What Is RTD ?

Resistance Temperature Detectors (RTD), as the name implies, are sensors used to measure temperature by correlating the resistance of the RTD element with temperature. Most RTD elements consist of a length of fine coiled wire wrapped around a ceramic or glass core. The element is usually quite fragile, so it is often placed inside a sheathed probe to protect it. The RTD element is made from a pure material whose resistance at various temperatures has been documented. The material has a predictable change in resistance as the temperature changes; it is this predictable change that is used to determine temperature.

RTDs are popular because of their excellent stability, and exhibit the most linear signal with respect to temperature of any electronic temperature sensor. They are generally more expensive than alternatives, however, because of the careful construction and use of platinum. RTDs are also characterized by a slow response time and low sensitivity, and because they require current excitation, they can be prone to self-heating.

RTDs are commonly categorized by their nominal resistance at 0 oC. Typical nominal resistance values for platinum thin-film RTDs include 100 W and 1000 W. The relationship between resistance and temperature is very linear and follows the equation

For < 0oC RT = R0 [ 1 + aT + bT2 + cT3 (T – 100) ]
For > 0oC RT = R0 [ 1 + aT + bT2 ]

Where RT = resistance at temperature T
R0 = nominal resistance
a, b, and c are constants used to scale the RTD

The most common RTD is the platinum thin-film with an a of 0.385%/oC and is specified per DIN EN 60751. The a value depends on the grade of platinum used, and also commonly include 0.3911%/oC and 0.3926%/oC. The a value defines the sensitivity of the metallic element, but is normally used to distinguish between resistance/temperature curves of various RTDs.

Materials Used In RTD :

  • Nickel
  • Copper
  • Balco 
  • Tungsten
  • Platinum

Platinum Resistance Temperature Detector (RTD) :

316 Stainless Steel
Temperature Limits -320°F and +900°F
Calibration Curve: Per Sama Standard

Wire Configuration in Wheatstone Bridge :

Two – Wire Circuit :

Shown is a 2-wire RTD connected to a typical Wheatstone bridge circuit. Es is the supply voltage; Eo is the output voltage; R1, R2, and R3 are fixed resistors; and RT is the RTD. In this uncompensated circuit, lead resistance L1 and L2 add directly to RT.

Three – Wire Circuit :

In this circuit there are three leads coming from the RTD instead of two. L1 and L3 carry the measuring current while L2 acts only as a potential lead. No current flows through it while the bridge is in balance. Since L1 and L3 are in separate arms of the bridge, resistance is canceled. This circuit assumes high impedance at Eo and close matching of resistance between wires L2 and L3. TEMPCO matches RTD leads within 5%. As a rule of thumb, 3 wire circuits can handle wire runs up to 100 feet.

Four – Wire Circuit :

4-wire RTD circuits not only cancel lead wires but remove the effects of mismatched resistances such as contact points. A common version is the constant current circuit shown here. Is drives a precise measuring current through L1 and L4; L2 and L3 measure the voltage drop across the RTD element. Eo must have high impedance to prevent current flow in the potential leads. 4-wire circuits may be usable over a longer distance than 3-wire, but you should consider using a transmitter in electrically noisy environments.

If necessary you can connect a 2-wire RTD to a 3-wire circuit or 4-wire circuit, as shown. As long as the junctions are near the RTD, as in a connection head, errors are negligible.

Types Of RTD :

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