For the month of January 2017 I analyzed the accuracy of 5 barographs by comparing its their recorded sea level values with values with either calculated sea level values from my Henry J Green mercury observatory barometer or with values from the O'Hare airport NWS station near Chicago, Illinois.  O'Hare is 22.7 km WNW of my location but is at a significantly different elevation.  Unfortunately, the analysis is somewhat flawed in that two of the barographs were calibrated to the O'Hare value while three were calibrated to my mercury barometer.  For the month of March, I will recalibrate all of the barometers to the mercury barometer value and I will repeat this analysis in April 2017.

​Four statistics were calculated:

1. The average error was calculated.  It is the average difference calculated as the mercury barometer or O'Hare pressure minus the pressure read from the barograph.
2. The standard deviation of the individual error values was calculated. By definition, 68% of the error values fall within +/- one standard deviation of the mercury barometer value. The average error can be reduced by recalibration of the barograph, while the standard deviation generally cannot, it is more inherent in the performance of the barograph.
3. The average drift was calculated. This is simply the slope of a straight line fitted to the the scatter of error data points plotted against day of the month. It is an indicator of how the error changes with time.
4. Finally, the average non-linearity was calculated. This is the slope of a straight line fitted to the scatter of error data points plotted against the mercury barometer sea level pressure, in mb per mb of barometric pressure. It is an indicator of how much the error changes on average with the local pressure.

Most of my barographs are quite old and performance can be affected by the condition of the instrument as well as it's design. It is not my intention to pass judgement on the design of the individual barographs. Take these results with a word of caution.
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The results of the analysis for five barographs are presented in the table below.  The results are in no apparent order.

​I will repeat this analysis with March barometric pressure data in April 2017.  For the month of March, I will recalibrate all of the barometers to the mercury barometer value and I will repeat this analysis in April 2017.  I will also disconnect the dashpots from the two ML-3A microbarographs and will try a different 31 day chart on the second ML-3A.  I may also extend the analysis to three other barographs:

• A WeatherMeasure B211 microbarograph
• A Meteorological Service of Canada Type B microbarograph
• A Dr A Mueller/R Fuess barograph

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