Calculation of Local Pressure for Mercury Barometers
Three additive corrections are used to calculate the local pressure from mercury barometer data. These are
The local pressure is just the mercury column height plus or minus the three correction terms. The equations for the calculation of these terms, as well as numerical examples of the calculation, are shown in the following PDF document (2 pages):
- The Calibration Correction: If your lucky, the mercury barometer you bought came with a card indicating how much the instrument scale differed from the true pressure when it was originally calibrated by the manufacturer. In the case of my barometer, the instrument reads 0.005 inHg too low. If you don't have this value, you need another source of calibrated pressure data to determine it for your barometer. Once determined, this is a constant for all future barometer readings. This is usually the smallest of the three corrections.
- The Temperature Correction: Both the mercury column and the barometer metal tube and scale expand and contract at different rates with changes in barometer temperature, affecting the instrument reading. The temperature used is from the thermometer mounted on the barometer. I use the temperature correction equation presented in the 1963 WBAN Manual of Barometry, which may be found in the miscellaneous downloads section of this website. If you don't want to calculate it, tables of values for this correction can be found in the Smithsonian Meteorological Tables, also available for download on this website. This is usually the largest of the three corrections and will vary from barometer reading to reading with the varying temperature.
- The Gravity Correction: The height of the mercury column also depends on the local acceleration due to gravity. This is a function of your latitude (north or south), the ground level elevation above sea level at your location, and the height of the barometer cistern above ground level. Again, I use correction equation presented in the 1963 WBAN Manual of Barometry, which may be found in the miscellaneous downloads section of this website. If you don't want to calculate it, tables of values for this correction can also be found in the Smithsonian Meteorological Tables, also available for download on this website. Once determined, this correction is constant unless the barometer is moved.
The local pressure is just the mercury column height plus or minus the three correction terms. The equations for the calculation of these terms, as well as numerical examples of the calculation, are shown in the following PDF document (2 pages):
I use a Microsoft Excel spreadsheet to calculate these correction terms and determine the local pressure from the mercury barometer readings. The following is a copy of this spreadsheet, which should work in Excel 97 or newer:
Note that the temperature correction equation used here is for vintage inHg mercury barometers which were standardized at a temperature of 62 F. Metric barometers are probably standardized at 0 C and require a different correction term. See the values tabulated in the Smithsonian Meteorological Tables.
Here is a picture of the spreadsheet with data from August 2016:
Here is a picture of the spreadsheet with data from August 2016:
