Ahhh, a topic on which I can contribute with some authority. I know bilge pumps
and their switches well. Why you ask? Well....
"Hello. My name is MedSailor and I was a wooden boat owner. I have now been 224 days without owning a wooden boat"
If you want to skip the following treatise, the short answer to your question is that they are FANTASTIC and REVOLUTIONARY and I can not recommend them highly enough.
So my poor 31ft lapstrake converted lifeboat longed to live at the bottom of the sea. Many MANY times it tried to go there and each time what stopped it from doing so was one of my bilge pumps
. I always had 3, each with its own switch, each with separate wiring, 2 to the house bank (one wired direct one not) and one wired direct to the starting battery.
In my 6 years of owning her I went through about a dozen bilge pump
switches and several pumps
. Since the bilge was always wet and nasty, the switches were always immersed (even if just partially) and failures were seen early.
All of the mechanical float style switches I used had problems. Everything from wire chafe as they move up and down, to cycling when going through a rolling or pitching sea (depending on orientation) to having bilge flotsom or wires mechanically obstruct the lever arm of the switch.
Also ALL of the mechanical switches at some time or other would be found in the up position, but would not have activated the pump. If you pressed the lever down and it floated back up, "presto" they pumped. Never figured out what that happened but it happened to all of them at some point or other.
The only mechanical switches worth their salt were the ones with a metal ball on a track that rolls INSIDE the arm of the switch. Rule makes these and others might also. The arm is heavier (though still floats of course) and is much less prone to cycling in a pitching sea. They seemed to last much longer and not have nearly as many problems at the mercury switches. They still had issues though and were also prone to mechanical obstruction (though not as much). They can be found at the store by listening as you tilt the switch and you'll hear a marble sized metal ball roll down the arm of the switch and go "clunk".
Now for the solid state ones. I would always eye these with suspicion (better the devil you know) but would invariably try them out of frustration. There is one (can't remember who makes it) which was white and had a clear tube on the bottom that claimed to optically "see" water. It also had no test swtich. What a piece of crap. I never got it to work once.
The johnson switch on the other hand can be tested by touching your fingers to both sensors on the switch. You can then remove your finger from the top one and see if it keeps pumping, which it should. This switch has worked flawlessly for me for the 1.5 years I've had it on a SERIOUSLY sinking wooden boat. Also, because of its low profile it can essentially be mounted in a very low spot in the bilge. Just be careful not to mount it lower than the pump can pump out as it will cause the pump to run forever. This just takes some experimentation.
The johnson switch also doesn't cycle in a pitching sea as it has to detect water for one full second before beginning to pump. So water sloshing by it will not activate it.
The only claim of the product which was not met was that after the bottom sensor is dry it is supposed to keep the pump on for a further 20 seconds or so in an attempt to dry the bilge. It would be a great feature but mine did not do this it simply turned off when the bottom switch was dry.
So this is one of the very few products I have ever come across that I can absolutely recommend without reservation.
PS When it comes to bilge pumps
remember "One is none. Two is one, and three is two. You should have two bilge pumps