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from the brewing department...
level transmitter calibration
Continuous process industries, DCS questions. topic
Posted by MSE on 3 November, 2010 - 4:08 am
hi all,

we have a water tank with level transmitter DELTAPILOT_S - DB50L {ENDRESS + HAUSER} containing FEB 22 insert and FHB 20 display. it is connected to PLC and SCADA. we recently replaced the transmitter with a new one and also changed the address on PLC as its slot got damaged.
the problem is that we calibrate it as per operation manual and it gives correct reading when empty and when full giving 4 & 20mA respectively. but when empty its gives an error in SCADA indicating the current is below 4mA or -9% level, while the transmitter is giving 4mA signal. when the tank reaches certain level the error goes away.
when the tank is full the trasnmitter reads correctly and gives 20mA, but in the SCADA it reaches 50%level.

what could be the reason for this? kindly assist. thanks


Posted by Sam on 3 November, 2010 - 9:50 am
Hi there,

I can try to help but since I cannot be sure what have be done where, it is not possible to give you exact answers to your problems. I will tell you how I would tackle a problem like this and you can then follow those steps to find the cause or causes.

Setting the level up in the PLC and SCADA could be the problem if you have not set the new address up in the PLC exactly as before(the faulty one), so check that first. In a case like this, look at anything you have change and see if it is correct and the same as before, before looking at new possibilities.

There is a difference in setting up a level to setting a temp , flow or pressure in the software since level is only concerned with getting a 4 to 20 mA in and it will use that 4 to 20 mA signal to represent a 0 to 100% level indication in the PLC and SCADA. With pressure and temp you have to set values like 4 to 20 ma in = 0 to 100 Degrees or 4 to 20mA in = 0 to 10Bar and so on. Level have no reference and will use what ever 4 to 20mA is a 0 to 100%. Check this in the PLC and SCADA or use a different application to see how it is done there and see if yours is set to the same.

Even if your calibration is done completely wrong on the transmitter that will still not give you any of the errors as you have now, so the problem is not on the transmitter side but in the PLC or SCADA setup. Again the transmitter only send a 4 to 20mA signal representing the 0 to 100% level. The PLC have to assume that that 4 to 20 is the correct level and display a corresponding 0 to 100% level indication, so does the SCADA. They have no reference other than the 4 to 20 mA.

Also keep in mind that if the transmitter goes below zero it might still give a zero indication on the transmitter display but the output to the PLC might be 3,6mA, so measure that and don't believe the transmitter display. It is sometimes setup like this to avoid flashing of the display when below zero, and questions from production.
If the signal goes to 3,6mA the PLC and SCADA will read it as the -9% on the SCADA display. Have a look if there is a feature to limit the display on the SCADA to zero and not below zero or have a look at another application to see how it was done there.

The reading of 50% when the transmitter reads full scale is definitely a scaling problem in the PLC or SCADA. Do some testing to determine where.

Break the job up in three phases.
1)
Get the transmitter right first and check that the output is 4 to 20 out for 0 to 100% level.

2)
Simulate a input to the new slot in the PLC of 4 to 20 and see if the PLC reads the 0 to 100% level correctly.

3)
If PLC readings all ok, keep simulating at the PLC and have a look if the SCADA reads the level as 0 to 100%.

a Bit of legwork to do but you will find the problem soon enough.


Posted by Carl on 3 November, 2010 - 10:36 am
Assuming SCADA is reading the correct point (the analog input I/O point has changed from one slot to another), then SCADA must be scaled wrong for this point, since your level device correctly reads 4mA at empty and 20mA at full.

How does the PLC 'view' the level? Is its interpretation of the level signal correct?

Presumably SCADA is fetching the level from the PLC. Is SCADA reading PLC A/D counts or values in eng units?


Posted by Ultima Weapon on 3 November, 2010 - 2:50 pm
Hi,

if you performed the calibration as per manual as you said, then did your plc has been also re-adjust the min & max value (URV,LRV)? I assume the Analog Input are used by PID block as Process Variable. you better check the PID block. I also have usually seen that if the LRV (4ma) under 0% value at HMI and the URV (20ma) are approx 50%, there's something at the installation and wiring that cause it. you better double check it. maybe other person here can give some more good advice.

thank you.

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