Coolant lever warner

[CDK]

The expansion tanks on my Bowman exhaust manifolds have a threaded hole for a temperature sensor which is too small for any level switch.

I want to be informed when the level drops; it does happen and because there is no visible leak I assume that under pressure some fluid disappears to the raw water section. To avoid having to lift the floor every now and then I came up with the following solution:

The sensors are the original plugs in which I drilled a small hole, pressed in a piece of RG-58 coax insulation sleeve and half of a tungsten welding electrode. These you cannot bend, but they break like glass.

The submerged tungsten provides a little over 200 mV against the alloy of the manifold. I feed that to a simple circuit comparing the input with a 100 mV reference; the current is almost zero, so the tungsten isn't eaten within the next 10 years.

If the level drops or the wire comes off, a LED lights and the buzzer in the engine bay draws my attention. The 1"x1" circuit including the LEDs I molded into a small module using some leftover resin from another project.

[finnracing]

CDK

Thanks for all your valuable input. Just a couple of questions. What transistor have you chosen in this design? Can this device be used to monitor water & fuel tanks?

[CDK]

Any transistor (NPN) that can carry the buzzer's current will do; I used a 2N2222.

The principle applies to conducting (=ionized) liquids only. In a fuel tank it can be used to warn for water in the fuel.

[finnracing]

CDK

I cannot quite make out what you have attached to the back of the tungsten electrode & How have you attached it?

[CDK]

Sorry finnracing, I forgot to mention that.
Tungsten electrodes are a sintered material, you cannot solder a wire to it or crimp a terminal on it. So you need something with spring pressure.

I used nickel plated bronze terminals from Hirschmann (MBU-1) with a solder lug, they are part of a 1 mm contact program for test equipment.