Probe type low water cut off 1

[skbutler]

My experience is mainly in high pressure steam boilers but am doing more and more with low pressure hydronic boilers. I have a MDM probe type low water cut off the is giving me fits. The low water cut off is tripping every hour or so. I cleaned the probe (it wasn't very dirty) but the problem didn't go away so I replaced the McDonnell & Miller control and the problem is still there. I measured about 2 meg ohm, from the probe to the ground. It occurred to me that I didn't have nor could I find what the resistance should be with TDS in the normal range, ie, 2000 to 4000. Anyone have an idea what resistance a low voltage probe should see?
Thanks

 

[quark]

For water, roughly, Conductivity (in microSiemens/cm)x0.6 = TDS

This online calculator will be useful.

http://www.lenntech.com/unit-conversion-calculator/tds_engels.htm

Can you try calorimetric switches by IFM?

 

[skbutler]

Thanks quark... That was very helpful.


skbutler

 

[RossABQ]

By low pressure hydronic, do you mean 15 psi steam or a HW boiler?

If it is a LP steam boiler, dirty water could be causing frothing or unstable water levels, and the trip could be "legitimate".

 

[TBP]

If it's a steam boiler, check the feedpump. I've seen lots of small boilers with pumps that didn't have enough head to move sufficient water into the boiler at high loads.

 

[skbutler]

Let me expatiate: For those who are not familiar with probe type low water cut offs... it is a simple relay circuit where the relay coil current passes through a probe extended into the boiler water and through the boiler water itself. The energized coil closes normally open contacts and completes the boiler interlock circuit. If the water level falls and no longer makes contact with the probe, the coil circuit is interrupted and the coil de-energized.

Since the boiler water is part of the current path, its resistance is a factor. Obviously if the resistance of the water was high enough to limit the coil current to something lower than the coil's minimum pull in current, the coil will not pull in or drop out if already energized.

To my case...

The boiler is a 30 hp firetube low pressure steam boiler. There is nothing unusual about it. I have of course checked the usual suspects: corroded probe, loose electrical connections, fluctuating water level.

As part of my troubleshooting, I wanted to simulate the probe/water part of the circuit using a potentiometer. To do this I need to know what pot value I need to use; 100K pot, 50 ohm pot, 10Meg pot, what? I thought this would be an easy thing to find. I'd find some reference stating that the normal resistance for boiler water in a low pressure steam boiler with average water treatment chemicals and TDS (total dissolved solids) is in the range of 100 ohm per inch or 2K ohm per inch.... at least a ball park guess but nothing.

I am now communicating with McDonnell & Miller who makes the control I am using and who should have the exact information I need. In the mean time I greatly appreciate the folks who have responded. Thanks

 

[grantak]

I have had the same problem on a number of bolers and found that the soldered joint between the female spade connector and the stainless steel probe had become highly resistive. A simple solution is too remove the spade terminal and file the solder off the probe end back to the stainless. Then use a double screw connector from an earth BP connector to join the probe to the wire again. One screw onto the probe and one to secure the wire.