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Hi everybody,

We have a sewage tank and we need some level switches as a back up for hydrostatic level transmitter which generates alarms and start/stop commands to pumps, valves, etc. The question is what type of level switch is proper for this application. Some facts regarding this issue are as follows:

1. In some cases the tanks are agitated (no calm surface)
2. The media is sludge and other thick and slurry stuff.
3. We need multi point switches (up to 5 points)
4. The level range is around 10m and less.

To me, float switch is not proper for this application at all. I know capacitance, thermal and vibrating level switches are good. I am also aware of bubbling type but it cannot be used as we have no compressed air available. I know some optical level switches are also used in for sludge but I doubt they are trouble free! I try to avoid ultrasonic as well, since the tanks are agitated.
I want to avoid the complexity of nuclear, radar and microwave type as well.

Personally I think capacitance level switches are best for such an application.

I would appreciate expert comments about this issue concerning the technical and commercial aspects.

Beat Regards,
Hamid

Hamid,

I think your comments regarding capacitance make sense. You will have some build-up on the probes that will need to be addressed through routine maintenance.

I do question the idea of backing up a level transmitter with five independent level switches. It seems a more reliable and trouble free approach would be to perform the back-up with an additional transmitter that utilizes a different technology. If you are comfortable with capacitance, why not use a transmitter instead of five switches? You mention concerns with the complexity of radar. I think the complexity issues with the technology have largely been resolved. The particular application described would be well suited to thru-air or guided wave radar transmitters. To avoid the maintenance, thru-air may make the most sense. Most vendors should be able to demonstrate the technology on your application to give you a good idea of how it would work and how complex it is to set up in your application.

Hope this helps.

Bill Hendrick
Magnetrol International, Incorporated
www.magnetrol.com

Hamid,
I worked in mining for many years. Ultrasonic meters work well on agitated tanks provided there is not much foam. Have you considered a submersible pressure transmitter eg Druck, you would probably need to mount it at the bottom of a pipe for support and easy removal. I agree that capacitance will work if the agitation is not too violent (bend the probe). I agree also float switches are a bad choice you will end up with a tangled mess.

Conductivity probes can work well in some cases i.e. one probe for each level point but again they can get tangled in an agitated tank.
What about a second DP cell either flange mount or regular connected to the bottom with a water flush?

Good Luck
Roy