DIY liquid sensor (bilge pump switch) circuit.

[rob2]

I seem to recall a piece in PBO about a liquid sensor circuit, which received a "correction" from another reader the next month to make the circuit electrically fail-safe. I've spent most of a day going through copies, but have obviously missed it. Can anyone advise me of the relevant issues, please?

Since a rather neat unit from Rapid Electronics with gold plated contacts is available for £3.86, I am rather tempted to use this type. http://www.rapidonline.com/Electron...w/78-2090?source=googleps&utm_source=googleps

Rob.

 

[ghostlymoron]

The Rapid unit sounds like a good idea. Making up a suitable DIY version would probably cost more and it's always difficult to produce a housing that looks OK.
Having checked the Spec for the Rapid unit (see below) just make sure that the switching capacity is adequate for your pump. 10w seems a bit low.

Contact form Form A normally open
Switching voltage 100V DC max.
Switching current 0.5A max.
Switching capacity 10W max.
Contact resistance 0.5Ω
Operating temperature –5°C to +80°C
Case material White plastic
Liquids Non corrosive. Not heavy oils
Cable Twin core insulated. 1.8m

Afterthoughts:
1. Salty John has a floatless switch for bilge pumps - don't know how much though
2. Anything is better than the Rule float switch

 

[rob2]

Yes, the capacity is rather low for a direct switch. It was the switch circuit I was after. Being a compete ignoramous in circuit design, I was hoping to find something to copy. I was thinking of banking three sets of contacts to give off/on/alarm, using a relay for the power feed to the pump.

Rob.

 

[ghostlymoron]

1. Why would you need a signal for 'off'?
2. Relays are adding to the cost and need to be 'marine quality'
3. You might be better to source a higher rated switch

 

[rob2]

Off? Well, I was thinking hysteresis. Middle contacts switch pump on, lowest contacts switch it off and the top pair tell you to man the bilge pump - the electric one isn't coping!

Rob.

 

[ghostlymoron]

Hysteresis. Thereoretically you wouldn't want the pump cutting in/out too often but with a reasonably sized bilge I don't think a seperate contact for off would be necessary.

 

[NB Willawaw]

Unless you really like spending time making DIY circuits, why bother.

I fit electronics on boats and just use a readymade solid state sensor and microprocessor which costs less than £30 and will outperform a float switch.

 

[NickRobinson]

or-

washing machine pressure switch. Remotely located above water level with small pipe to bilge terminated in a plenum chamber (inverted plastic bottle on Magna)

Good switch on/off differential, adjustable if required. Reliability? Put more than one in parallel if you feel the need. I twinned mine to a 'marine' float switch which failed in less than a season! No relays used on my setup.

Nick

 

[stuhaynes]

and also

washing machine pressure switch. Remotely located above water level with small pipe to bilge terminated in a plenum chamber (inverted plastic bottle on Magna)

Good switch on/off differential, adjustable if required. Reliability? Put more than one in parallel if you feel the need. I twinned mine to a 'marine' float switch which failed in less than a season! No relays used on my setup.

Nick

We got fed up with our float switch clogging up and sticking. Changed it for a 20(?) amp solid state switch, very clever... Two years later, absolutely no problem. 3 year guarantee too. I think it cost under £20

 

[davidpbo]

I bought one of these but have never used it as we don't have a bilge as such. I was going to replace the sounder with a relay or transistor switch and relay if the relay pulled too much current.

 

[rob2]

Thanks for the replies. I was looking at several different types and the washing machine pressure switch would work well on my boat as I have a sump above the encapsulated fin keel. The reason I was also interested in the resistance type or the reed switch type is that A colleague with the lifting keel version nearly sank after launching due to developping a continuous drip from a skin fitting. He has only 3" under the cabin sole, so a really compact detection system is called for in his case (I believe the pressure sensor typically requires a vertical tube at least 6" long).

Rob.