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I was checking the temperature inside my oven using a multimeter with a temperature probe. I had set the oven's thermostat to 190o C, and it had reached equilibrium, but the measured temperature was well below what I had set the thermostat for. Having not previously used this temperature probe (new meter!), I was wondering if my probe and multimeter were accurate. So I put a pot of water on to boil, to see how close it would be to 100o C. The boiling water turned out to be 98o C. This made me reasonably sure that my oven's thermostat was either wildly out of kilter, or incapable of heating to the desired temperature. So I baked the item longer than the recipe called for. Problem solved, except for wondering why the water was boiling at 98oC instead of 100oC.

I figured that it must have something to do with atmospheric pressure, and is probably also wrapped up with the elevation of my house above sea level. The question I have, then, is this: is there a way to calculate the atmospheric pressure from the temperature I measured, or alternatively, determine what the boiling point of water should be at this moment -- to determine how accurate my meter is? My house is 25 m above mean sea level, and the current atmospheric pressure is 1040 hPa.

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  • $\begingroup$ Check here: en.wikipedia.org/wiki/… $\endgroup$ – user139175 Dec 28 '16 at 19:11
  • $\begingroup$ Thermocouples, especially simple ones on multimeters without temperature stabilised junctions are typically only good to 1-2deg C $\endgroup$ – Martin Beckett Dec 28 '16 at 19:24
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    $\begingroup$ Water has to be at a full, rolling boil in order to be at 100 ˚C. If you measured the water temperature while it was boiling at a level less than that, then that would explain why you measured 98 ˚C. Another possibility is that, as Martin wrote, your multimeter's probe may not have been accurate enough in this temperature range. I use food thermometer with a small probe tip which is designed for high accuracy in the room-temperature to 100 ˚C range. It consistently agrees with the temperature of the water in my sous vide water bath cooking device within 0.5 ˚C. $\endgroup$ – Samuel Weir Dec 28 '16 at 20:38
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Steam Table says that the saturation temperature for Water @1040 hPa is 100.7°C. Conversely, the saturation pressure at 98°C is 943.9 hPa. Saturated Steam Table

Check the accuracy of your temperature probe as well as that of your barometer.

For the connection of Temperature, Pressure and Elevation, see Barometric formula

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