# measure barometric pressure using boiling water

I have set a cup of water in a bell jar, with a thermometer. I am able to lower the pressure until the water boils and I am able to measure the temperature of the water simultaneously. I then looked at a phase diagram for water, and found where the temperature axis line intersects the liquid to vapor phase change line. I then used the Antoine Equation to solve for the pressure at which the water was observed to start boiling.

I have several electronic barometers (which measure in inches of water column) and are showing several inches difference between then. I am looking for a good way to set them to a known absolute pressure reference.

Is this a good way to determine barometric pressure? Is it prone to a bunch of esoteric sources of errors? Is there a method available that I can follow to do this experiment with uncertainties?

• What's the range of pressures you need to measure? "[...] and are showing several inches difference between them". Inches of what? – Gert Sep 25 '15 at 23:54
• How are your significant figures? It can throw off your calculation if you round your numbers too early. – CoilKid Sep 26 '15 at 0:01
• @Gert He says. "(which measure in inches of water column)" – CoilKid Sep 26 '15 at 0:15
• @CoilKid: ooops! I don't think he's rounding anything, though. – Gert Sep 26 '15 at 0:19
• my absolute pressure range is 0 to 800 inches of water @4°C. The barometers that I'm fighting with have an accuracy of ±0.3 inches of water, and a resolution of ±0.01 inches of water. I am boiling water around 9.6 inches of water, with a room temperature of about 20.1°C. – beaglebreath Sep 26 '15 at 1:06

To measure absolute pressure, take advantage of the ideal gas law, $$PV = NkT.$$ For an isolated pressure system (constant $N$) you should be able to change the pressure $P$ by a known amount by changing the temperature $T$ or the volume $V$. You could for instance put your pressure vessel in the freezer for a few hours (265 K or so) then pop it in some water and boil is (373 K or so) for a ~30% pressure change. You'll need to check in advance whether your barometer is temperature sensitive.