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Temperature Sensors

 
 

What does a temperature sensor look like?

Temperature sensors can take many forms. In most industrial and commercial applications, the actual sensing element is placed inside a sealed stainless steel sheath, which is attached to a head assembly that contains screw terminals for wiring the sensor to a measuring device. The sensor head may also contain electronics to interface the sensor to the measuring system (e.g. a 4-20 mA transmitter).

For some applications it is more practical to use unsheathed sensors. This particularly applies to the more robust sensors, such as thermocouples. Advantages include a more rapid response, smaller size and lower cost.

When measuring very high temperatures, above 600°C, the sheathing material often becomes critical. It is chosen for its chemical tolerance to the medium being measured and its contamination effects on the sensing element. Atomic diffusion is greatly accelerated at elevated temperatures.

Measuring Temperature

Check out the Temperature Measurement page for hints on practical temperature measurement.

Selecting a temperature sensor

There are many sensors that are able to measure temperature. The choice depends on a number of factors, including: the accuracy, the temperature range, the access to the point of interest, the speed of response, the environment (chemical, physical, electrical) and cost effectiveness. Selecting the appropriate sensor is not always easy. One method is to follow the lead of others in the field. Particular sensor types almost become traditions in a field (although not always the most appropriate). The following table may provide a guide:

Field Traditional Sensor
Agricultural Research Thermistor, Type T thermocouple, Semiconductor
Automotive Thermistor, Pt100, Bimetallic
Chemical & Materials Processing Pt100, Thermocouples
Cryogenics Metal oxide resistor
Environment Research Thermistor, Type T thermocouple, Pt100, Semiconductor
General Industry Pt100
Hobby, Education Semiconductor, Thermistor, Type T thermocouple, Paint
HVAC Ni1000, Thermistor, Pt100, Paint
In Manufactured Goods Semiconductor, Thermistor, Pt100
Metallurgy Type K or Type N thermocouple

The table in the introductory section of this article may also help in making a decision.

A good starting point is to define the following requirements:

    • The accuracy and resolution required
    • Your sensor interchangeability needs
    • The temperature range
    • The measuring equipment capability
    • The cost
    • Whether individual calibration is practical
    • Media compatibility and other environment issues that might impact the sensors reliability and survivability.

The following table provides a rough comparison between the temperature sensor types:

Sensor Type
Output
Range° C
Accuracy±°C
Robustness
Cost
 Thermocouple
40µV/°C
-270 to 2300
1.5
high
low
 Platinum RTD
0.4%/°C
-200 to 600
0.2
medium
medium
 Nickel RTD
0.4%/°C
-200 to 600
0.3
medium
low
 Thermistor
5%/°C
-50 to 200
0.2
high
medium
 Semiconductor
10mV/C or 1µA/°C
-40 to 125
1.5
medium
low
 Non-Contact
millivolts
0 to 6000+
2
low
high
 Fiber Optic
various
-100 to 200
1
medium
very high
 Cryogenic
various
-273.15 to -200
various, to ±0.001
various
various
 Bimetallic
displacement
-100 to 300
2
high
low
 Paint
colour change
-30 to 1200
1 to 20
medium
low

The above table is general in its summary and should not be taken as the definitive statement on temperature sensors. This particularly applies to the accuracy column, which shows the 'off the shelf' or inter-changeability accuracy. Generally the accuracy of all sensor types can be greatly improved by individual calibration. For more information, refer to the appropriate page on each sensor type (in the left-hand menu) and the Selecting a Sensor section below.

Temperature Sensor Manufacturers

There are many manufacturers of temperature sensors. There are three to five steps to producing a useable temperature sensor:

    1. Starting from raw materials and produce the sensing material such as high purity metals or semiconductors

    2. Fashioning the sensing material into a useable form such as a wire, foil or powder

    3. Producing the sensing element

    4. Assembling the sensing element into a protective case and wiring

    5. Provision of signal conditioning, buffering or translating, depending on sensor type

Few manufacturers are involved in all steps, the majority deal only with the last two. The quality and performance of a sensor is dependent on all process steps.