Help scale an RTD with a 4-20mA SIG Conditioner

skyfox · Jun 21, 2021

willxfmr said:
Keep in mind I'm just a 3rd shift knuckle dragger.
It looks to me like you need dip switches 5,6, and 8 on. In your analog card set the scaling to 4ma/-200C 20ma/850C. I gather this from the box on the lower left that gives the ranges of PT100 RTD.

Bubba.

I actually tried this myself. It only works for RTD's rated for -200C to 850C. I think that box on the lower left just tells you the min max ranges available for RTD's TC' etc. However, you would think even with this setting -200 to 850, an RTD rated for -30 to 260 should be outputting the same resistance as an RTD rated for -200C to 850C at 30C.

skyfox · Jun 21, 2021

danw said:
Setting the Range of an RTD transmitter

1. RTD element range

>We use OMEGA's -200 to +850C RTD's

I can't find an RTD with a 850Â°C top end, the usual high end for wire wound platinum elements is 600Â°C. This looks more like a spec for a thermocouple.

I should have been clear. We do use -200C to 850C RTD's for a different application. The reason I brought up this RTD is it reports the correct temperature with this very same signal conditioner with temperature within 0.4 Deg. C of what is reported by the Mass Flow Controller.

But I do not get accurate results with the -30C - 260C rated RTD with the Signal conditioner set to -25 to 255C and the 4-20mA signal scaled to this range value in the PLC. 30C is well above and below the min max range of the RTD. So therefore one should expect it to report the temperature correctly.

The RTD transmitter's output is 4-20mA. The transmitter's output range is scaled over some temperature range according to what makes sense for the application.

I have already done this. However, what I have not tried yet is narrowing the range even further and see if it would yield accurate results.

5. What output range makes sense in this application?

The perceived problem is that the transmitter's output range DIP switches does not have a -30 to 260Â°C range. Some other range has to be selected, and the PLC's analog input range changed to match the selected transmitter output range.

For me, range of 15C 40C would be more than enough for purpose of testing.

The issue I am trying to understand is why the RTD with a min max range of -30 to 260 does not report temperature correctly when signal conditioner is set to a range of -25 to +255C. Which is within the min max range of the RTD. The 4-20mA signal is also scaled in the PLC to this signal conditioner range setting and not the actual min max range of the RTD. RTD should be reporting the correct temp with these settings.

In post #1, the process temperature range is described as
>the process loop which can never be above 32 C or below 28 C.

During testing, ambient gaseous Nitrogen is flown thru the system. We neither heat nor cool the gas. However, customer does and we are not privy to what gases and what temperatures their heated or chilled to. This happens after this subassembly get installed inside their process equipment.

Also, in post #1, it states
>RTD - OMEGA 4 Wire -30 to 260 C PT100 RTD

The -30 to 260Â°C range suggests some recommended range.

What is the source of the -30 to 260Â°C range?
Is -30Â° to 260Â°C the receiver's analog input range?
Is -30Â° to 260Â°C a customer specification?

-30C to 260C is a customer spec'd RTD. We cannot change that.

It seems to me -30Â° to 260Â°C to be an overly wide range for a process that has to maintain a temperature of 28 to 32Â°C.

If I had to pick, I'd use 0-50Â°C because the process temperature range of 28 to 30Â°C falls near the middle of a 0-50Â° range, but I don't know all the process details.

Agreed. I would try narrowing down the range.

The suggestion that the RTD's sensor range of -200 to +850C is not appropriate because of the stated 28 to 32Â°C process range; a wide range such as -200 to +850C would result in low resolution at the process operating range.

-200C to 850C RTD was used for testing. Signal condition range set to this value and 4-20mA signal scaled accordingly, this RTD was reporting the temperature correctly.

danw · Jun 21, 2021

skyfox said:
The 4-20mA signal is also scaled in the PLC to this signal conditioner range setting and not the actual min max range of the RTD. RTD should be reporting the correct temp with these settings.

Where do these min/max ranges come from? Is this a thin-film construction temperature range limit stated by the mfg?

Regardless of where the limit comes from, the limits are just that - limits. One should be able to scale the transmitter over any portion of the RTD's range and get a valid, proportional 4-20mA signal.

So the PLC AI input scale/range is identical to the transmitter output range settings, right?

skyfox said:
-30C to 260C is a customer spec'd RTD. We cannot change that.

I thought that might be the case.

skyfox said:
-200C to 850C RTD was used for testing. Signal condition range set to this value and 4-20mA signal scaled accordingly, this RTD was reporting the temperature correctly.

Out of curiosity, where to do get/source RTD's that go up to 850Deg C?

Does this combination provide sufficient resolution and uncertainty over the working temperature range?

Uncertainty is percentage of span and -200 to 850 Deg C is quite a wide span.

cjd1965 · Jun 21, 2021

you should consider what your trying to measure (range min to max) because a wide span of 16mA means you lose resolution

Also do what i said and start measuring stuff. you can check the RTD resistance for example and decide if it is 'off' /not accurate

A few measurments that take 10mins will show you what is wrong

BryanG · Jun 22, 2021

danw suggested the solution way back in this thread. At the moment the Signal Conditioner is scaling the RTD to -25C to +250C, you just need to re-scale it in the PLC to -30C to +260C. Your PLC should have a Scaling Function, but if not then use this universal scaling maths:

Code:

Iv_In val        Input
Ish_In Hi        Input High value
Isl_In Lo        Input Low value
Osh_Out Hi       Output High value
Osl_Out Lo       Output Low value
Ov Output_Scaled Output scaled

Ov = [(Osh – Osl) * (Iv - Isl) / (Ish – Isl)] + Osl

skyfox · Jun 22, 2021

danw said:
Where do these min/max ranges come from? Is this a thin-film construction temperature range limit stated by the mfg?

Correct. -30 to +260C specified as the working range by OMEGA.

Regardless of where the limit comes from, the limits are just that - limits. One should be able to scale the transmitter over any portion of the RTD's range and get a valid, proportional 4-20mA signal.

Completely agree and precisely what I am trying to do. It is not reporting the correct temperature. However, if I set the range switches to -200C to 850C and use an RTD rated for that range, I get the correct temperature reading that correlates with temperature readings from the 3 MFC's. I have not tried setting a narrower range with this RTD and see if it would still report the correct temperature. I will get some time to play around with the system later this week.

So the PLC AI input scale/range is identical to the transmitter output range settings, right?

Correct. Signal conditioner is ranged from -25 to +255C and the 4-20mA input signal from the signal conditioner is scaled back to -25 to +255C in the PLC using a scaling function. Same scaling function that is used for all other analog inputs (pressure, flow etc.) in the system.

Out of curiosity, where to do get/source RTD's that go up to 850Deg C?

It is an OMEGA RTD. Omega Website Item# 1PT100K2515. (Class B Accuracy)

https://www.omega.com/en-us/search/?text=1PT100K2515

If you look at the attachment on my first post, OMEGA's RTD signal conditioner has this range as the largest range available for RTD's.

Does this combination provide sufficient resolution and uncertainty over the working temperature range?

Uncertainty is percentage of span and -200 to 850 Deg C is quite a wide span.

We are mostly utilizing it for temperatures in the range of -195C to +1C for a cryogenic application. Basically, these act as cheap level sensors for detecting liquid nitrogen levels in a vessel. High precision or Resolution is not a concern in this application. Someone picked these because Cryogenic level sensors were cost prohibitive. These RTD's do not use an external signal conditioner per say but rather attached directly to an RTD Input card that is part of the PLC rack.

skyfox · Jun 22, 2021

cjd1965 said:
you should consider what your trying to measure (range min to max) because a wide span of 16mA means you lose resolution

Also do what i said and start measuring stuff. you can check the RTD resistance for example and decide if it is 'off' /not accurate

A few measurements that take 10mins will show you what is wrong

That is my next step if narrowing down the range does not yield correct results.

skyfox · Jun 22, 2021

BryanG said:
danw suggested the solution way back in this thread. At the moment the Signal Conditioner is scaling the RTD to -25C to +250C, you just need to re-scale it in the PLC to -30C to +260C. Your PLC should have a Scaling Function, but if not

I am not sure scaling the PLC to -30 to +260C is the right way to go when the input signal of 4-20mA represent -25 to +255C. But other than that, yes I am using a scaling function and I am scaling it in the PLC -25 to +255C which is what the input 4-20mA supposed to represent. Actual temperature is around 30C but this range configuration is reporting around 21 Deg C. I will try measuring the actual resistance when I get back to this system later this week.

drbitboy · Jun 23, 2021

Another country heard from

Post #2 by @cjd1965 seems to me the right answer: measure the elements along the way the next time you are back on site.

There are several places in this thread where the measured (not rated) range is stated as [-25 to 255], and several other places where it is quoted as [-25 to 250]. The signal conditioner image in post #1 has an "End Temp." (upper limit) configuration for 250 but not for 255, so I assume all of the 255s in the thread are typos. Even if they were not, using an upper limit of 255 in the PLC would raise an actual temperature of 31.5 by just a bit over a 1°C, and post #1 says it is off by 2°C.

I am sure OP has made the calculations several times, correctly, but I will provide an octuple-check, with a silly number of significant digits:

31.5°C is at fraction 0.20545 (20%+) of the range [-25°C:250°C].
0.20545 * 16 = 3.287, so that would be 7.287mA in the [4mA:20mA] range.
We don't know the count range of the PLC input card, but 0.20545 = 841/4095 = 421/8191 = 210/16383 = 105/32767 = 53/65535*
- In any event this particular input card seems to be okay as it gave a correct reading with the wider-range-rated RTD with the same signal conditioner and conditioning range [-25°C:250°C].

Because of the test with the [-200°C:850°C]-rated RTD, the signal conditioner and PLC input card appear to be okay. So it is most likely that the problem is on the RTD side of the signal conditioner: i.e the RTD sensing element and or/the wiring provide the wrong resistance to the signal conditioner. An ice-in-water (not the other way around) bath would be the canonical test, requiring no polynomial evaluation to determine the correct resistance for another temperature.

* I am not sure if these should be 4095 vs. 4096, 65535 vs. 65536, but that is out in the fourth significant digit and therefore negligible for the problem at hand.

BryanG · Jun 23, 2021

I can't find what value you are actually getting, you said it should be 31.5 and you are reading about 2 degrees off. Just for fun I thought I would do as I suggested and re-scale your values with the standard scale formula I gave above.

Code:

[FONT=Fixedsys]Input High Value    250
Input Low Value    -25
Output High Value  260
Output Low Value  -30[/FONT]

Trying to get an output of 31.5 I have to use an input of 33.3, which is about 2 degrees off what you were expecting.
So what value are you actually getting, if it is about 33.3 I am looking like a real smarty, if it isn't, not so smart

. But either way you have to admire a tryer.

Help scale an RTD with a 4-20mA SIG Conditioner

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