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Temperature Transmitter

01/08/2017 10:38 AM

Recently i am working on a data sheet of a PT100,3 wire RTD for the selection purpose of a TT which is best suited for my process.If my process temperature range is 0-200deg then what will be the Range limits and calibration range of the transmitter??is there any standard for selecting/determining those parameter based on process data??

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#1

Re: Temperature Transmitter

01/08/2017 10:47 AM
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#2

Re: Temperature Transmitter

01/08/2017 11:04 AM

Don't you think the intelligent course of action would be to go to the manufacturer and ask their experts, instead of asking total strangers on an anonymous forum?

3-Wire RTD Measurement System Reference ... - Texas Instruments

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#3

Re: Temperature Transmitter

01/08/2017 11:05 AM

Do you have a QA/QC (ISO e.g) or operational standard for your instruments/equipment?It must have been stipulated there.

If not, then you should recommend a safe calibration range or a standard for such. Maybe +/-10 to 20% the limits or whatever critical points for the process and media. Have it approved by your superiors.

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#4

Re: Temperature Transmitter

01/08/2017 12:40 PM

What are the requirements for your process? The RTD accuracy needs to be better than that required by your process in the required temperature range.

Here is a sample calibration table for a pt100.

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#5

Re: Temperature Transmitter

01/09/2017 3:09 AM

A1) 0-200degC.

A2) The Process Description document contains that information. It cannot be seen from here.

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#6

Re: Temperature Transmitter

01/09/2017 11:11 AM

If the process is 100% contained in the 0-200 C range, then you need a temperature transmitter that will at least cover 0-200 C range, but better would be -10 to 220 C, with a suitable error limit at each end of the range.

Look at the standards put forth by your vendor.

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#7

Re: Temperature Transmitter

01/13/2017 7:35 AM

Let's see. I used to have this problem back when we were trying to figure out how to get our ISO9000 certification twenty-some years ago. This is how we thought it should be done.

Range Limits:

Say, your product needs to be heated to 85°C ±5°C. Then it needs to be cooled down to 40°C ±3°C. Those plus or minus figures are the process tolerances (our jargon, others might use different terms). This means that the process range is 37°C (40-3) to 90°C (85+5). That range of 37 to 90°C now helps us determine the range the instrument we will use.

Now, how to select the range limits for this transmitter.

Exactly 37 to 90°C. This will allow our controller to use the entire 0 to 100% span. This may allow tighter control of your process.

35 to 95°C. The difference from the above choice is only small so controller performance may be about the same. The only other reason for selecting this range is that people like rounding things to the nearest 5s.

0 to 100°C. We like to keep transmitters in stock that are pre-ranged and pre-calibrated so that we can immediately replace a defective transmitter instead of having to configure and calibrate before installation. To reduce the number of transmitters we need to keep in stock, we configure them to have certain ranges that are common in our process. We have 0 to 50, 0 to 100, 0 to 150, and 0 to 200°C.

Calibration Range

If you're asking about "range", the range is the same as the range limits. So, if I use a 0 to 100°C transmitter, we calibrate it to 0 to 100°C. If you're asking about "tolerance", that's different.

If you remember the product temperature requirements of 85°C ±5°C and 40°C ±3°C. The plus and minus figures give us tolerance figures of ±5 and ±3.

A rule of thumb we used to have is that the calibrator needs to be more accurate than the instrument by a factor of four. In this case, the instrument needs to be more accurate than the process by a factor of four as well.

If the process needs to be ±3°C (the smaller of the two tolerances), the instrument needs to be accurate to 3÷4 or ±0.75°C. This becomes our calibration tolerance, or the allowable deviation of the instrument before it needs to be recalibrated.

Caveat:

When we asked our process engineers for their process tolerances, all they could tell use was, "the error should be zero" or "it should be as small as possible."

Since we lack this crucial piece of information, we just put ±5% for level, ±2% for temperature, ±2% for pressure, and ±5% for weight transmitters.

The auditors nodded their heads and signed our certificate, the process people didn't question our choice, and we happily discarded our range calculation methods.

I hope your company does better.

regards,

Vulcan

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#8
In reply to #7

Re: Temperature Transmitter

01/17/2017 9:47 AM

tnx a lot.Now i got it properly.

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