Temperature Transmitter Configurations

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Recently, we replaced our old digital temperature transmitter with a Baumer Flextop 2201 digital temp transmitter. The application of the digital temp transmitter is for a hydraulic press and the output is fed to the Siemens S95-U plc. The Flextop was able to react properly promptly when there is increase in temperature but whenever there are sudden drops in temperature it is slow to react. The analog transmitter (dial) was already at 25 C when the hydraulic press was shut down while the Flextop was still at 35 C. We used a three wire configuration for this. Does this have to do with our wire configuration? I know there are 2 wire ,3 wire, and 4 wire configurations for 4-20 ma sensors but I'm not sure what are their differences among them. Please see attached configurations below.




Flextop Configs.PNG

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What you are showing is 2, 3, and 4 wire configuration for a Pt100 RTD temperature sensor. This gives a millivolt signal that needs either a converter to change the signal to 4-20mA, or a special input card on the PLC. I haven't dug into the different wiring configurations on RTDs, as I mostly work with thermocouples.

There are also 2, 3, and 4 wire configurations of 4-20mA devices also. In this case, 2-wire means that the devices receives it's power from the loop. You supply 24V on one wire and receive the current signal on the other. Unless your analog input card does it for you, you need to inject 24V into the loop. Newer Siemens cards have this capability. With 3-wire devices, you supply power separately to the device. One wire for supply power, one for common, one for signal. 4-wire is the same, except that the supply and signal commons are separated. In my experience, 4-wire are much more common than 3-wire.

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1.  RTD types

    2 wire RTD's have a constant offset error due to lead wire resistance.

    3 wire RTD's are probably 97% of the industrial marketplace with sufficient accuracy for industrial use.

    4 wire RTD's are for critical accuracy applications like BTU calcs when accurate measurement for invoicing/billing is involved or for laboratory use.  Most transmitters all

2.  Temperature transmitter

I cannot find a Baumer model 2201 in a quick web search.  The 2202 is a 2 wire loop powered, blind (non-indicating) RTD temperature transmitter (sometimes called a hockey puck in North America).

It appears to be a modern competitive unit that is highly likely to work out-of-thebox when wired and configured properly.  My experience says that the probability of a fault or error is very low.   But, there are several things that can make you think the transmitter is in error.

If you wired the 3 wires as shown and you get a 4-20mA signal, then the transmitter probably working OK.

The 4-20mA output represents a specific temperature range.  The default 2202 output range is 0-120 Deg C (32 - 248 Deg F).

The receiver (PLC/controller/indicator analog input) that takes the 2202 4-20mA output must be ranged to the same range (and units) as the 4-20mA represents.

Is your transmitter also ranged 0-120 DegC (factory default) or some other range?  

What is transmitter's 4-20mA connected to?   Did you configure that for 0-120 Deg C?

Mismatch between the temperature represented by the transmitter output by the receiver is the most likely source of error.

3.  Error of 10 Deg C

Dial indicator shows 25C, Flextop shows 35C, an error of 10 Deg C

a)  The dial temperature is probably a bi-metallic thermometer?  

How do you know it is giving an accurate value?  You're using it as a reference.  Why can you trust it?  

Many bimets have an offset (screw) adjustment on the back that could have been bumped.  If the probe has been bent, a bimet will indicate a false temperature due to the distortion of the mechanical linkage from the spring to the dial.   In other words, just because the dial shows some temperature, how to do you know it's valid?

b)  Is the dial measuring the same physical point as the RTD?  

Temperature is not the same everywhere in a process.   Even in an office room, there are temperature gradients in elevation (floor is cooler than the ceiling; door gaps allow outside air at different temperature).  How close are the probes to one another and what exactly does each 'see'?

c)   What influences the heat transfer at that point?

Both devices have some thermal lag.  But lags are on the order of fractions of a minute.

On a bi-met thermometer, heat transfer has to penetrate the probe sheath, saturate the entire length of the bimet 'spring' to drive the dial.   
On an RTD, the heat transfer has to penetrate the probe sheath and the MI insulation powder used to electrically isolate the RTD element from the sheath.  

But the surrounding area that heats or cools the probe can have a dramatic effect on response.  Insertion into 100Kg of steel that has its own slow thermal response is different than insertion into a tank with agitated liquid (about the fastest response).

4.  slow to react.

The 2202 transmitter has adjustable damping, meaning the values are filtered or averaged over time to reduce a fluctuations that produce a jumpy/noisy output.

The damping time base is 0 (no filter) to 30 seconds (heavy, heavy filtering).   The spec sheet does not say what the default value is.   A highly damped signal will take some time for input (RTD temperature) changes to be seen at the output (4-20mA).

Edited by DanW
haven't had my coffee yet

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Hi JRoss and DanW,

Firstly, thanks for the quick and comprehensive response. Actually, I do not have the configuration device for the Flextop so I cannot check what the default configuration is. I thought replacing the old temperature transmitter with a new one will immediately solve the problem but apparently the Flextop still has to be configured. What I did was adjusted the output value in the PLC so that it will match the readings of the dial indicator. I can confirm the Flextop partially works because it can react well when there is increase in temperature and will  but when temperature drops, it is very slow to change. Maybe it has to do with the damping value as DanW mentioned.

DanW, you mentioned that the dial indicator is not an accurate way to validate the readings so can you suggest a more accurate way to test the validity of the readings of the Flextop? It is measuring incoming steam temperature by the way.



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There's nothing inherently wrong with bimet thermometers - they need no power and process grade models are good to plus/minus 1% full scale. 

The problem is relying on the value it shows without knowing how good that value is.   Even if it was another instrument with a sensor like an RTD or thermocouple my question would still be, how do you know the reading is accurate (enough for your purposes)?

Industries where accuracy is important (pharmaceuticals, for instance) have spares of sensors/instruments like RTD's or bimets that are periodically checked for calibration to see how much error is indicated.  Those that 'pass' are exchanged for ones in service, so that there's a degree of confidence that the reading is good.


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