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PLATINUM
RTDs and THERMISTORS vs THERMOCOUPLES
The amount of study, experiments, and effort that has been applied to the
measurement of temperature is perhaps far greater than that devoted to any other field of
industrial instrumentation. More
than 50% of all
measurements of important variables in industrial and related fields are measurements of
temperatures. The following is a discussion of
three of the most popular temperature sensor types in use today.
| THERMOCOUPLES |
These are easily the most often used sensor. They are
small, low cost, and relatively user friendly. They lend themselves to installation,
where they cannot be removed after testing, or where they may be destroyed in
testing. Thermocouples are very rugged and will withstand far more abuse than other
sensor types. Electrically, the thermocouple acts as a low impedance source,
allowing the measuring instrument to have a high impedance, thereby minimizing noise
pickup on longer runs. Compared to other sensor types, thermocouples are relatively
inaccurate and not as repeatable. Because of low-level output, non-linear e.m.f.
versus temperature and the need for reference junction, the readout instrumentation is
more complex than for other sensor types.
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| PLATINUM RTDs |
RTDs (Resistance Temperature Detectors) are excellent but
fragile tools for RTD Calibration.offering a wide range (useful from –200 to +660ºC) and higher
repeatability, stability, and accuracy than other sensors over this range.
They are electrically superior to thermocouples because a higher e.m.f. output can be
obtained, thereby producing higher sensitivity and better resolution. Because of the
higher e.m.f. and no need for reference junction, the readout instrument is less complex
and usually less expensive. Initial probe cost is high and care must be
exercised in their application and use, and they must include a greater sensitive
area. To obtain optimum accuracy, the probe and readout instrument must be
calibrated as a system, thereby reducing probe interchangeability.
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| THERMISTORS |
Thermistors are considerably more accurate than thermocouples
from below the freezing point to just above the boiling point of water. They are highly accurate and may be selected to
close interchangeability. Other advantages
include the relative low-cost and small size. The
useful range limits the application for this sensor.
Although satisfactory for industrial use, they are not as rugged as thermocouples. |
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