RTD question

ceilingwalker

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ceilingwalker
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Mar 2010
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Good day all. I have a SLC5/03 with a 1746-NR4 RTD Module. I'm using 100-Ohm RTD's in acid baths. When the calibration tech measured the tanks with his meter, the temperature varies from what the PLC is displaying. I put in a simple ADD instruction to create an offset. The problem is, they will match for about 3-4 hours, then when checked again, the offsets must be removed because with the second check the temps are now out of whack with the calibration device. The device goes to the manufacturer for calibration on an annual basis and was just calibrated 2-months ago. Any suggestions or ideas as to why this is happening? Thanks much.
 
Are these two, three, or four wire RTDs? How are they connected? Do they use a plug connector to the RTD? How are the connections protected from fumes from the acid bath?
 
Going thru this offset thing myself. Really wonder why we need it.
In our case we are greatly affected by ambient temperature and drafts etc so the temperature of heated device will vary with ambient heat loss. The offset is desired to compensate for this. I think an exercise in futility and frustration.

I think most important are what are the allowable temperature ranges on your tanks? Are you able to hold within that criteria?

NEXT what is error tolerance on your measurement setup probe, wire instrument etc?
and what is the total error?

Will your ambient heat loss change thruout the shift?
Are you reading temperature just after adding cold parts or adjusting the solution?

Dan Bentler
 
>The problem is, they will match for about 3-4 hours, then when checked again

Checked against what and how?
 
please tell me more i like this type of questions.
like what is resolution, is there a software filter in the plc
when is temperature measured?
what is the speed of measurement.
is the tech also using a pt100? (when using this infrared thing give him a coldboot) use a sensor like the one used in food, for yourself and check manual who is correct.
 
I found it interesting that the manual for a Fluke data logger I have calls out for a mercury thermometer inserted in a thermous to use as standard for checking temperature measurements.

Yes kind of old fashioned, definitely not solid state nor leading edge technology.

Dan Bentler
 
calibration is done without electronics as they influence the result. i still have a mercury close to 0 celsius with me, when ever there is somebody coming over to give a calibrationreport for USDA
 
Alaric said:
Are these two, three, or four wire RTDs? How are they connected? Do they use a plug connector to the RTD? How are the connections protected from fumes from the acid bath?
Click to expand...

They are 3-wire, they are wired to each RTD which has a head on the top of CPVC which protects them from the acid fumes. The RTD's themselves have a teflon coat on them to protect the RTD sheath from the acid. Where the teflon terminates we wax the only other part that would be in contact with the fumes with a substance that is used to wax areas of parts we do not want to come in contact with, in the normal process of cleaning parts.
 
I think most important are what are the allowable temperature ranges on your tanks? Are you able to hold within that criteria?

The calibration device and the actual read-out can vary by only 1-degree.


Will your ambient heat loss change thruout the shift?

Yes, the atmosphere in the room is controlled by an evap cooler. In the morning room is always much colder than in the afternoon.


Dan Bentler[/quote]
 
danw said:
>The problem is, they will match for about 3-4 hours, then when checked again

Checked against what and how?
Click to expand...


When the same test procedure is performed: place calibrated device into tank, check HMI read-out, allow for a 1-degree deviation. I will set offset, everyone is happy. A few hours later, perform the exact same test, now the temps don't match again, sometimes by the exact amount put in the offset.
 
leitmotif said:
I found it interesting that the manual for a Fluke data logger I have calls out for a mercury thermometer inserted in a thermous to use as standard for checking temperature measurements.

Yes kind of old fashioned, definitely not solid state nor leading edge technology.

Dan Bentler
Click to expand...

There are very few materials that can be placed into these tanks, glass is not one of them, that the mixture of the acids wont disolve almost immediately.
 
ceilingwalker said:
There are very few materials that can be placed into these tanks, glass is not one of them, that the mixture of the acids wont disolve almost immediately.
Click to expand...

OK acid dissolves glass - not unknown - you must have some pretty nasty stuff - to my knowledge glass is impervious to most acids exept hydroflouric and aqua regia (mix of HCl and H2SO4 - was it??)

I would suggest
1. get a mercury or alcohol thermometer calibrated to NIST traceable with a cal cert sheet. Better yet two (now that you can break one you wont).
2. Calibrate the field unit ie secondary standard against the primary. Now you know it is telling the truth - at least for next five minutes.
3. Take secondary to tank and check tank.
4. Go back and verify secondary is still telling the truth.
5. I liked Flukes suggestion of a thermous and hot water in thermous - effective and cost conservative. Looks a little more scientific than my two styrofoam cups trick.

Now at least you know who is NOT lying (watch #1 or #2).

What is allowable error for both your field cal unit and for the tank temp indications. You have to allow for worst case error ie add them together ie if both are +/- 2 then total error can be 4.

I would surmise you are pumping solution thru a heater. You may be able to add thermowells in the piping to monitor tank temp visually (now you can use glass thermometer). All you have to do is leave pump running to constantly check - I guess you are doing that already.

Hmm know much about electroplating copper to carbon fiber cloth?
Dan Bentler
 
leitmotif said:
OK acid dissolves glass - not unknown - you must have some pretty nasty stuff - to my knowledge glass is impervious to most acids exept hydroflouric and aqua regia (mix of HCl and H2SO4 - was it??)

We use Hydrochloric, Phosphoric, Sulfuric, and another I can't remember right now. These acids are all mixed together with water, in different combinations for each tank. Also, tanks are heated to 130-degrees or less, for each.
Click to expand...
 

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