Industrial RTD Probes
Encapsulated probe is basically a resistance thermometer’s standard configuration usually for industrial process control as well as protection of the machinery. Most of such probe cases tend to be of stainless steel or Inconel for withstanding extreme temperature, though other material provides better benefits in the mid-range. Standard diameter of the probe ranges from 0.125. to 0.250. Small sized probes tend to react faster when they are inserted directly, however large sized probes fit much tightly into the standard protective covers. The length-range of a probe tends to vary between some inches to even 10 feet or even more than that.
A wide range of mounting parts as well as accessories tends to assist in the installation of sensors. The choice is largely based upon the nature of the detected carrier and the cost needs. Direct probe immersion needs a threaded connection that can be fitted well or welded to the probe.
Flexible Resistance Thermometers
The encased probe is not very suitable for detecting flat surfaces. Unlike the junctions in thermocouples that could be soldered straight to metal surfaces, RTDs provide some mass; & heat loss to the surrounding air, which can tend to affect the reading. Small-sized flattened elements like thin films could also be mounted to the surfaces, but sensitive elements as well as wire connections tend to make the process installation much more difficult.
Resistance Thermometer for Special Purposes
A resistance thermometer tends to adapt easily to most models of process control & thermal equipment. The users can specify axial lead enclosures for the mounting of the circuit boards, flat packages in order to clamp to the surface(s), mini enclosures for embedment in metal blocks, and any fittings & sheaths that can all be produced by a machine shop. Additionally, windings of the wires could possibly be configured for detecting larger regions.
Applications of Resistance Thermometers
In general, resistance thermometer offers the maximum number of benefits over other types of thermometers in the following situations:
- High standardization is desirable
- Sensing of area, instead of point, betters control
- Accuracy should be extending over a vast range of temperature
- Accuracy & stability being the most important application goals
Advantages of Platinum Resistance Thermometers
- Lesser drift
- Higher accuracy
- Wide range of applications
- Suitability for specific and precise applications
Limitations of Platinum Resistance Thermometers
- A resistance thermometer in industrial application is rarely used above 600 degree Celsius. At a temperature above 660 degree Celsius, it tends to become highly difficult to avoid contamination of the platinum with the metal shell of the thermometer. Therefore, standard lab thermometers replace the metal sheath with a glass construction. At a very low temp, say somewhere around -270 degree Celsius (i.e. 3K) or even lower than this, the reason being quite a lesser no. of photons, the resistance is principally measured by the impurities as well as boundary diffusion; and therefore, essentially temperature independent. Resultantly, RTD sensitivity is largely zero, so it is not quite useful.
- Compared to a thermistor, platinum resistance thermometer is less sensitive to minute temperature alterations & has a much slower reaction time. But a thermistor has a much smaller range of temperature as well as stability.
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