RTD Sensors

Resistance thermometers, also called resistance temperature detectors or resistance temperature devices (‘RTD’s), are sensors used to measure temperature by correlating the resistance of the RTD element with temperature.

RTD – Resistance Temperature Device

rtd-bulb1Due to the need in Industry for greater accuracy and stability of the Resistance Temperature Device(RTD) are more widely used.
The Platinum RTD, having a normal resistance of 100 ohms at 0°C and a temperature coefficient of 0.0385 is regarded as a world standard.  RTD’s are constructed in two categories.

Thin-film elements are cheaper and more widely available because they can achieve higher nominal resistances with less platinum.

Their disadvantages, compared to the Thermocouples, are:

Detector tolerance and Accuracy

The RTD Bulbs used in D&N’s assemblies conform to BS 1904:1964 and DIN 4376 and in standard form have a tolerance of 0.1% of resistance. (Other tolerances of 0.05%, 0.03%, 0.02% and 0.01% are available on request). Also available are nickel 100 ohms and Platinum 130 ohms, 500 ohms and 100 ohms. The most common type pt100 has a resistance of 100 ohms at 0°C and 138.4 ohms at 100°C. There are also pt1000 sensors that have a resistance 1000 ohms respectively at 0 C.

Platinium RTD tolerance values

Temperature °C Resistance Value Ω Class A°C (Ω) Class B°C (Ω) 
-200 18.49 ±0.55-(±0.24) ±1.3-(±0.56)
-100 60.25 ±0.35-(±0.14) ±0.8-(±0.32)
0 100.00 ±0.15-(±0.06) ±0.3-(0.12)
100 138.50 ±0.35-(±0.13) ±0.8-(±0.30)
200 175.84 ±0.55-(±0.20) ±1.3-(±0.48)
300 212.02 ±0.75-(±0.27) ±1.8-(±0.64)
400 2477.04 ±0.95-(±0.33) ±2.3-(±0.79)
500 280.90 ±1.15-(±0.38) ±2.8-(±0.93)
600 313.59 ±1.35-(±0.43) ±3.3-(±1.06)
650 329.51 ±1.45-(±0.46) ±3.6-(±1.13)

RTD Types

Within the above types gold and silver are rarely used within the industry, copper is used in an economical situation but the bulb tends to be longer than that of a platinum bulb and the max temperature is only 130°C. Tungsten has a high resistance and is used in very high-temperature situations, it is brittle and difficult to work with. In most cases, the RTD is made from Platinum or nickel which tends to have a drifting problem and is non-linear in its scale over time.
The most common RTD used is the Platinum pt100 ohm which offers excellent accuracy over a wide temperature range from -200 to 850 C.

The advantages of RTD’s include:

Connecting RTD’s

There are three main methods to connecting RTD’s to the measuring instruments. Each method should be matched to the measuring instrument or control device.

Two Wire RTD Connection 2 wire RTD 
Three Wire RTD Connection 3 wire RTD 
Four Wire RTD Connection 4 wire RTD 

 

RTDs vs Thermocouple

The two common ways of measuring industrial temperatures are with resistance temperature detectors (RTDs) and thermocouples. The choice between them is usually determined by these four factors.

The D&N team can lend their 35 years’ experience to assist you in choosing the right RTD sensor for your needs. We also carry a range of stocked RTD sensors for immediate shipping.

 

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