Sunday, 22 May 2011

Float Switches Don't Float My Boat

I've never been greatly impressed with the float switches that are used in conjunction with bilge pumps to give automatic operation.

Many older models use Mercury tilt switches inside them. These are usually a small tube with electrical contacts at one end of the tube. When the float tilts, the mercury collects at one end of the tube and creates a conductive path to complete the circuit.

Mechanically activated float switches, which are often advertised as Mercury-free, use a steel ball in an enclosed run. When the float switch is tilted by the water level rising, the ball runs to one end and operates a lever, which in turn activates a micro-switch and makes the electrical connection to switch your bilge pump on.

Anything mechanical will eventually give problems.

I am now using a solid state switch for my bilge pump switching and bilge alarm applications.

These operate on electrical capacitance and are VERY reliable. So reliable, they come with a 5-year warranty. An added advantage of these is that they are very small, don't actually float up and suffer from unwanted buoyancy problems (never a good feature on a water levels switch) and they don't get triggered by an oil leak (which would result in pumping oil into the cut).

I've used these for a while now and am very impressed with them.

So much so, that I've started using them on my Intelligent Bilge Alarm design (my Mk3 version).

I also sell my surplus ones (and the bilge alarm) on my family Ebay site.


  1. I never feel terribly confident with my float switch. I get very little water in the drip tray and as a result it rarely has to operate (if ever) so I always fear it has jammed with old oil and grease. Mind you I keep a very close eye on the stern gland and see the automatic pump and a second line of defence.

  2. You should be able to test it by lifting the float - the pump should operate. The solid state sensors don't wear or deteriorate like float switches.


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