VDO cylinder head teperature gauges

I have a VDO cylinder head temperature gauge that I believe reads high.

With the thermocouple end in boiling water it reads 240 deg F, 30 deg F above the boiling point of the water. I believe that these gauges are calibrated in the factory at 75 deg F, so that has been taken into account, and I only live 25 meters above sea lever so I don't think that that will have a major effect on the reading

Does anyone know if this will be a consistant inaccuracy, 30 deg F above throughout the range, or will the accuracy get worse as the temperature increases, i.e a percentage error?

Any help appreciated