I had a couple of spare, inaccurate original senders so I decided to see if I could re-calibrate them to make them accurate enough to use.
I put them in my lathe and cut away at the top until I could get it off.
There are two “calibration” mechanisms, one for zero pressure and one for running pressure.
The zero pressure point is established by adjusting the position of the upper contact pad on the bimetallic strip. Depending on the age of the sender this is adjusted with either a threaded post or a rotary cam. You can see the brass threaded post at 10 o'clock.
To calibrate you power up the gauge and set this so that the contact pads never actuate at zero pressure. I did this by finding the lowest point at which the cams could contact then wound them back a fraction more.
The running pressure calibration is set using a resistor that connects in parallel with the coil of wire around the bimetallic strip. These were fitted at the factory.
To calibrate, I removed the old resistor. I then connected the sender to 40psi of air pressure. I used a switchable variable resistor to alter resistance until the gauge read accurately. This was surprisingly easy. One sender needed 22 ohm, the other needed 47 ohm.
In the end, for further tweakability I decided to install a 100 ohm trim pot rather than a simple resistor. This is the blue box in the picture.
I’ve now got two senders that, at least on the bench, will read zero and 40 psi accurately. I haven’t checked them for linearity because I can’t accurately apply lower air pressures with my setup. But as the linearity ought to be a function of the bimetallic strip, which I haven't changed, I’m hopeful that it won’t be any worse than it was originally.
Caveat. I haven’t allowed for expansion with heat. I’m not sure whether this will be an issue or not. If anything it might affect the zero point, but shouldn’t affect the running pressure. I might run a heat gun on one of the senders while on the bench and see what happens. At worst I might have to back the zero adjuster off a bit.
