posted February 17, 2005 11:38 AM            

Specifically, does anyone see the need to remove a thermocouple and calibrate it separate from the instrument to which is attached? What I am referring to is the practice of removing a thermocouple from the equipment where it is used and testing it for accuracy as a stand-alone unit.

I am aware of Block calibrators and Ice Point calibration chambers being used in the testing of thermocouple accuracy. However, since there are no adjustments to be made, I cannot see the need for calibration or checking. Indeed, the very act of removing them for testing may cause failures.

I am also aware that thermocouples come in “Standard” ANSI limits of error of +/- 4deg. and “Special” limits of error that cut those tolerances in half. Much wider tolerances than most instrument manufactures claim for the controllers themselves.

What I would like to establish is a “best practices” policy, but need some input.

Thanks in advance for your help.

Louis

posted February 18, 2005 11:19 AM               

I can't recall ever seeing any easy official calibration or verifcation methods, just the ones you mentioned. The most common problems I've seen are; thermocouple breakage which results in a run-up of the dispaly value to maximum, thermocouples which are not bottomed in the well; the wrong selection of wire or plugs which results in additional junctions and bad readings; the wrong type (J,K,T)thermocouple hooked up to the readout.

Some of the thing I've seen employed are;
a second thermocouple and readout mounded in a control zone for verification. A second thermocouple well mounted in a control zone so you can walk up with a portable unit and plug in to check the zone. The reason for a portible unit is to check for the effect of all the wiring juntions for the fixed units. A portable unit which provides a voltage to drive the controller unit to verify it's temperature readout. If I remeber correctly the pyrometer I had could read temperatures and generate voltages and it had some type of certification itself.
www.davis.com

seems to have all kinds of certified temperature measurment and calibrations stuff.

Sorry I wasn't so fast!

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Best Regards,

Tom Cunningham

Extrusion Technical Services

posted February 18, 2005 12:41 PM            

Thanks for the reply - no apology needed - just trying to drum up some interest.

What I am trying to get to the bottom of is the "need" or "requirement" top calibrate the thermocouple as a stand-alone unit, for instance completely disconnected from the instrument and the barrel.

We have some who are advocating just that and I do not see the need.

Besides in the sterilization process, have you ever run accross that requirement?

posted February 18, 2005 01:00 PM               

From my expereince taking a thermocouple out of a process to calibrate it is a waste of time. When a thermocouple goes bad, usually the resistance go way up, and the control unit displays "Open" or something like that.

There are so many influences on the thermocouple's accuracy while installed that any errors in the thermocouple will likely become lost in the errors of the installation. That is why I focused on checking a control zone using a whole seperate pyrometer,thermocouple, and well.

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Best Regards,

Tom Cunningham

Extrusion Technical Services

posted February 20, 2005 03:44 PM               

However, - in special segments of this Industy (e.g. Medical) they may have a Standard Operation Procedure (SOP) in place already, as it relates to Good Manufacturing Procedures (GMP) and Good Laboratory Procedures (GLP).

I'm sure Barber-Colman (Rockford, IL) still holds formal classes on this, as well as sells calibraation instrumentation as well.

The Thermocouple problems that I've tyically seen though, are the bottoming out of the T/C in the well, as well as passing over an inspection interval.

Worse yet, is the trimming back of talented people who do this thye of work in a Plant.

posted February 22, 2005 12:53 PM            

quote:

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Originally posted by zabielski:
From what I've seen in the Industry is to calibrate the Instrument (Temperature Controller) with the Thermocouple as a single unit.

However, - in special segments of this Industy (e.g. Medical) they may have a Standard Operation Procedure (SOP) in place already, as it relates to Good Manufacturing Procedures (GMP) and Good Laboratory Procedures (GLP).

I'm sure Barber-Colman (Rockford, IL) still holds formal classes on this, as well as sells calibraation instrumentation as well.

The Thermocouple problems that I've tyically seen though, are the bottoming out of the T/C in the well, as well as passing over an inspection interval.

Worse yet, is the trimming back of talented people who do this thye of work in a Plant.

[This message has been edited by zabielski (edited February 20, 2005).]

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That is the problem Zeb - I'm leaning really hard to NOT!