RTD Thermowell Response Time

[MSalem]

Is it possible to ascertain the thermal response time of RTDs in thermowells installed in a particluar application without conducting any testing? Are there any first-order or second order differential equations which can give the response time?

We've used ABB Temperature transmitters on SIL-2 and SIL-3 rated loops and due to the various factors influencing the actual response time such as thermowell dimensions, weight, material used, spacing between the RTD insert and thermowell, heat transfer coefficient and process parameters like velocity, heat capacity etc., all research I've done so far on the subject suggests that it's hard to determine the actual response time.

The problem is that ABB has provided some of the dimensions in their GA drawings and for the response time, they've conducted tests of immersing the sensor in flowing water at 0.4 m/sec in line with IEC 60751, however this only gives a rough guide of the thermal response time and the actual response time can vary significantly depending on the application.

ABB provides response times in the range of 60+ secs for the sensor-thermowell combination we've used but in practice the sensors could take longer to respond and hence can become critical where the process safety time is low.

I was wondering if mathematical analysis can be done to estimate the actual response time for a particular application, as this may obviate the need for in-situ testing of temperature sensors for response times?

Any inputs will be appreciated.

 

[danw2]

A web page entitled "Thermowell wakes, vortices, vibrations & more" references work done by David S. Bartran,

http://www.temperatures.com/twellwakes.html

Thermal response is just mentioned as a calculated time constant.

You might want to be aware that another paper of his, "Are your thermowells safe?" is here,

http://tappi.micronexx.com/JOURNALS/PDFS/02APR10.pdf

(While resonance is not the specific topic of your question, I thought it might be of interest in the theme of safety.)

 

[roydm]

Wouldn't it be easier to just take a thermowell and plunge it in boiling water, that would give you the true picture.

 

[temperaturetalk]

Your research is correct, it is very hard to predict response time in a given application without doing in situ testing. As they say, the proof of the pudding is in the tasting.

That said, Tom Kerlin's book on Industrial Temperature Measurement does the best job of accomplishing what you are after of any book I have read. The calculations are too long and complex to state in their entirety here, but I was able to find an old copy on Amazon for < $50

http://www.amazon.com/Industrial-Temperature-Measurement-Thomas-Kerlin/dp/0876646224

Depending on your application's characteristics, you can do a number of things to speed your response time for an RTD-thermowell assembly. One simple method is to use a step shank thermowell instead of a straight or tapered well.

 

[FrancisL]

Is there a particular reason that you want this information?
I ask because generally the response time of an RTD in a thermowell is so much faster that the actual process response time that the influence of the thermowell is insignificant.
Where exactly is the thermowell? And what do you use the temperature measurement for?

Francis

http://www.controldraw.co.uk

http://www.s88control.blogspot.com