Friez 620 Barograph Accuracy
For the month of January 2017 I analyzed the accuracy of my Friez 620 barograph by comparing its recorded sea level values with calculated sea level values from my Henry J Green observatory mercury barometer. The 620 used Belfort #1068 charts. Values were determined once-per-day near midday.
Four statistics were calculated:
- The average error was calculated. It is the average difference calculated as the mercury barometer pressure minus the pressure read from the barograph.
- 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.
- 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.
- 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.
For the Friez 620, the calculated average error for January 2017 was -0.890 mb, while the standard deviation of the error values was only 0.975 mb. The drift in the error values was only 0.38 mb/month. The calculated nonlinearity was 0.048 mb per mb of barometric pressure. Plots of the error values showing the scatter, drift, and nonlinearity are presented below.
Drift
Data for January 2017
Nonlinearity
Data for January 2017
My conclusion is that my Friez 620 barograph works quite well but needs recalibration. I will recalibrate it to match my mercury barometer sea level value before the beginning of March 2017. The standard deviation, drift, and nonlinearity were quite good. I will repeat this evaluation in April using the March data.
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