What is RTD? It’s Types.

RTD is the acronym for resistance temperature detectors. RTDs are sometimes referred generally as resistance thermometers.   

A resistance Temperature detector (RTD) is a popular choice of temperature sensors which measures temperature using the principle that the resistance of a metal changes with temperature.

Principle:

RTDs work on the basis of relationship between metal and temperature. As the temperature of a metal increase, the metals resistance to the flow of electricity increases. In the same way as the temperature of RTD resistance element increase the electrical resistance measured in OHM increase.

RTD elements are specified according to their resistance in ohms (Ω) at zero degree Celsius (°C).

The most common RTD specification is 100 Ω, which means that at 0°C the RTD element should display 100 Ω of resistance.

Materials used most commonly in resistance thermometry are platinum, Nickel, Copper and Semiconductors. Platinum metal is widely used in the temperature measurement. Copper and Nickel are less used compared to platinum.

Construction: There are three major RTD construction types.

  • Wire-wound RTDs (Coiled Element Design)
  • Wire-wound RTDs (Outer wound Design)
  • Thin-film Design

Wire-wound RTD (Outer wound Design):

Wire wound resistance temperature detectors consists of a length of fine coiled wire of platinum and is wrapped around a ceramic or glass core. They are relatively fragile and are generally supplied with a metal sheath for protection. They have demonstrate greater accuracy over a wider temperature range than flat film detectors, but they are expensive.

Wire-wound RTD (Coiled wound Design):

Its housing is a hard fired ceramic oxide tube with equally spaced bores that run horizontal to the axes. The coil is inserted into the bores and packed with a very finely ground ceramic powder. This arrangement provides good contact with the process.

Thin-Film RTD:

These are manufactured by placing a fine layer of platinum wire on to a ceramic substrate, the element then coated in epoxy or glass which provides protection.

They are cheaper alternative to wire wound detector and have fast response time. But they provide less stability and have a lower temperature range than wire wound design.

Platinum RTD:

Resistivity of platinum is six times that of copper. Platinum is relatively un-reactive has high melting point (1769 °C) and, because it has a well-established temperature coefficient of resistance.

Temperature Range: It is a common choice for precise measurement of temperature between -200 °C to 1000 °C. However, in industrial applications, the range is limited to 400C. This is because, poor accuracy at high temperature measurements.

Advantages:

Platinum metal is the most commonly used for RTD elements due to a number of factors, which includes

  • Chemical inertness
  • Nearly linear relation between temperatures versus resistance relationship.
  • Temperature coefficient of resistance is high enough to give readily measurable resistance change with temperature 
  • Stability (in that its temperature resistance does not drastically change with time)

What is Pt100 and Pt1000?

Pt100 is Platinum resistance of 100 Ω at 0°C and Pt1000 means platinum resistance of 1000 Ω at 0°C. Pt 1000 have higher resolution, generally highly accurate than Pt100 rtd.

Pt100 is used in many industrial and commercial applications.

Temperature Co-efficient of Resistance or Alpha Value.

The relation between resistance and temperature can be approximated by a relation is

            RT = R0 (1+ αT)

            Ro = Resistance at 0°C

            RT = Resistance at T (Ω)

            α (alpha) = Temperature Co-efficient of resistance (Ω /Ω per °C)

            T = Temperature (°C)

It is an important characteristic of a metal. It is curve drawn between resistance versus temperature relationship between 0°C and 100°C. This temperature coefficient of resistance is called alpha.

(Alpha) α = R100 – R0 / 100 °C * R0

              R0 = resistance of sensor at 0°C

              R100 = resistance of sensor at 100 °C.

Alpha (α) is the slope of the resistance (Ω) between the 0°C and 100°C.

Alpha (α) value= temperature coefficient of platinum 0.003925 Ω/Ω/°C at 0°C to 100 °C range.

Alpha (α) value used in Europe, is 0.00385 average between 0 to 100 °C.

Nickel RTD

Next to platinum metal, second metal in usage is high-purity nickel, which provides the highest temperature coefficient, second highest temperature range after platinum and an advantage of lower assembled cost than wire wound platinum at high resistance values.

Maximum temperature range of the Nickel RTD is -80 to 320 °C. Practical applications it is limited to -200 to 550 °C.

Advantages:

  • Compared to platinum, Nickel RTD have double the sensitivity.
  • The Nickel and its alloys are relatively low in cost.  
  • High resistivity and values of temperature coefficient.

Disadvantages

  • Variation in electrical resistance with temperature is non-linear.
  • Resistive to strain.
  • Nickel RTDs are declined to use due to their limited use.
  • Its operating range is limited, and poorer stability.

Application:

Used in low cost applications, where accuracy is not an issue.

Copper RTD

Copper is sometimes used for the range of -100 °C to +100 °C and is relatively cheap. 10 ohm are normally used for lower temperature range, but 100 Ohm and 1000 Ohms are also in use for better resolution. Maximum temperature range of the Copper RTD should not exceed 260 °C. They have a useful range of -50 to 250 °C.

Advantages:

  • Copper RTD used to measure in the range of -100Cto + 100C is relatively cheap with any other type.
  • It is more stable than Nickel RTD.

Disadvantages:

  • There is no internationally recognized standard curve for copper RTD.
  • Copper RTD is susceptible to chemical reaction.

Application:

  • Copper RTDs are most commonly used to sense the winding temperature of motors, generators, and turbines. Connecting them to an alarm trip provides an over-temperature shutdown activity.

The criteria for selecting conductor material for RTDs:

  • Change of resistance of materials per unit change of temperature should be as high as possible. This property provides high sensitivity and better response to the changes in temperature.
  • The material should have high value of resistivity so that minimum volume of material is required for the construction of the sensor.
  • The material should not react, display chemical stability. It should not get corroded with temperature.

Disadvantage:

Concern with all types of RTDs that the error produced due to its self-heating. 

Author: PSS Bapu Rao

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