PT100 step 7 strang analog value

toont

Member
Join Date
Sep 2014
Location
Den Bosch
Posts
56
Dear all,

I'm trying to figure out the following situation:

A PT-100 temperature sensor is measuring water. When I measure the water temperature with a handheld sensor it measure about 40 degrees, the correct temp. as the water feels slightly above body temperature.
A PT-100 (4-20mA, 0-100 celcius) is giving analog value of 20000, which scales to about 70 degrees celcius.

The scaling and specifications indicate the analog value is correctly scaled, but the real temperature is much lower. What could be the problem?

I have looked at other threads (http://www.plctalk.net/qanda/showthread.php?t=79915) and documents which discuss wiring, but I don't know much about that. Is a real temp of 40 degrees and analog signal temp of 70 degrees indicating any wiring or other hardware problem?

Thanks for your help!
 
Maybe the PT100/4-20mA converter is 0-50ºC not 0-100ºC ??
Your scaling looks correct. 20mA=27768
 
If it's PT100 then it's not 4-20mA. Input value should be unscaled temperature. The input module has to be correctly setup i the hardware config and it has to be wired correctly.
 
PT 100 is the sensor element.

The device the PT 100 is wired to is called a temperature transmitter.

The transmitter's 4-20mA output can scaled/ranged/configured for any range, 0 to 50, 25 to 125, -100 to +100, 0 to 100, whatever, as long as the range meets a minimum span requirement.

The PLC's input scaling has to match the output range of the transmitter.

In your case, the scales/ranges do not match. It is probably easier to re-range your PLC than to change the configuration of the transmitter.

Remember that when 20mA = 32678 counts, then zero degrees at 4.0mA, is around 6535 counts.
 
Last edited:
PT 100 is the sensor element.

The device the PT 100 is wired to is called a temperature transmitter.

The transmitter's 4-20mA output can scaled/ranged/configured for any range, 0 to 50, 25 to 125, -100 to +100, 0 to 100, whatever, as long as the range meets a minimum span requirement.

The PLC's input scaling has to match the output range of the transmitter.

In your case, the scales/ranges do not match. It is probably easier to re-range your PLC than to change the configuration of the transmitter.

Remember that when 20mA = 32678 counts, then zero degrees at 4.0mA, is around 6535 counts.


Maybe I misunderstood. To me a PT100 sensor is just the sensor and nothing else.

On Siemens you can wire a PT100 sensor to a compatible analog input module. In that case you get the temperature directly (1/10th degrees).

If you say transmitter then yes, it's 4-20mA output (or fieldbus). And you to wire it to a compatible analog input and then you have to scale it.

I've seen people mix up the terminology and concept before and trying to wire a pt100 sensor to a 4-20mA input and vice versa. Which obviously doesn't work.

.
 
Last edited:
Maybe I misunderstood.
So do I, so don't apologize.

Toont's terminology does not make clear what he's doing. He uses both 4-20mA and PT100.

Is he direct connecting an RTD to an RTD input?
Or connecting an RTD to a transmitter?

There's no way to determine from the information he provides.
 
Hi! Thanks for you respones. Some more info:

The sensor is a TMR31, specs as on label 0-100 degrees C and 4-20mA. It's connected to PLC with M12 plug, pins 1 and 3 (10V and 0V)
In Step 7 hardware config it's setup as:

Measuring Type = Current
Measuring Range = 4-20mA
Scaled with Step 7 FC111: analog min=0 max= 27648, minOut 0 maxOut 100.

This is from an original program I'm editing, but I suspect the readings have never been correct.

I did some readings with a working infrared thermometer. The TMR31 with scaling 0-100 gives a temperature which is roughly 27 degrees to high. So scaling will be: -27 to 73 degrees C.

So this means it was a pure scaling problem? A range of 0-100C is preferred, that means the sensor needs to be configured properly. But why would the lable give a range of 0-100 while the actual range is -27 to +73?
 
Last edited:
OK, a search says that it is a:

Easytemp TMR31
Compact thermometer Pt100, Class A. Optionally with integrated 4 to 20 mA transmitter, programmable via PC.
http://www.endress.org.ua/pdf/TMR31.pdf

You could have one or several problems.
1) There is a problem with the measuring PT100 sensor (for instance covered in muck, not mounted deep enough etc).
2) You are not measuring the same thing or are using the wrong reflectance setting with the IR thermometer.
3) Something is broken in the TMR31.
3) You don't have the right analog input module.
4) It's not wired correctly.
5) Analog input is broken.
6) Temperature range on the transmitter has been changed with PC software.
7) You haven't configured the analog input module correctly.
8) You are not scaling the values correctly in step7.

Start troubleshooting from one end or the other.

You need an instrument that can generate a 4-20mA current loop to test the analog in. You need an instrument to measure the current loop from the transmitter.

You need to determine if you have a hardware or software problem.
.
 
Last edited:
Pete S and others, I really appreciate your help. Meanwhile some more info is gathered:

1)a TMR31 with identical label connected to the M12 plug of the original sensor gives the same readings. The comparison with the thermometer was done in a bucket of clean water, no muck etc.
2)My mistake: it's actually a dual infrared/foldback thermometer, I used the foldback part. http://shop.ebro.com/tlc-730.html?___store=en&___from_store=de
3.1) not likely as another TMR31 gave the same readings with existing wires, so:
3.2) analog input module is a Siemens SM331 AI 8x13bit, which should work.
4), 5), 6), 7) will investigate
8) likely. The adjusted scaling seems to work but is not ideal.

Again, thanks! Really helpful info.
 
The sensor is a TMR31, specs as on label 0-100 degrees C and 4-20mA. It's connected to PLC with M12 plug, pins 1 and 3 (10V and 0V)

In Step 7 hardware config it's setup as:

Measuring Type = Current
Measuring Range = 4-20mA
Scaled with Step 7 FC111: analog min=0 max= 27648, minOut 0 maxOut 100.

This is from an original program I'm editing, but I suspect the readings have never been correct.

I did some readings with a working infrared thermometer. The TMR31 with scaling 0-100 gives a temperature which is roughly 27 degrees to high. So scaling will be: -27 to 73 degrees C.

So this means it was a pure scaling problem? A range of 0-100C is preferred, that means the sensor needs to be configured properly. But why would the lable give a range of 0-100 while the actual range is -27 to +73?

This makes no sense to me at all.

1) is the the transmitter a 2 wire transmitter (DC power on the loop wires)? All 2 wire transmitters are 4-20mA.

3 or 4 wire transmitters with separate power supply connections can be 0-20mA output.

2) How can any device with "live zero" output (4.0mA is a 'live zero") have an "analog min 0 ?"

3) what is the relationship of analog input pin 1 being 10V and Pin 3 being 0 V to a field device output that is (presumably) 4-20mA?

Does the AI have a built-in dropping resistor for current? Is the dropping resistor 500 ohms which creates a 2 to 10Vdc signal for a 4 to 20mA input?

If this were my stuff, the first thing I'd do is get any milliammeter and a DC power supply and check the output of the transmitter with
- a glass of ice water (near zero C)
- ambient (whatever the wall thermostat says it is)
- and a cup of coffee or boiled water (close to whatever temp your IR says it is)

to see if the ice water 4-20mA value is close to 4mA and whether the ambient and coffee mA values are close to the temperatures they should be. That will confirm that the transmitter is in fact scaled 0-100C over 4-20mA.

Then I'd use the 4-20mA soure/calibrator that someone mentioned and wire it into the PLC input, scale the PLC 4.00 to 20.00 and see what the indicated results are. If the calibrator value is not very close to the indicated PLC reading, then figure out what's wrong with the scaling factors or configuration.

Dan
 
The "normal" block supplied by siemens is FC105, example call below:

Code:
      CALL  FC   105
       IN     :=PIW256
       HI_LIM :=1.000000e+002
       LO_LIM :=0.000000e+000
       BIPOLAR:=FALSE
       RET_VAL:=MW10
       OUT    :=#rTemperatureDegC
Your FC111 does not sound like a copy of this block given your terminology for the block I/O. Copy your FC111 to a library and post the archived library.
 
The FC111 I use is a protected block, but the scaling works. It's just the standard FC105 with added alarm functions.

danw, in a few days the analog input signal of the analog card will be measured with the source/calibrator, I think that will clear up a lot of things. The sensor (with 2-wires) is connected in a non-standard way (it gets power from an external power supply, but sends signal to analog input card of PLC, so its power source is not the analog card).

1) it's a 2-wire 4-20mA
2) my mistake, the analog minimum value is lower then zero, but zero is the lower limit below which the (non-standard but working correctly) FC111 activates an alarm.
3) I used the names the pins were given in the TMR31 datasheet.
Whether AI has a dropping resistor and what kind will be checked in a few days.

Tests were done with hot and cold water. With hot water 90 degrees the analog value goes above the normal Siemens upper limit of 27648. The mA calibrator has to clear up this issue first before getting proper scaling factors. This is most likely caused by the non-standard wiring or other wrong wire connections.

Before the wiring issue is cleared up not much else can be done. Thanks all for helping, I'll post an update when the problem is solved :)
 
Make sure that the analog input module channel group in the hardware configuration is set to 4-wire current NOT 2-wire current.

2 wire current setting means that the analog module supplies 24VDC to the sensor. 4 wire current means it does not supply 24VDC.

some analog input modules in the S7-300/400 have jumper blocks that must be set to correspond with the hardware configuration!
 
Last edited:
check type: TMR31-A1AA....(0-100)
put a milliamp meter in between, reading should be 0.25*16+4= 8 mA at room temperature. the power supply must be >15 Volt.
now what type of input you are using? (type of card)
how is connection?
FC111 is only a thermostat.

I am not a siemens man, better for me is codesys, where all these functions have proper names.
do you know oscat.de they have a nice library with lots of functions also for S7
You may contact me as i am close to you (0736139136)
 

Similar Topics

Have you seen a PT100 sensor reading going slowly up, even after heating power turned off ?! and it keeps rising for several minutes ... It looks...
Replies
11
Views
4,433
Im having a 3wire pt100 connected to a temperature transmiter. Im reading the current in the loop and I have 22mA, so is overrange. What could...
Replies
6
Views
1,651
Hi. Almost every time when working with PT100 probes the customers ask me "have the probes been calibrated?" The reason for the customers to ask...
Replies
18
Views
4,985
Hello, can someone help me with scaling PT100 RTD useing Schnider m211 PLC? I can see the analoog value on the controller but once the temperature...
Replies
16
Views
4,264
Hi all, Not sure how to call this, but my client wants a module that offers a dry contact, if one of three pump/motor bearing temperatures...
Replies
15
Views
2,959
Back
Top Bottom