selecting a temperature sensor for air/solvent vapor

[clandestinho]

hello,

I need to select a temperature sensor for an air flow containing solvent vapours. My criteria are
- fast response. This is top priority
- fair accuracy and repeatability. I would choose a 3-wire pt100 if it had a faster response
- good chemical resistance. I would prefer an exposed sensor but any polymer such as thin film or pvc covered wire will be attacked by the solvent vapors
- should withstand a significant airflow, around 300 m3 /h (88 L/sec)in a 2" pipe.

the relevant temperature range is -10C to + 45 C.

Any ideas?


Claus

 

[xnuke]

It's hard to help you when you use qualitative terms like "fast," "fair," and "good." Do you have numeric values in mind?

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[controlnovice]

Try these:

https://www.emerson.com/en-us/catal...e-sensors/paine-215-01-120-temperature-sensor

https://www.emerson.com/en-us/catal...rs/paine-225-10-0x0-digital-temperature-probe

or this, but be careful of velocity and strength of element casing

https://www.emerson.com/en-us/catal...ure-sensors/rosemount-1067-temperature-sensor

Possibly with a Rosemount 1097 Thermowell (thin wall, 3mm)

Consider using 4wire pt100



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This is normally the space where people post something insightful.

 

[clandestinho]

Xnuke,
Yes, I was a little vague there, i'll try to be more specicif.

response time
I have found a fast response RTD with a time constant of 2 sec. @ 63.2% and with a 2" inserted length and in stainless steel for direct immersion.
I think this would work but the 2 seconds are measured in moving water, and I don't know what to expect when the sensor is used for air instead. I suppose the response will be slower but by how much? I asked a supplier and they couldnt provide any useful info.

Accuracy / repeatability
Both thermocouples and RTDs would be Ok for this application. I have not used thermistors, they are said to drift over time and to self heat. They are not my first choice unless one of you convinces me otherwise.

chemical resistance
a thermocouple could give me the response time I need but I think they would be exposed elements or quite thin, sheathed elements and I doubt they are the right choice for the aggressive environment and/or the air flow.
I Can't used copper tipped probes either. The ideal materials are stainless steel and/or ptfe.

I was hoping that someone could see some options that I don't know about.

Thanks for helping

 

[ScottyUK]

Plenty small-geometry thermocouples available which will survive in solvent-laden air. Some have stainless sheaths only 1mm or so in diameter, so little mass and fast response time. If they aren't subject to mechanical impact they should have no difficulty in the application you describe.

 

[roydm]

I agree with ScotyUK, a stainless steel probe without a well.

I don't think it will make a difference what type of element you use but RTD will be more accurate for sure. The velocity will be beneficial in reducing the time constant.

 

[georgeverghese]

The Paine 225 unit from Emerson @control novice has posted looks good for fast response.

 

[controlnovice]

You can go without a Thermowell for faster response, but just remember if you ever need to replace the element, you will have to shut down your process!


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This is normally the space where people post something insightful.

 

[clandestinho]

Controlnovice, thanks for suggesting the Emmerson sensors, they look interesting and I would not have found them otherwise.

Roydm, good point about the volocity, I had so far only seen the problems which are that the velocity will make a very thin element vibrate until it fails. But indeed, the sensor may respond almost as fast as in water.

Thanks !

 

[danw2]

a grounded thermocouple can provide a fast response in a thin sheath, as the tables show (E is Exposed, G is Grounded, U is Ungrounded):

RTD's require electrical current pumped through them in order to take a measurement so commercial RTD's are not grounded to the sheath.