AMS2750E Offsets procedure

S

speck

#1
I would like to see if anyone can shed light on how they "account for and reintroduce any intentional offsets" per AMS2750E 3.2.4. I perform all our internal SAT's and TUS's but we have never used offsets but now that we have class 2 furnaces I find that the simple thermocouple or instrument error can put me outside of the ?2?F tolerance unless I use the offsets. I need to make a procedure to document the use of offsets and not sure what the above requirement is asking, (reintroduce). My TUS and SAT forms/reports are very thorough so I can easily amend them. Any help would be greatly appreciated. Thanks
 
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MTProcessing

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#3
We document for each furnace:
  • Calibration offset: Anything our instrument calibration technician (outside source) needs to put in. There are no limits here.
  • SAT offset: We put this in to make SATs pass. Limited to ?5?F for class 2 furnaces.
  • TUS offset: We put this in to center our TUS results. When doing an SAT, this amount has to be removed from the instrument reading before comparing to the probe reading. Limited to ?5?F for class 2 furnaces. Independent of the SAT offset.
  • Total instrument offset = Cal + SAT + TUS and needs to be the offset actually programmed into the instrument at all times (including when doing surveys).

That particular paragraph 3.2.4 is not very clear. It's talking about both manual (operator programs the furnace for a different temperature than the router calls for) and internal (programmed into the controller). It mentions calibration, SAT, and TUS offsets, when all of these are treated differently than each other. I can't really make out which part is talking about which kind of offset. Possibly "reintroduce" is referring to putting the TUS offset back after removing it to do an SAT. (Although we just remove the TUS offset mathematically on the SAT form and don't mess with the instrument programming).

I have also heard of companies specifying "half special limits" tolerance on all their thermocouples to try to avoid the need for offsets. I'm told the price difference is not very large compared to normal "special limits" tolerances.
 
S

speck

#4
Compliments to you for your help! I have a good direction now and will also look into 1/2 special limits specification on my T/C's. This is probably what I need to do most because I do not necessarily need to add offsets for the instrument calibrations, SAT or TUS but when I use the same thermocouple lot for control and SAT and the correction factor is 1.7?F at 250?F and 3.9?F at 1200?F (?.4% allowed) it puts me out of tolerance obviously when I can't add in the 3.9?for the controller T/C error.

Which brings me to my final question on this....? If there are different calibration errors for different temperatures on the instrument calibration report, SAT report or calibration certificate, how do you enter 1 offset without having the operators enter the correct offset based on the set temperature; or can avoid this? This would create a possibility for many compounded errors in the shop by having to independently calculate the proper offset for each temperature.
 

MTProcessing

Involved In Discussions
#5
If there are different calibration errors for different temperatures... how do you enter 1 offset without having the operators enter the correct offset based on the set temperature?
The only requirement to account for all certified errors / correction factors is on the SAT thermocouple and thermometer when doing an SAT, and on the TUS thermocouples and logger when doing a TUS. The furnace instrument and thermocouples do not have to be corrected for any of their errors.

I agree that having operators enter different offsets based on the set temperature, while allowed, introduces more error possibility, and I have so far :cfingers: avoided needing that. We've been able to get all the TUS and SAT to pass at all temperatures with a single offset. (Well, a multi-component offset, but the "total instrument offset" is a single number for each of our furnaces regardless of temperature.)
 
S

speck

#6
Thank you very much again for your time. The direction is most appreciated. I believe I can take a stab at writing the procedure now and will also be ordering 1/2 special limit T/C's.
 
#7
Hi can I check what is the purpose of tus offsets (&purpose of centering tus results)? Isn't a temperature as accurate and close to actual conditions better?
 
#8
fmp

A TUS offset is used to account for the fact that the control sensor is not actually placed in the center of the control volume. By performing a TUS you can document the characteristics of the control volume with regards to high and low temperature at the extremities of the volume and their relation to the center of the volume. You actually don't want an accurate reading of conditions at your sense point to drive your controller unless your sense point is very close or in contact with the material being processed, generally doing so is impractical so a TUS offset is used instead.

The purpose of a TUS offset is to make the controller control the actual mid-point of the control volume's temperature range so that the absolute deviation of the high and low temperatures are as close to the same value as possible.

For example an TUS reveals that the high deviation is +8°F and the low deviation is -4°F. The deviation to the center is -2°F. this oven is ±8°F

Correcting for the center gives a +10°F high -2°F low and 0°F center deviation. this oven is ±10°

Correcting for the high and low areas gives +6°F high -6°F low and -4°F center deviation. giving an oven that is ±6°F

hope that explains it.
 
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