I have some different temperature senders to try.
Hi James, As Mahesh says, these senders are very temperature sensitive. As an example my original Smiths sender reads 682 ohms at "room temp " ~ 68 deg F right now but hold the sensor element for a few seconds and it will rapidly drop to 610 ohms.
And the resistance is not linear with temperature.
Fro memory it is related by something callec "B" value which is different between sensors.
Now the one sender I do like is the 3rd original one:- like mine from your photos.
Can you check the d.c resistance from the copper bulb to the screw thread is <1 ohm please.
I see you have quite a clever dvm. Does it have a thermocouple accessory ?
If so can you check with the sensor in an oil bath at various temperatures please ?
( If not I can help with this )
It would be interesting to compare all 4 units.