How to Specify an RTD Sensor ~ Learning Instrumentation And Control Engineering Learning Instrumentation And Control Engineering

How to Specify an RTD Sensor

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When a Resistance Temperature Detector (RTD) is required for a given application, many parameters need to be accurately documented for the particular RTD to be procured from the manufacturers. Since there are many different manufacturers of RTDs, there will be several different styles of RTDs in the market. Each manufacturer has their own way of specifying their product. In any case, when specifying an RTD you will always be required to select the following:
  • Sensor element to meet your operating/ process conditions and specifications
  • Sensor tolerance/accuracy
  • Lead wire configuration. This should correspond with the number of leads your application requires
  • RTD sensor sheath material and size
  • Termination style of RTD
  • Mounting configuration/connection
  • Sheath length and much more depending on the manufacturer of your RTD sensor. 
  • RTD with Transmitter or RTD sensor with direct cabling.
To specify your RTD sensor correctly, you need to follow the checklist below and marry it with what your manufacturer requires of you. Checklist for Specifying RTD Sensors Correctly
RTD sensors specification is broken down into the following:

RTD Element Specifications:
In the RTD sensor element specifications, you will be required to specify the following:
  • Element material
  • Reference resistance (100 ohms resistance is usually common)
  • Number of lead wires per sensor element
  • RTD sensor temperature coefficient of resistance (TCR)
  • Element configuration (usually either single or double)
Operating temperature range of sensor:
Here you specify the temperature range the RTD sensor is expected to sense while in operation in the particular process in 0F or 0C

RTD sheath/probe Specifications:
Most RTDs in actual process application come in the form of probes which in conjunction with a thermowell is inserted into the process of interest. Here you will specify:
  • sheath/probe configuration. This is usually of the penetration type
  • sheath/probe diameter in inches
  • sheath/probe sensitive length (This is the length that goes into the actual process whose temperature is required to be measured)
  • sheath/probe material e.g 316SST, 304SST etc.
Process connections(Sensor mounting connections):
Most RTD sensor assembly come with threaded NPT fittings that are either brazed, welded or soldered. Specify:
  • NPT size for conduit connection
  • NPT size for process or instrument connection
Sensor termination type:
Specify whether the RTD sensor termination will be bare or insulated lead wires

Application environment:
Specify whether the RTD sensor is to be weather proof or is to be explosion proof which ever is applicable.

Vibration Survivability:
Is your RTD sensor being used in an environment where there is a lot of vibration? How much vibration can your RTD sensor withstand without being damaged in the particular environment? Specify the vibration survivability of the RTD sensor in grams (g)

Special process conditions/Application:
Is the RTD sensor you require to be used in special process conditions or application environment? You should let your RTD manufacturer know this to enable them give you the right RTD sensor. Therefore specify any special process condition or application environment your RTD sensor is required to be used.

RTD with Transmitter or RTD sensor with direct cabling:
Where the RTD is installed will dictate the type of RTD required. If the control/measuring equipment  is relatively close to the installed sensor, then it is practicable to cable directly to the equipment. In that case specify an RTD sensor without transmitter.
For long distances of the control/measuring equipment from the installed sensor, where it has been ascertained that the lead wire impedance is too large, the use of a 4 - 20mA transmitter is inevitable. In this case specify an RTD with a transmitter configuration.

Electrical characteristics:
The RTD sensor requires an external power source. So you need to specify:
  • The rated voltage of the sensor (AC or DC)
  • Rated Current in Amps
  • Rated frequency in Hertz
The above checklist for RTD specification is by no means exhaustive but could serve as a general specification guide for ordering an RTD sensor from any manufacturer. Most manufacturers have their own specification guide and ordering code. To be able to specify and order an RTD sensor correctly, consider the checklist above in conjunction with what your manufacturer requires. By so doing, you won’t miss anything out in the RTD sensor specification.

For more information on RTD Sensors, check out: