Resistance Temperature Detectors - RTDs

Overview

Resistance Temperature Detectors, also known as RTD's, are wire wound and thin film devices which incorporate pure metals or certain alloys that increase in resistance as temperature increases and, conversely, decrease in resistance as temperature decreases. RTDs are similar to thermocouples in that they convert changes in temperature to voltage signals by the measurement of resistance. As a RTD gets hotter the resistance measured on it's element becomes larger, as it cools the resistance becomes smaller.

The Platinum Element RTD

The most commonly used type of RTD element is Platinum, these models are often referred to as PRTs or PRT100s. Platinum elements are popular because they can be used over a wide range of temperatures and feature a quick response time. Platinum's coefficient of resistance is nearly linear, as such, by using a platinum element resolutions of ±0.1 °C or better are possible.

Other Elements

Other elements can be used in RTD's which are less costly, the most commonly used alternatives are Copper and Nickel. The advantage to this is that the RTD will cost less money to manufacture and have a lower replacement value, but the drawback is that these elements have more restriction on the ranges they can be used due to non-linearities in the resistance vs. voltage curve. Also, there are problems with wire oxidation problems which are known to occur when copper is used as the element.

Why choose an RTD?

The advantages of RTD's include: stable output for long period of time, ease of recalibration, and accurate readings over relatively narrow temperature spans. When compared to thermocouples their disadvantages are: higher sensitivity to vibration, smaller overall temperature range, and a greater initial cost. For more help in determining if an RTD is the correct choice for your application, please consult our knowledgeable sales staff.

Prolonging Life and Eliminating Measurement Errors

To get the maximum life from your Resistance Temperature Detector it is recommended that you use a protective well and terminal head. The well serves to protect the RTD from damage by the gas or liquid being measured. Protecting wells are available in stainless steel, carbon steel, Inconel, or cast iron, and they are used for temperatures up to 1100°C. To eliminate errors in taking measurements it is necessary to compensate for the length of leadwire used to connect the RTD to the readout. The calculations to correct this are relatively simple, and there are also 3-wire and 4-wire designs which help to minimize such errors. An example of a typical 3-wire measuring circuit is shown in the RTD Specifications Section.

Features of ARiDET Mineral Insulated Resistance Temperature Detectors

Time Constant of 2.2 sec or less

Temperature Range: -200 to +600 °C

Pressure Range: Vacuum to +50,000 PSIa

Shock Resistance to 1000G in all 3 axis

Withstand Vibration of ±50G, 60 to 2000HZ

Accuracy:±1 ohm @ 0 °C (±0.1% for 100 ohm)

Reproductibility: ±0.1% of resistance for 100 ohm element

Very Long Lengths. Entire length can be exposed to high temperature

Available with 4-20ma temperature transmitter/amplifier

Superior corrosion resistance with low carbon 316 ST/ST sheaths

Can be sharply bent with no change in performance

Full Technical Specifications

LABRATORY ACCURACY WITH INDUSTRIAL RUGGEDNESS

Platinum resistance temperature detectors are used for interpolation between the IPTS-68 fixed temperature points of ice and antimony (0°C to 630.74°C), generally used where accuracy and stability are most important. The ARiDET® approach to RTD's will bring ±.05° laboratory accuracy to the toughest industrial processes - without special handling considerations. ARi Industries Inc. has combined the inherent accuracy of a precisely wound Platinum resistance element with AeroPak® compacted ceramic insulated cable. The result is a low cost yet rugged sensor that will take factory handling - retaining the stability needed for today's precise process control.

Available with either the 99.999%+ high purity JIS C-1604-81 or 99.99% DIN 43760 temperature vs. resistance curve.

See the temperature vs. resistance tables in the RTD charts section for how to obtain laboratory accuracy.

For a more detailed explanation of how resistance temperature detectors work please visit our technical specifications page

RTD Model's and Descriptions

Style PRT-14.3
Basic unit, constantan wires bared for direct attachment to terminals

Style PRT-22.3
Teflon insulated and shielded lead wires attached to basic probe, epoxy potting compound, US Patent #3,144,507 strain relief.

Style PRT-90.3
with quick disconnect, sealed, vibration resistant connector with extra pin for shielding, mate supplied overall connector length 3.25" when mated.

Style PRT-96.3 or PRTX-96.3
with standard size connection head, Industrial weatherproof alumin head with internal screw terminals 3/4" NPT exit thread for hard wiring. PRTX-96.3 has temperature transmitter.

Style PRT-50733.3
with miniature size connection head. Miniature weatherproof aluminum connection head with internal screw terminals. 1/2" NPT exit thread for hard wiring.

Style PRT-52400.3
with fixed hex nipple. Replacement element with 1/2" NPT hex nipple silver brazed to sheath

Style PRT-50735.3
with spring loaded hex nipple. Replacement element with 1/2" NPT hex nipple spring loaded

Style PRT-91.3
with 3 prong quick disconnect, quick disconnect and mate.

Style PRT-296.3 or PRTX-296.3
with easy access hinged cover. Weatherproof snaplocked hinged cover connection head with 1/2" NPT exit thread. PRTX-296.3 has temperature transmitter.

Style PRT-396.3 or PRTX-396.3
with nylon connection head. Nylon screw cover connection head with 1/2" NPT exit thread. PRTX-396.3 has temperature transmitter.

Calibration:

All assemblies shown can be supplied with a different calibration and 0°C resistance value.

Lengths:

Standard lengths for the 3 diameters is 24" (610mm). Longer lengths of compacted MgO insulated lead portion can be supplied up to the following.

Duplex elements at 0.250" diameter will be approximately the same length.

Duplex Elements:

Can be supplied at 0.250" diameter only where suitable terminations can be made. Temperature transmitters for duplex circuits are not available for internal mounting in connection heads. The following styles can be made with duplex elements. Specify the 6 wire style for duplex elements.

3 or 6 wire RTD's

Single Circuit 4 Wire:

PRT Type assemblies can be made with 4 leadwires with the following circuit.

4-wire RTD circuit