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ContinuousWave: Small Boat Electrical
Bilge Pump Float Switch
|Author||Topic: Bilge Pump Float Switch|
posted 08-09-2013 11:07 AM ET (US)
My very old switch failed. I purchased the closest match Rule switch in stock at my local West Marine store and installed it.
I initially suspected the switch in-line fuse. When opening the fuse holder the tubular end terminals were stuck to the holder so the fuse came apart. Then I needed a fuse holder, too. The old float switch wiring included a fuse holder so I decided to buy a new switch. The new switch didn't include a fuse holder but the mounting pattern looked the same as the old switch so I bought a switch, blade type fuse holder and fuses. That was a good decision, because when I tested the old switch it was inoperable. At least my bilge pump was only a couple of years old, so now the system is virtually all new.
Installation was straight forward with only a couple of problems. First, the mounting holes for the old switch were not perfectly aligned with the new one. The were close enough to use but slightly closer together. I didn't want to drill another hole in the deck so used the old holes by carefully removing a little material from the new switch base. Second, none of the wiring was of the same gauge and the Ancor butt connector selection a West didn't include any with different sizes at each end. I dealt with this by using connectors to fit the largest wires (the fuse holder) and doubled the ends of the other wires. It was not my preferred option but an acceptable field expedient method.
A problem with the way my pump is set up in the Sport 13 is the pump sump. There is room for a small pump but not a pump and a float switch. Consequently when I bought the 13 it was set up with a pump in the sump and a switch adjacent to the sump. The switch requires a rise of two inches to come "on" and shuts "off" at three quarters of an inch. This is not a bad thing when the 13 is in a slip but is not wonderful when using the boat as water sloshing around aft is not convenient. From the rear thwart this not a big deal because I can reach back and manually raise the float and hold it up until there is only water in the sump. To eliminate the problem I'm going to experiment with using a small nylon wire tie to attach a small fishing float to the end of the switch float. I could have installed a pump with a built-in switch but didn't want to risk a depleted battery where I have no shore power.
posted 08-09-2013 01:00 PM ET (US)
Your method of modifying the new float switch mounting base to fit to the existing holes in the boat hull was a good decision. The fewer fastener holes in the Unibond hull of a Boston Whaler, the better the hull integrity.
When controlling a sump pump with a float switch, you have to take precautions that the pump can actually lower the water level below the OFF threshold of the float switch. It is possible to arrange the float switch in a position that allows it to come on when water appears in the sump, but if the pump cannot evacuate the water in the sump to a level below the float switch's OFF position, the control system will never be able to shut off the pump. This results in a control system that can only turn the pump ON but cannot shut it off.
posted 08-10-2013 10:42 AM ET (US)
Thanks for the comments. I'll be wary while trying to "tune in" my anticipated modification to the switch lever.
posted 08-12-2013 12:16 PM ET (US)
In a well-designed control system there will be hysteresis in the control circuit. That is a fancy way to say that the threshold used to make the ON decision will be different than the threshold used to make the OFF decision. If the same threshold is used for both ON and OFF functions, the control system can become prone to oscillation between ON and OFF states. This is particularly true in a sump pump because of the tendency for water to flow back into the sump once the pump is shut off. For this reason, the control system should have hysteresis.
In float switches there is usually a weighted element in the switch which moves to a different position when the float is being buoyed up compared to when the float is not buoyant. This shift in the weigh distribution affects the buoyancy of the float. Typically, when the sump is dry and the float is at rest, the weight shifts outward from the pivot, making more buoyant force necessary to raise the float. Once the float becomes buoyant, the weight shifts toward the pivot. This makes the float more buoyant than it was at rest. This means the pump must lower the water level below the ON threshold level in order to reach the OFF threshold. Or, the pump stays OFF until the water rises far enough to trigger it on, but the pump stays ON until the water is lowered below the initial level. Because of this it seems like there will always be some water in the sump the pump either has not tried to remove yet (because it is below the ON threshold) or that the pump cannot remove (because its OFF threshold is higher than this level to insure it can shut off).
posted 08-13-2013 08:22 AM ET (US)
Thanks for the additional information.
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