AN ELECTRONIC BOILER WATER LEVEL GAUGE FOR A STANLEY STEAM CAR ASSEMBLY

                                    Capacitive Water Level Gauge: Assembly Notes
                                                                Part Two
Probe
The probe is an insulated thin wall brass tube.  The insulation I have used is Teflon (PTFE) heat shrink tubing.  To prevent some problems experienced with earlier versions, the brass tube should be perforated about every half inch.  I suggest rotating the tube while heating the heat shrink tubing to obtain uniform shrinkage.  I have tried several samples of heat shrink tubing and think McMaster-Carr catalog number 75665K62 is particularly suitable.  A four foot length costs less than eight Dollars. 
A second method of insulating the probe is to wrap the tube with overlapping layers of teflon tape made for pipe joint purposes.  I have not tried this method, but it has been used successfully by another user  of this gauge.

Probe Mounting
The bottom end of the probe is inserted in a Teflon insulator which is in turn inserted in a drilled hole in a quarter inch brass pipe plug.  Red high temperature silicon rubber joint compound seals the probe from water leakage.  As the probe is open at both ends, the silicon rubber is not required to be pressure resistant.
The top end is centered by its contact wire on the sparkplug used as the gauge's signal connector.  I have used both the Autolite 4194 and the Champion J99, which is an industrial furnace igniter.   The Champion is both easier to use and more reliable, it having a long electrode to which it is easy to braze an extension.   The extension serves only to make electrical contact with inside of the probe tube and may be as thin as twenty gauge copper wire.  A few slight bends in this wire will insure adequate contact.  
As the usual spark plug gasket works poorly with steam and to obtain a grounding point for the cable, an O-ring retainer made of a tenth inch thick brass or steel is fitted on the plug.  If the seat hole is 0.75 inch diameter, a 206 Viton O-ring will work.  The grounding point is tapped 6-32.

Sensor Case
I have used a length or one inch black iron pipe with ends and suitable connections welded on.  The ends may be made of three eighths thick steel plate. A 1 1/2 inch hole saw can be used to make the end pieces. The top end can be tapped to take the sparkplug directly, 14 by 1.25 mm. thread for the Champion plug.  The bottom  end should be tapped one fourth National pipe thread.
The side fittings may be five eighths round steel rod, or short bits of 1/4 inch black iron pipe..  Drill them and the body of the gauge one fourth of an inch so a bit of threaded rod will hold them in place during welding.  Afterwards, they can be drilled five sixteenths and tapped one eighth pipe thread.
For a ten inch gauge, a twelve inch length of pipe is suggested.  Thus the side fittings can be an inch from the end and still be ten inches apart.

Assembly
To keep the probe centerd while inserting it in the case. I used a long piece of thin (18 gauge) steel wire inserted first.  After the bottom fitting is in place, the steel wire sticking out of the top hole will keep the probe tube in view.  One may then insert the end of the connector wire in the probe, remove the steel wire and screw the sparkplug home, remembering to have the ground connector and O-ring in place first.

Checking
An assembled gauge should have an infinite electrical resistance whether empty or full of water.  The empty gauge should have a low capacity, probably about thirty picoFarads.  When full of water, the capacity should be roughly ten times as great.  If the gauge is over filled so the water goes above the end of the insulated probe, it will short out and have a relatively low resistance.  This will cause no damage either to itself or the circuit to which it is connected.  As soon as the water level drops and the plug insulator dries, the gauge will work normally.

Calibration
The circuit board has two potentiometers on it for calibrating the gauge.  with the gauge connected but empty, adjust the potentiometer marked R2 so the meter reads zero.  With the gauge full of water, adjust the potentiometer marked R4 so the meter reads full scale.  ​