Instrumentation and Sensors Package

 

Temperature Measurement

Temperature Sensors

Thermocouple

Basic montage

made out of two strips or wires of different metals (alloys) and joined at one end. The joint is called thermo-junction. A thermocouple produces voltage as a function of temperature. The voltage generated rises with temperature although it is not linear. A thermocouple does not need power to operate thus it is a passive sensor.


Thermocouple code


Thermocouple leads are color coded. In the U.S. the color code is Red

and some other color, where the red lead is the negative (low) lead.

 

Take Care for other countries

What to make a junction ?


Every time you connect two different metals/alloys a thermocouple

junction is formed.

Copper

Constantan

Copper

Copper

Junction

Junction

+

+

-

Dealing with Thermocouples

You cannot extend a short thermocouple wire using a regular wire or

using a different type of thermocouple to make a longer thermocouple

wire.

Connecting a thermocouple wire to an instrument in reverse will not

harm the instrument. It will however give you an incorrect reading.

 

Thermistors

RTD and thermistor are both non-linear

RTD/Thermistor Circuit

A fixed, DC current is supplied to the sensor, the voltage developed

RTD (Resistance Temperature Detector) is made out of metal (Platinum wire wrapped around a mandrel and covered with a protective coating) Thermistor is made out of ceramic semiconductor.

across the sensor is a function of temperature.

RTD and Thermistor

RTD (Resistance Temperature Detector) is made out of metal (Platinum wire wrapped around a mandrel and covered with a protective coating)

Measure Voltage Here

Constant Current Source

Infrared Thermometry

non-contact temperature sensor. It senses the amount of radiated

energy, in the infrared band, by the targeted object.

Realisation



Emissivity

You must know the emissivity of the surface being measured in orderto be able to measure its temperature correctly. The emissivity of a material is normally listed in a table that comeswith the instrument.If you don't know the emissivity of the surface you are targeting, youcan put a masking tape on the surface and use 95% as the emissivitynumber. This approach won't work in a gin stand, will it? You can"fudge" the emissivity number by using another instrument tomeasure the temperature and then use the infrared to measure thesame temperature and make the readout correct by trying differentemissivity numbers.

Wires of compensation


 

Pyrometer