Temperature Measurements

 Temperature Measurements











Temperature is one of the most common process variable measured in industries.

  •       Motor winding
  •       Boiler

Different technology are made to serve different application


Temperature sensors

 













  • RTD 
  • TC




RTD " Resistive Temperature Detector"

The RTD wire is a pure material, typically platinum, nickel, or copper which change their electrical resistance as a function of temperature.

RTD is a passive device  need external electrical current

PT100 is the most common RTD 

"PT100 " 

Means : PT for Platinum 100 Ω at 0°C

Connection types  : 2 wires , 3 wires , 4 wires

         R=R0(1+αT)

               where R0 =100 , α=0.0038 for Platinum

     For PT100

  1 Ω =2.63°C (above 100 ohm)

For PT1000  (above 1000 ohm)

1 Ω = 0.263 °C


PT100 2-wires


 











With a 2-wire connection, not used with direct connection to controller 

because the resistance of the cable is added as an error in the measurement.

 for example a copper cable with a cross-section of 0.22 mm2, the following guide value applies: 0.162 Ω/m → 0.42 °C/m for Pt100.

that mean if we has a distance 100m to the controller there will be 42 °C error by cable resistance

 

 





Temperature Transmitter














so PT100 2-wires need a Temperature transmitter connected closer to the PT100 and send 4:20 mA
signal to controller represented the temperature readings.









PT100 3-wires










With a 3-wire connection, the maximum length of the connection lead depends on the conductor cross-section

Maximum cable distance up to 300 ft / 61 m

If longer cable lengths are required, then it is best to use temperature transmitter which transmit  4:20mA



PT100 4-wires










 4 Wires connection cable : 4 wire RTD is more expensive than 3 wire more accurate and high response.

Completely eliminates the influence of the connection lead on the measuring result

 use whetstone bridge to detect small variation


TC "Thermocouple"






Thermocouples (TCs) use the thermoelectric effect (Seebeck effect) to measure

the temperature difference between a measuring point and a reference junction with known temperature. 

It is a simple device consisting of two dissimilar wires joined at their ends , it generate (mv) when heat is applied to hot junction 


Thermocouple TC act as a DC battery one metal (Alloy) act as a positive side and the other metal (Alloy) act as the negative one.

by using different metals (Alloys) we built another type of Thermocouple

Every type has its standard colors according to type and country 

Constantan "Copper + Nickel alloy"




Temperature Transmitter may connected to the Thermocouple TC and send 4:20mA to the controller or it TC send mV signal to the controller through a TC module 



RTD vs TC




Temperature Gauge


Based on the volumetric temperature expansion of the gas filling in the measuring element. The measuring element is firmly connected by a capillary tube with a measuring spring. The spring transfers the expansion of the gas filling by a movement on a dial.

 


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