Pt100 temperature sensors

[Roll No Three]

Hello Sirs and madams!!!

I am working on an application in which temperature varies between 20°C to 160°C and am using pt100 sensors for this.

I have observed one thing that at room temperature the sensors shows the correct reading but as the temperature rises the readings deviate from the standard values, i.e, for temperature 100°C(as per the thermometer) the sensor readings are for around 85°C and for temperature 140°C the readings are for 120°C.

Any idea why such behavior??

 

[FvM]

Wrong circuit, wrong calculation?

 

[Roll No Three]

measuring pt100's resistance directly from multimeter and then converting it to temperature using this link:
**broken link removed**

Also I tried a ready made digital temperature display that uses pt100. Same issue in that case too.

 

[FvM]

Simple explanation, the sensor is not heated to the assumed temperature, e.g. you have insufficient heat transfer between heated object and sensor and Pt100 wires are conducting the heat away.

A typical beginners fault with temperature sensors.

 

[embpic]

post your schematic.

 

[Roll No Three]

@Fvm
Exactly!! thats what I observed..
I heated them in water,both the thermometer and sensor gave concurrent reading but when I am trying to heat them in a closed chamber under a lamp the sensor's reading deviate ;hence heat not being properly transferred to it.
Any ways to get around with it?

@embpic
I am measuring the sensors reading directly through mumtimeter

 

[FvM]

Yous can be pretty sure that the sensor temperature readings are correct. But the sensor has a different temperature than you are expecting. It's not a problem of Pt100 sensors, it's a problem of how to measure temperature of objects or media with real thermometers.

You didn't describe the measurement problem in detail. If it's about gas/air temperature measurement, you should consider that you have heat transfer through sensor wires and sensor surface, with quite different thermal resistance.

 

[crutschow]

Of what exactly are you trying to measure the temperature? The probe must be in direct contact with that item.

 

[Roll No Three]

Well Its a closed chamber heated by an IR lamp and am measuring the chamber ambient temperature(air temperature inside the chamber).
Pt100 is completely inside the chamber beneath the lamp with its lead coming outside the chamber for measurement.

I was verifying sensor's reading when I observed that the temperature shown by thermometer was 140°C while the resistance of pt100 was that of 120°C.

 

[FvM]

In this setup, it's not so easy to tell which value is the right temperature. IR radiation heats objects, those with high emission index (black ones) more than others, but not the air. Although there are some means for accurate air temperature measurement, I'm not sure if they help in this situation.

 

[GUMY]

Sensor should not be directly under IR lamp, it should be in the "shadow". It is like measuring outside air temperature: You do not put thermometer on direct sunlight.

 

[Roll No Three]

@GUMY
Well the entire chamber area is under the lamp so there is nothing like shadow inside it .

@FvM
Lets c what can be done. Thanks for your help. Will let you know if I get any success.

 

[GUMY]

If there is no shadow, then You should make one. Try it and You will see the difference.