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Temperature Measurement and Control
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Overview.
Since the properties obtained by heat treatment are determined by heating and cooling, accurate temperature measurement and highly precise temperature control are required.
01 Types of Thermometers
As shown in the figure above, there are two types of temperature measurement methods, contact and non-contact, and they are used according to the temperature, type of substance to be measured, and measurement environment.
Contact thermometers include the following table Thermocouple Thermoelectric thermometers use a thermocouple to measure Resistance thermometers Most of them are used in heat treatment work.
Non-contact thermometers include radiation thermometers, which measure by the amount of infrared radiation energy, and optical pyrometers, which measure by visible light. Among heat treatment operations, radiation thermometers are used for high-frequency quenching and ion nitriding.
02 Thermocouples and their characteristics
| Type symbols | Constituent materials | Wire diameter mm | Normal use limit °C | Heating use limit °C | |
| + leg | -foot | ||||
| B | Pt-Rh alloy containing 30% Rh | Pt-Rh alloy containing 6% Rh | 0.50 | 1500 | 1700 |
| RADIUS | Pt-Rh alloy containing 13% Rh | Pt | 0.50 | 1400 | 1600 |
| SADIST | Pt-Rh alloy containing 10% Rh | Pt | 0.50 | 1400 | 1600 |
| N | Alloys mainly composed of Ni, Cr and Si | Alloys consisting mainly of Ni and Si | 0.65 | 850 | 900 |
| K | Alloys mainly composed of Ni and Cr | Ni-based alloys | 0.65 | 650 | 850 |
| 1.60 | 850 | 1050 | |||
| 3.20 | 1000 | 1200 | |||
| E | Alloys mainly composed of Ni and Cr | Cu and Ni based alloys | 1.00 | 500 | 550 |
| J | Fe | Cu and Ni based alloys | 1.00 | 450 | 550 |
| T | Cu | Cu and Ni based alloys | 1.00 | 250 | 300 |
Thermoelectric thermometers measure temperature by means of thermocouples, which are made by joining the tips of two types of metal wires. Thermocouples measure temperature by the emf generated by temperature differences, and various combinations of thermocouples are used. Among them, K thermocouples and R thermocouples are commonly used in heat treatment work.
K thermocouples are made of Chromel (-) and Alumel (+) and are commonly used for heat treatment up to 1000°C. R (formerly PR) thermocouples are made of PT (-) and Pt-13%Rh (+) and are commonly used for high temperature heat treatment work up to 1400°C.
03 Thermoelectric thermometers and temperature control methods
Thermoelectric thermometers are based on a combination of a thermocouple and a measuring instrument. When used in a large area such as a heat treatment plant, the thermocouple and the measuring instrument are connected by a compensation conductor that matches the type of thermocouple.
TEMPERATURE CONTROL METHODS INCLUDE BASIC ON-OFF CONTROL, PROPORTIONAL CONTROL (P CONTROL), AND PROPORTIONAL, INTEGRAL, AND DERIVATIVE CONTROL (PID CONTROL). ON-OFF CONTROL IS THE SIMPLEST AND MOST COMMONLY USED, BUT PID CONTROL IS EFFECTIVE FOR PRECISION CONTROL DUE TO ITS LARGE CONTROL RANGE.