Hello from the Conwy valley in North Wales

Hi @thornogson,

re: I would like the original gauge to show the tank level of the fuel selected by the changeover switch . Fuel tank sender is 0-30 ohm , gauge is 30-0 ohm but LPG sender is 0-90 ohm . What is the simplest ( most robust) way I can show the correct level of each type of fuel? 

Sorry, I don't have an immediate solution - simple or otherwise. But perhaps I can offer a few comments to open up the discussion:

• I think you should (re)start this question on a new thread of its own, with an appropriate title. More people will see it, so there is a better chance of a good reply, plus it gives some sort of 'order' to the forum.
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• Fuel tank senders usually take electrical power into the fuel area of the tank, which makes them a potential safety issue. In particular, there are various safety standards to limit the amount of power (e.g max voltage and max current) that could possibly flow ithrough the tank. By possibly, I am including 'worst-case' assumptions that components, insulation, etc can fail, without exceeding these limits. Regardless of any advice from this forum,  ensuring safety is totally your responsibility.
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• As for the challenge itself, to be in a position to consider, and to 'test' a proposed solution by simulation or otherwise, I think it is necessary to have a specification of the gauge. For example, what resistance is the gauge, and what currents through the gauge represent 'empty' and 'full'
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  • By 'empty', I mean the level when there is enough fuel for it to continue to flow to the engine, but the flow will cease in a short time. I note fuel gauges sometimes move to a position below empty when the power (ignition) is switched off, presumably because the sender is (say) 30 Ohm, not infinity, when the tank is empty, and hence some current will flow through the gauge.

Good luck with your project. Dave

@thornogson I also have an RV and gas (we call it propane in NA). My remote is bluetooth so I doubt that is much help.

As a resolution, FYI I have recently found that a 47 ohm 5 w resistor in parallel with the tank rheostat (ie between sender wire and gnd) gives a fair approximation of the tank contents which will do for now. Thanks for your input.