In a pot, 2 qt or larger, of water.
Get a battery, lawn mower/motorcycle would be fine.
Thermometer you trust.
Sending unit and gauge.
You might even add a thermostat - suspend it in the water, don't let it touch the pan.
Wire up the battery to gauge to sending unit in water to battery, just as if it were in the car, minus the switch. You might have hose clamp the ground wire to the unit body. Heat the water and watch the gauge & thermometer. Water at sea level boils at 212°F, and that's a calibration fact.
Compare all items, gauge reading, thermostat start to open, then t/stat full open, usually 10° later.
I have the electic kettle sitting on the hood catch, remove the sender, attach a ground wire from the battery and clip to the base of the sender, thermoter in the jug, turn the jug on and start heating up, making notes on a ruff pic of the gauge, of temps
Like everett says 212F (100C) is boiling point of water in an open container..no matter how hard the kettle boils.
If you want temps above that add antifreeze or salt...but remember anti freeze is poisnous if using the kitchen jug.
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