Stuck float switch overdoses tank

[johnfr]

For all you DYI'ers out there.

Here is a lesson to learn from me. I have a peristaltic pump that I use to add lime water topoff. To activate the pump I have a float switch in my sump to turn on my pump when the level drops below a given level. This switch energizes a 12 volt DC relay that turns on the pump's 110V source.

As an experienced Electrical Engineer I should have remembered that when a relay de-energizes it generates a large voltage spike that can easily weld the contacts of small reed type switches closed. Last night I came home to find my sump and refugium on the verge of overflow and my limewater resevour nearly empty.

The solution is that one should always connect a diode across the relay coil (connect to the banded end of the diode to the relay coil that is connected to the positive voltage. The unbanded end connects to the other side of the relay coil.) This diode suppresses the voltage spike and prevents welding of the switch contacts. I used a 1N2003 diode.

Randy, a question for you.

When this happened I added 3 cups of white vinegar to reduce the PH ASAP. Previous to this my Cal was running around 420 and Alk around 2.45. Tonight (24 hrs after the overdose) I tested the water and found the Cal at 390 and Alk at 2.3. I estimate that the welded switch added about 15 gal of pure saturated kalk water. My overall system contains about 220 gallons of water. I would have expected this to significantly raise the cal & ak levels but instead they dropped. Does the rise in PH as Kalk is overdosed cause calcium to precipitate out and therefore cause an overall reduction in Cal & Alk levels?

Thanks,

Johnfr

[johnfr]

Randy, one other detail I failed to mention. Tonight the PH was right at 8.2 where it normally runs.

Johnfr

[Randy Holmes-Farley]

Thanks for the info, John. :)

In my case, I use a float switch that can handle 15 amps directly, so I don't use a relay.

I would have expected this to significantly raise the cal & ak levels but instead they dropped. Does the rise in PH as Kalk is overdosed cause calcium to precipitate out and therefore cause an overall reduction in Cal & Alk levels?


Did the water turn white like milk? That's the usual result when overdosing limewater, as calcium carbonate precipitates from solution. That precipitation actually drops all of pH, calcium, and alkalinity.

15 gallons of saturated limewater would raise 200 gallons by 60 ppm calcium and 3 meq/L alkalinity. It is the pH rise that is the true problem.

With the pH at 8.2, the only thing left to worry about is possible low O2 from bacteria consuming the vinegar. So just keep the aeration up. :)

[goodfortune]

Johnfr, that has happened to me, twice :eek:. Kalk dumps are truly no fun. However, almost everything, with the exception of very sensitve livestock survived on both occasions.

Dr. Holmes-Farley, what type of float switch are you using, that is able to handle standard household voltage? If you don't feel comfortable posrting brand(s), you can PM me.

Thanks so much for your help so far :).

- Michael

[johnfr]

Thanks Randy

We arrived back home this afternoon and all seems to be well. My system has strong aeration so the bacteria consuming the vinegar should have had plenty of O2.

Regarding the float switch, the one I have is also rated for 10- 15 amps. When controlling inductive loads like motors it is always a good idea to provide some spark suppression for the switch. Float switches use reed type switchs which are more prone to contact welding when arcs occur.

Regardless of whether you are switching a DC load as I am, or an AC load, when the switch opens it can create a arc that can potentially weld the switch contacts closed. If you are controlling a pump directly with AC I would recommend placing a .1 ufd capacitor across the switch contacts to suppress any arc that might occur. It would probably be wise of me to place a capacitor across the relay contacts that switch the AC voltage on my system.

A 2 cent diode or capacitor is cheap insurance to avoid the type of disaster I nearly had.

johnfr

[Randy Holmes-Farley]

Dr. Holmes-Farley, what type of float switch are you using, that is able to handle standard household voltage?

Here it is. It is the Omega Engineering LV1201

http://www.omega.com/ppt/pptsc.asp?ref=LV1100_1200&Nav=grek12


bottom in picture:

http://www.omega.com/Green/images/LV1100_1200_l.jpg

specs:

http://www.omega.com/Green/pdf/LV1100_1200.pdf

It says it is designed to directly control pumps, so I didn't figure I needed to do anything else. Thanks,John, for the additional info!