Temperature Measurement

Resistance Temperature Detector, RTD

Introduction

Temperature measurement is one of the significant functions in the field of measurement, which has been most frequently conducted in meteorological observation, environmental research, laboratories, and various other production processes. Temperature measurement is fundamental and important in terms of manufacturing products in a specified condition and maintaining consistent quality in the industry. Therefore, this describes the measurement principles of the thermocouples and resistance temperature detectors (RTD) in temperature sensors most often used in temperature measurements carried out by the field of industry.

 

1. What are Thermocouples?

Thermocouples are temperature sensors. They operate under the principle that the junction of two dissimilar metals (forming a closed circuit) produces a measurable voltage (electromotive force) when the two ends of the thermocouple are at different temperatures (see Figure 1). Because thermocouples have simple construction and are superior in reliability, they have been used as industrial temperature sensors in a wide range of fields. Moreover, connecting a measuring instrument (recorders, etc.) to one end of a circuit allows you to measure potential difference (electromagnetic force) (see Figure 2).

There are many types of thermocouples to measure diffrent range of temperature. Commonly-used types with superior characteristics have been standardized by JIS, IEC standards, and others. The following summarizes the typical thermocouple types (generally represented by symbols) and their features (advantages and disadvantages).

Type Advantages Disadvantages
B 1. Suitable for high temperature measurements of 1000° or more
2. Has very low thermal conductivity at room temperature. No compensating lead wires required
3. Offers excellent acid resistance and chemical resistance.
1. Has low thermal conductivity in intermediate- to low-temperature ranges, disabling measurements at 600° or less
2. Poor sensitivity
3. Poor thermoelectromotive force linearity
4. Expensive
R/S 1. Has good accuracy and less variations and deterioration
2. Offers excellent acid resistance and chemical resistance
3. Usable as standard
1. Poor sensitivity
2. Susceptible to reduction atmosphere (particularly, hydrogen or metallic vapor)
3. Causes a large compensating lead-wire error
4. Expensive
N 1. Has excellent thermoelectromotive force linearity
2. Offers excellent acid resistance at 1200° or less
3. Little affected by short-range ordering
1. Not appropriate to reduction atmosphere
2. Subject to large secular changes in comparison with precious-metal thermocouples
K 1. Has excellent thermoelectromotive force linearity
2. Offers excellent acid resistance at 1000° or less
3. Has excellent stability among base metal thermocouples
1. Not appropriate to reduction atmosphere
2. Subject to large secular changes in comparison with precious-metal thermocouples
3. Subject to error caused by short-range ordering
E 1. Is the most sensitive among the current thermocouples
2. Offers better heat resistance than type J
3. Both legs are non-magnetic
1. Not appropriate to reduction atmosphere
2. Shows slight hysteresis.
J 1. Usable in reduction atmosphere
2. Has a thermal conductivity approx. 20% greater than type K
1. “+”-leg iron is rust-prone
2. Shows large variations in its characteristics
T 1. Has excellent thermoelectromotive force linearity
2. Offers excellent characteristics at low temperatures
3. Small variations in quality
4. Usable in reduction atmosphere
1. Low usage limit
2. “+”-leg copper is prone to oxidation
3. Causes a large thermal conductivity error

Source: Japan Electric Measuring Instruments Manufacture’s Association; “Proper Usage of Thermometers – New, “Japan Industrial Publishing Co., Ltd.

2. What are Resistance Temperature Detectors (RTDs)?

Resistance temperature detectors (RTD) are temperature sensors. They operate on the principle that the resistivity of a metal increases in proportion to its temperature.
A Platinum RTD uses platinum (Pt) for a resistance temperature sensing element, which has good temperature characteristics and is linear and stable.

Among the various types of temperature sensors, Platinum RTDs have been used widely with their high accuracy. In particular, Pt100 (resistance value at 0° is 100 ohm) has been popular worldwide. Nickel and copper are also used for RTDs. Thermistors are employed as resistors.

Three types of wiring techniques are available: two-wire, three-wire, and four-wire.

Moreover, a measuring circuit on the measuring instrument side differs depending on each wiring techniques. Figures illustrated below explain the principles of the three-wire technique most frequently used in the industrial measurement, and the four-wire technique used for precision measurements.

a) Principle of the three-wire technique

Lead resistance of r1 and r2 are perfectly matched, and therefore canceled in a bridge circuit. Therefore, keeping the resistance of the three leads low and uniform allows you to perform temperature measurements with few errors, even if the leads between Rt and the measuring instrument are made longer.

b) Principle of the four-wire technique

A constant current is passed through r1 and r4, and voltage is measured at the terminals of RTD, which is free from the effect of lead resistances in your measurement. Therefore, this system allows accurate temperature measurements.
If a RTD of the four-wire technique is connected to a measuring instrument of the three-wire technique, disenabling one of the RTD leads in the four-wire technique provides a simple configuration of temperature measurement. In this case, keeping the resistance of the three leads low and uniform is required in the same way as the three-wire technique. the unused lead must be terminated (insulated) to avoid the effects of noise and others.

Reference:
(1)Japan Electric Measuring Instruments Manufacturer’s Association; “Proper Usage of Thermometers – New,” Japan Industrial Publishing Co., Ltd.
(2) Tamotsu Ishii; “Instrumentation Control System,” Denkishoin Co., Ltd.