Pressure-temperature graph without zero y-intercept

I am a high school physics teacher and I've been trying out an experiment for my students. I used a manometer (a plastic tube in a U-shape partially filled with water) to measure the pressure of the air in a boiling tube. I placed the boiling tube into a beaker of water and varied the temperature. I recorded the head difference in mm and converted this to metres.

I used (pressure = density of water x 9.81 x head) to work out the pressure (relative to atmospheric pressure). I then added the local atmospheric pressure (sourced from BBC Weather in millibars and converted to Pa) to the relative pressure to get the absolute pressure of the air.

I plotted a graph of temperature (in Kelvin) against absolute pressure (in Pa) and got a nice straight line relationship. However, my y-intercept should be at zero, according to the ideal gas laws, but is actually crossing at around 90,000 Pa.

Either my gradient is much too shallow (my points are very close to the line of best fit) or I have an absolute error that has shifted my results up the y-axis. The most likely source of this is the atmospheric pressure.

I'm very puzzled and have probably made a silly error somewhere. If you're interested in checking my numbers, please have a look at the attached spreadsheet.

Thanks,

Rob

Attached Files

Trial results.xlsx (18.9 KB)