Choosing the right temperature sensor
Temperature is probably the most measured physical quantity in industry, it is an essential parameter in many processes. A temperature sensor transforms temperature information into an electrical signal that can be used by a measuring instrument, display or automation.
Among the different existing temperature sensor technologies there are two very different types:
- Contact sensors, which make up the majority of temperature sensors, have the sensitive element located at the point of contact between the sensor and the temperature to be measured.
- Non-contact sensors use infrared technology to measure a surface temperature remotely.
How to choose a temperature sensor?
In order to choose the best temperature sensor for your needs, you should ask yourself the following questions:
Should I choose a contact or infrared sensor? Contact sensors can come in different forms, there are sensors suitable for measuring the temperature of a surface, or sensors for measuring the temperatures inside the material. In any case, the quality of the contact between the sensor and what is to be measured is essential.
Infrared sensors can only measure the temperature of the surface they are aimed at. Even if they do not require contact, you must pay attention to the nature of the atmosphere between the sensor and its target, which can influence the measurement.
In the case of contact sensors, what technology should you use? There are several different options: thermocouples, resistance sensors, etc. It cannot be said that one technology is better than another, each has its advantages and disadvantages:
The response time of the sensor may also be important to consider if temperature variations are to be measured, with thermocouples having a faster response time than RTDs.
The temperature range you need to measure is obviously important when choosing the sensor itself, as is its technology. Thermocouples and RTDs are classified according to the materials their sensitive element is made of and they have different measuring ranges.
Why choose a thermocouple temperature sensor?
A thermocouple sensor is based on the Seebeck effect, it consists of two different metal wires welded together at one end, called a hot weld. By connecting the two remaining ends called reference welds to a voltmeter, an electrical voltage is measured when the temperature of the hot weld is different from that of the reference welds.
There are several types of thermocouples that correspond to different metal pairs. Each metal pair has a different measuring range:
|T||copper / constantan||-250°C to 400°C|
|J||iron / constantan||-180°C to 750°C|
|E||chromel / constantan||-40°C to 900°C|
|K||chromel / alumel||-180°C to 1,200°C|
|S||platinum-rhodium (10%) / platinum||0°C to 1,700°C|
|R||platinum-rhodium (13%) / platinum||0°C to 1,700°C|
|B||platinum-rhodium (30%) / platinum-rhodium (6%)||0°C to 1,800°C|
|N||nicrosil / nisil||-270°C to 1,280°C|
|G||tungsten / tungsten-rhenium (26%)||0°C to 2,600°C|
|C||tungsten-rhenium (5%) / tungsten-rhenium (26%)||20°C to 2,300°C|
|D||tungsten-rhenium (3%) / tungsten-rhenium (25%)||0°C to 2,600°C|
The use of one type of thermocouple over another depends strongly on the temperature range it will have to work on but also on the type of environment:
- E: Recommended for constantly oxidizing or inert atmospheres.
- J: Appropriate for vacuum, reducing or inert atmospheres.
- K: Recommended for constantly oxidizing or neutral atmospheres.
- N: Can be used in applications where type K elements have lifespan problems.
- T: Can be used in oxidizing, reducing or inert atmospheres as well as in vacuum. Not subject to corrosion in humid atmospheres.
- R&S: Recommended for high temperatures. Must be protected in a non-metallic protection tube and with ceramic insulators. Type R is used in industry, type S in laboratories.
- B: Identical to R&S but able to measure higher temperatures.
- resistance and stability at high temperatures
- lots of options in terms of diameter and dimensions
- measurement at the end on the hot weld
- very short response time
- lower accuracy than other technologies
- requires expensive wiring, cold junction compensation
- weak electrical signal
- large temperature range
- short response time
- type K thermocouple
- type T thermocouple
Why choose a resistance temperature sensor?
A resistance temperature sensor, often called an RTD, is a contact sensor. It uses the variation of the resistance of a metal (platinum, copper, nickel or tungsten) according to the temperature. This type of sensor uses several metals that offer different measuring ranges:
- Platinum: -200°C to 600°C
- Copper: -190°C to 150°C
- Nickel: -60°C to 180°C
- Tungsten: -100°C to 1,400°C
The most commonly used metal for resistance temperature sensors is platinum because it offers an interesting measuring range. This is known as a platinum resistance temperature sensor. The most well-known are Pt100 (with a resistance of 100 ohms at 0°C) and Pt1000 (with a resistance of 1,000 ohms at 0°C). The Pt1000 offers better accuracy and a larger tolerance to long wire lengths than the Pt100.
Compared to thermocouples, resistance sensors offer better accuracy and a more linear response. They are more stable in measurement and have a wide temperature range. However, they have a longer response time and lower sensitivity.
- high response time
- weak sensitivity
- large measuring range
Why choose a thermistor temperature sensor?
Thermistor temperature sensors are another type of resistance sensor, they use the variation of the resistance of metal oxides according to the temperature. There are two types of thermistor sensors: NTCs (Negative Temperature Coefficient) which generally have a regular negative resistance variation and PTCs (Positive Temperature Coefficient) which show a sudden positive resistance variation for a narrow temperature range.
Thermistors have a fast response time and are inexpensive, but they are quite fragile and have a much narrower measurement range than other sensor technologies.
- high sensitivity
- good precision
- limited temperature range
Why choose an infrared temperature sensor?
An infrared temperature sensor measures the radiation of a surface in the infrared range to derive the surface temperature. The main advantage of this type of sensor is that it works remotely without any physical contact with the target surface.
The response time of these sensors is very fast, unlike contact sensors they don’t need to establish thermal equilibrium (the same temperature). As such, these sensors can measure moving objects, for example, on a production line, objects that are difficult to access inside a furnace, etc.
On the other hand, they can only measure the surface temperature of the target and the measurement can be influenced by the condition of the target surface (dust, rust, etc.), the cleanliness of the sensor lens (dust) and the environment on the optical path between the sensor and the target (dust, humidity, combustion gas, etc.).
- surface measurement
- very fast response time
- moving objects