Suggestions for H/L/LL level switches in a large tank? 1

[amazing azza]

Hello friends, I have a 30 m^3 (30 tons) polyethylene water tank. I need to sense water level inside said tank to turn on/off pumps that supply it. I am familiar with the various sensing technologies (tuning forks, ultrasonic, radar, etc) but I am running into limitations due to the tank size. There is about 5m height difference between full and empty, which is too long for most models.

What is the most practical method for sensing/controlling the fill level of a large tank like this?

 

[roydm]

I don't think a common ground would be the problem but I think you are saying the 220 VAC control signal is switched inside the transmitter, if that's the case I can see a problem.
Yes a 24 Volt coil will make a big difference, you should not have 220 VAC anywhere near the transmitter or cable to it, only Low Voltage DC, There is too much coupling between the AC and DC

 

[amazing azza]

Yes a 24 Volt coil will make a big difference, you should not have 220 VAC anywhere near the transmitter or cable to it, only Low Voltage DC, There is too much coupling between the AC and DC

Ah, I see. The internal relays are rated up to 220 VAC which led me to believe that 220vac control signal could be switched on them... So it is never a good idea to do this? How come the manufacturer does not specify DC-only for use with these relays?

 

[roydm]

Perhaps the manufacturer has never tried to commission one, sitting at a desk in a cozy office is one thing, they may have just copied the specs from the relay, it's hard for them to cover every situation.
The length of cable you have between the MCC and transmitter will also have some effect.

You could easily just disconnect the AC wiring jumper the pump and monitor the relay contacts with your Ohmmeter as a test but I'm fairly sure if you limit the transmitter/switch wiring to low current DC you will cure the problem.

Does the transmitter have a Start/Stop relay option e.g. close at 50% on falling open at 75% while rising, I tried figuring it out from the awful on-line manual but gave up. (I'm assuming the pump runs to fill the tank)

If you can use a solid state relay at the starter, these typically operate on a 3 - 30 VDC DC trigger signal and provide excellent isolation between different Voltages, otherwise a small low power DC coil relay.

 

[amazing azza]

Does the transmitter have a Start/Stop relay option e.g. close at 50% on falling open at 75% while rising, I tried figuring it out from the awful on-line manual but gave up. (I'm assuming the pump runs to fill the tank)

Indeed it does, you can use as pure high/low relays, or let the sensor keep track of that and issue a "give me water now" signal. I'm using both methods right now (one in each tank). Don't have any feedback yet on which is "better". The manual is quite awful as you mentioned and the useful information is actually in the manual for the software (rather than instrument), which somehow is provided separately.

Right now still dealing with the fall out from the theft (ordering replacements, repairing cable and panels) but will post here with the results of the switch to DC as soon as I can.

 

[amazing azza]

Hi guys, a little bit of an update.

Finally finished changing the ultrasonic sensor relays to use 24 VDC (from 220 VAC previously). Unfortunately, the phantom behavior still remains... There are a few other electrical gremelins too, which may be related.

I am going to start a new thread dedicated to control panel ghosts discussion, please join me there!