Meaning of the ‘dots’ under the temperature scale?

[SteveHall64Healey]

Does anyone know the meaning of the 4 ‘dots’ on the temperature scale at 130, 140 and just below and above 212F ?

 

[RAC68]

Hi Steve,

Its only a guess but I would suggest it indicates the effective range of the gauge/sender...the effective operating temperature.

Ray(64BJ8P1)

 

[dklawson]

On the electric gauges the dots represent calibration points (primarily used by the factory). I assume the same applies to the mechanical gauges but I have no information to support that claim.

 

[BJ8Healeys]

I agree that the dots are calibration points. The two at 212 are obvious: 212 is the boiling point temperature of water at sea level and is a constant known temperature quantity and therefore suitable as a calibration point. I don't have a clue what the two dots at 130/140 calibrate for but it seems desirable to calibrate a gauge at a high and low temperature rather than just one location. I used the two at 212 to re-calibrate my gauge when I suspected that it was reading too high and contributing to my engine "overheating" problem. With the sensor immersed in boiling water, I found that the gauge read significantly above the 212 mark (I live at sea level, by the way). With the sensor still immersed, I removed the gauge needle and replaced it exactly between the two dots. My gauge now runs where it should, and with the engine dead cold the needle sits at the peg at 90 F.