RTD question

[TConnolly]

Better yet two (now that you can break one you wont).

QFT! Funny how that happens isn't it? If you do not have a spare you'll almost certainly need one. If you do have a spare you most probably won't need it.


We used to have a process that used hydrofluoric acid. HF acid is some seriously dangerous stuff, it is easily absorbed through the skin and then it hangs around and messes with blood and bone chemistry. We found another way and ditched that potion.

I'll second Dan's suggestion - check the calibrator with a thermometer first in a neutral liquid, I suggest just plain water.

Is the acid bath part of an electrochemical process (plating, deplating, etching, etc)? Is the first check made before start of a batch and the second check made later after the current is switched on? If so then switch the current off for a few seconds if your process can tolerate it and check to see if there is a small but noticeable change in the RTD reading when the current is off. If the reading changes you might have a ground loop or an isolation problem with the RTD probes so that there is a common mode voltage noise problem on the RTD elements.

 

[danw]

When the same test procedure is performed: place calibrated device into tank, check HMI read-out, allow for a 1-degree deviation.

Is the reference test probe insertion depth into the process liquid defined and rigorously held to?

Is the reference check probe's submersion & dwell time in the process liquid defined ?

I can imagine this procedure is a pain. Could there be a tendency to hurry it along, because it is so routine and boring, and there are other things to attend to?

If the reference probe is not thoroughly heating to an isothermal condition with the bath, then it's merely a snapshot moment on a temperature gradient/transient.

 

[leitmotif]

I was thinking the same stuff as DanW did. If you do not give the probe adequate time to equalize then you are wasting your time. Yes I know about as much fun as watching grass grow or paint dry but if you dont have the time to do it right why do it at all?

The other thing that bugs me about this whole post is it focuses on the instrument and brings up nothing about operator error, what is added to tank, ambient temperature,
is the SAME person taking readings - IMPORTANT
Nothing about calibrating or checks against a standard.
Uhhh did anyone save the manual
did anyone read the manual

HISTORY Was getting funny readings on a FLUKE temperature monitor. New batteries changed the performance dramatically. AMAZING when instrument got 9VDC instead of 4 VDC.

Dan Bentler

 

[ceilingwalker]

We used to have a process that used hydrofluoric acid. HF acid is some seriously dangerous stuff,

We do have HF, however it is not used much, as a liquid. We use HF gas on a daily basis for CVD process, yeah, some pretty dangerous stuff.

Is the acid bath part of an electrochemical process (plating, deplating, etching, etc)?

No, it is used only to clean engine parts in preperation heat treat, etc... .

Is the first check made before start of a batch and the second check made later after the current is switched on?

These tanks are constantly in use, with temp maintained. The test probe is placed within 2" of the control temp RTD. Our procedure states that a reading is not to be taken until calibration device has reached a stable temp reading.

you might have a ground loop or an isolation problem with the RTD probes so that there is a common mode voltage noise problem on the RTD elements.[/quote]

I ran test cable direct from the RTD modules out to the tanks to bypass field wiring. I put my meter on DC and AC milli-volts and checked for a ground-loop, it didn't indicate anything significant. I'm new to this so I'm not sure what you mean by an isolation problem with RTD elements. I did try new RTD's and it had no effect.

 

[Gil47]

This is only a suggestion

Go to the temperature probe , remove the RTD then insert a piece of welding wire or similar to the end of the pocket, this is to get the length that the RTD should be going to get to the end of the pocket and also clears any obstructions, then make sure your RTD wires are twisted round each other this makes the wire stiffer, then insert the probe to the distance you measured on the welding wire.

I say this because I have seen probes only half way down a probe pocket to many time,
and its amazing how many times you are not measuring what you think you are.

Your RTD probe will be connected to a signal conditioner to say 4 to 20 mA,
that signal conditioner what is its calibrated range and next, is your PLC scaled to that same range.

 

[Calistodwt]

Expected error at 130 deg C could be in a range of 0.5 deg C to 1.5 deg C depending upon which standard your RTD and measuring instrument conform to. This applies to an ideal installation. Your installation will probably not be ideal. I would therefore expect that your tolerance of 1 deg C is optimistic at best. To approach the required acuracy I would expect that you would have to test RTD's and select the best possible ones for your installation rejecting those that were at the high end of the RTD specification limits. Also verify that your measuring instrument can conform to the required accuracy of measurement. Worst case scenario is that all errors in the loop will be additive. I would also check your installtion for loose wires / bad connections etc. Is this error you are experienceing a day one problem or has is recently developed? The repeatability of the measurement in your loop is poor and as such it is a waste of time applying a correction factor unless good repeatability can be achieved.

 

[Roy Matson]

I suspect there's something wrong in your wiring, as the interconnecting cable heats up the reading is drifting. One way to check this would be to substitute a 100 Ohm resistor in place of the RTD and watch the result over a 24 hour period.
I have also seen where the RTD was tracking across due to moisture in the head, enough to throw it off by a couple°

 

[leitmotif]

How much does tank level vary with number of parts in it?

Are you always doing the comparison readings at the same level?
Does level affect the temperature readout?

I wonder if the variation you see is more of an operator or methodology issue than wiring.
Dan Bentler

 

[danw]

It sounds like the teflon coated RTD sheath is inserted directly into the process and that there's no thermowell thermal lag to deal with.

What are the relative uncertainties of the process reading and the reference check reader?

What about warm-up time for the reference check (calibrator?) reader? Is the current procedure to turn it on and when the temp appears to stabilize after 90 seconds, that's the reading that is used as a reference? But the manual states that 15 or 20 minutes warm-up time is needed to achieve specified accuracy?

Is the PLC I/O card at a stable temperature? Or does the floor mounted, open (or missing) door control panel periodically get a good dose of frigid winter air when the shipping dock overhead door opens? RTD cards experience a phenomenon known as temp coefficient (tempco) which is an uncertainty as the electronics' temperature moves away from the reference temperature upon which the accuracy spec is based. Changing the I/O cards temperature will affect the I/O card's reading.

If it were my problem to solve, and I couldn't use Shooters' suggestion to compare to a mercury in glass thermometer, I'd start by buying two things:
- a dual element, Class A, highest accuracy available RTD
- power supply/charger for the 'check' calibrator, which given the frequency of having to do the check and the need to warm up the calibrator would be permanently plugged in near the test site.

The dual element RTD would replace the RTD in the process. One element wires to the PLC I/O card (protecting any and all terminal connections as needed in that nasty environment). An RTD plug that mates with the calibrator's RTD socket connector would be installed on the end of the other element's cable.

When it was time to make a temp check, I'd plug in the 2nd element RTD plug into the calibrator, plug in the calibrator's power supply/charger, turn the calibrator on and let it warm up for whatever duration its manual says is the required warm up time period. After the calibrator warm up time, I'd make the check.

A dual element high accuracy RTD installed in the same process location will provide the best duplicate sensing I can think of. Both elements will be at the same temperature, regardless of how the elements are inserted into the process and whatever thermal lag their common sheath 'sees'.

By eliminating sensing thermal lag, deltas due to location, and sensor/check calibrator warm-up time errors, any remaining reading differences issues reside with the readers.

the other Dan on this thread

 

[ceilingwalker]

Thank you all very much for the suggestions. I will use each suggestion and see what happens. Will post results.

 

[jdbrandt]

I had really funky issues with the same module (correct readings at one process temperature, incorrect readings at a different one.), until I changed the excitation current to 'the other setting'. You can choose 0.5 or 2.0 mA. The manual is unclear as to preference, but it all made the biggest difference. Its bit 11 or 12, as I remember.
Later in the book, I discovered that changing this setting also 'calibrates' the module, which would be <?>...wire distance?
Either way, it started working, and I was happy.
----
The key piece of information missing from the original account is: Is this a new installation, or has this BEEN working for a while, and NOW is messing up.

 

[Mickey]

I had really funky issues with the same module (correct readings at one process temperature, incorrect readings at a different one.), until I changed the excitation current to 'the other setting'. You can choose 0.5 or 2.0 mA. The manual is unclear as to preference, but it all made the biggest difference. Its bit 11 or 12, as I remember.
Later in the book, I discovered that changing this setting also 'calibrates' the module, which would be <?>...wire distance?
Either way, it started working, and I was happy.
----
The key piece of information missing from the original account is: Is this a new installation, or has this BEEN working for a while, and NOW is messing up.

You can cause/or correct "Self Heating" errors by adjusting this current.
See link below for a short discussion on the subject.

http://www.thermosensors.com/rtd/definitions.htm

 

[Roy Matson]

Reading some of the later posts reminds me of an unusual problem i once saw.
Several RTDs installed in large thermowells (~6ft long) from the top of an autoclave.
5 out of 6 read the same but one read about 10% low.
It turned out the thermowell had a little moisture inside this would boil off cooling the RTD then condense at the upper end where it was cooler and run back down to the tip repeating the process. Cracking the fitting allowed a jet of steam out and the problem fixed itself.

 

[ceilingwalker]

I suspect there's something wrong in your wiring, as the interconnecting cable heats up the reading is drifting. One way to check this would be to substitute a 100 Ohm resistor in place of the RTD and watch the result over a 24 hour period.
I have also seen where the RTD was tracking across due to moisture in the head, enough to throw it off by a couple°

We had that also. That's why each head is sealed with wax to protect it from the harsh environment it is subjected to.

 

[ceilingwalker]

I had really funky issues with the same module (correct readings at one process temperature, incorrect readings at a different one.), until I changed the excitation current to 'the other setting'. You can choose 0.5 or 2.0 mA. The manual is unclear as to preference, but it all made the biggest difference. Its bit 11 or 12, as I remember.
Later in the book, I discovered that changing this setting also 'calibrates' the module, which would be <?>...wire distance?
Either way, it started working, and I was happy.
----
The key piece of information missing from the original account is: Is this a new installation, or has this BEEN working for a while, and NOW is messing up.

System has been in place for a while, HOWEVER, the entire rack and modules were replaced a couple of months back because they had deteriorated to an unusable point. I took all of the original configuration info for the RTD modules and set it up identical to the original.