Testing temperature senders

As part of my investigation into high temperature I looked into the information being sent to the temperature gauge in the cabin.The sender was not the original, I'd removed that during the recommissioning as it was non-operable. Now it had a red-capped sender when it should be black - the red and black caps are supposedly calibrated for different gauges so might provide different results. Could this be the reason for the high gauge readings? It would certainly be a simple fix.

To test them I hooked up the sender so I could immerse it in a kettle. A long wire ran from the sender's connection to its normal connection in the engine bay and another wrapped around the screw threads of the sender to an earth. With the ignition and the kettle on, I read off the temperature and marked the gauge on the dashboard.

The result? They were pretty much identical.

A kettle with the wired-up sender and thermometer submerged
The test rig
The sender showing the red cap
Red capped sender
The sender showing the black cap
Black capped sender