Capacitive Level Transmitter

1. The manufacturer (which you don't state), is the best source of information.

2. Most likely this will create a problem. You are adding capacitance, inductance, and resistance in between the primary element and the transmitter.

3. If the transmitter is capable of sending a signal (0-10V, 4-20ma) to a PLC/DCS, then it would be better to tap into that line for the external display.

There is no way to tell without more details. It all depends on the interface between the instrument head and the capacitance sensor.

The interface is very simple. One wire each from the inner rod and outer pipe (electrodes of the capacitor) terminate on TB.

The executing group just want to extend these two wires by 20 meters.

My job is to give technical feedback about feasability of their "invention".

We have an old saying around here, "One test is worth a thousand expert opinions."

Are you sure that 20m of cable has zero capacitance, Mildred?

In that case, I would say it is unlikely to work as intended. In addition to adding capacitance (and inductance) of the cable to the capacitance of the sensor, you also have a good chance of picking up noise.

But I expect you will have to arrange an experiment to prove this point.

"My job is to give technical feedback".

You have the wrong job.

http://www.omega.com/Green/pdf/CAP_LEV_MEAS.pdf

It will probably induce an error but obviously you don't care at all about errors. If you did care about errors you'd tell us a lot more about this, like what is the capacitance when empty and full, what is the tolerance of the meter and lastly how critical is it to know precisely the level. A "full, half, empty" gauge will probably not care at all, but gauge with ±0.1% precision certainly will.

If the electronics head were disassembled from a high-end Siemens LC 500 and reassembled with intermediate cable, it would not function properly, but remote mounting can be done, like this model:

http://www.rawsonlp.com/brands/robertshaw/model-158a.html

It might or might not work. Try it and see. There's probably some adjustment tolerance that can adapt to intermediate cable.

There's also the question of how the connections are protected on the tank end where the head has been removed. Duct tape?

Anytime you disturb a component in a capacitance level system the system has to be calibrated. Do you have someone on your staff that knows how to do the calibration? If not, you need to contact the manufacture of the equipment. I could walk you through it but I won' t for the lack of information you've posted here and I sure wouldn't want to read about your failed attempt in HazardEX

If I visualize your setup, 20 ft will add about 600 pF (typical cable) to your sensor and result in a large offset you probably won't be able to calibrate out. A low-capacitance cable might help.

Try it, but I don't have a good feeling.

The user manual says nothing about this modification.

Don't you think there is a reason for this "omission"? They don't want you to do that.

That said, if and only if I could afford to replace the unit, and if and most definitely only if the process controlled is not hazardous to life, I'd give it a try and observe the error induced and see if a correction chart can be calculated. Of course, I'm also the kind of engineer who will disassemble anything, not always with success, so maybe you don't want to follow my advice.