weight);
(b) a large area for surface contact to permit heat-transfer between the
object being measured and the probe sensor,
(c) a minimum of heat conduction away from the sensor which will
reduce its temperature below that of the object being measured and create an
inaccuracy.
Surface probes are designed with the above three conditions as criteria, and
also include an insulating handle for your safety and convenience. Each
probe is assembled with a plastic handle made of high quality Delrin
(a) Thermally isolates the hand from the probe, preventing hand heat
from reaching the sensor, and sensor heat, in the case of high temperature
measurement, from burning the hand.
(b) Electrically isolating the hand from the metal probes and therefore,
potentially, from a voltage on the surface to be measured.
The handle is attached to a tangle-free retractable cord which extends 6' and
retracts to 12" (See Figure 2-1). Each Probe is labeled with its part number
and type.
For surface measurements, to obtain best time responses and accuracy, two
things are necessary;
(a) Good surface contact with all of the probe surface. Make sure of
this by holding it squarely on the surface. The 121 and 145 surface probes
are specially designed with a spring loaded tip to provide compliance with the
surface.
(b) A rough surface gives poor heat conductance to the measuring
probe. This can be overcome by applying a thin layer of heat conducting sili-
cone paste between the measuring point and the probe tip. The paste short-
ens response time up to 50%.
In liquids, move the immersion probe around the mix to get good contact. In
air, put the probe in the vicinity of a stream to get good velocity, otherwise
move the probe back and forth to induce velocity across the tip.
®
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