Digital Barometer Accuracy Evaluation - January 2017
I own six relatively common digital barometers:
- An ASI (Aquatech Scientific Instrument) DBX1. It has a stated accuracy of +/-0.007 inHg. It costs about $400 and is available on eBay.
- A Weems & Plath Electronic Marine Barometer with a stated accuracy of +/-0.5 mb, or +/-0.15 inHg. It does not appear to be available from Weems & Plath at this time. It cost approximately $300 US.
- Two (2) Dracal Technologies USB Precision Barometers, which is a USB dongle that comes with associated software. It has a stated accuracy of +/-0.15 kPa, or +/-0.044 inHg It is available new on eBay. It costs approximately $50 US.
- Two (2) Novalynx Model 355 digital barometers. I bought mine used and don't know what it originally sold for.
For the month of January 2017, once a day, I compared the local pressure as measured by the six digtal barometers with the local pressure I measured with my Henry J Green Fortin mercury barometer. For each digital barometer, four statistics were calculated:
- The 'average error' is the average difference between the digital barometer pressure minus the mercury barometer pressure.
- The 'standard deviation' is the standard deviation of the individual error values. 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 aneroid barometer, while the standard deviation generally cannot, it is more inherent in the performance of the barometer.
- 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 fitted against the mercury barometer local pressure, in mb per mb of barometric pressure. It is an indicator of how much the error changes on average with the local pressure.
Digital Barometer Accuracy Evaluation - January 2017 Results
In general, the observed average error is less than the inaccuracy stated by the manufacturer. All of the barometers except the Dracal were 'calibrated' by entering offset pressure values into the firmware based on previous testing. The Dracal software does not allow entry of an offset for calibration, though this could be done through post-processing if the data is exported to a spreadsheet.
The standard deviation data is too close to call for the first four digital barometers. The Dracal barometer is inexpensive but has the disadvantage that it requires a computer with a USB port to operate or log data.
The standard deviation data is too close to call for the first four digital barometers. The Dracal barometer is inexpensive but has the disadvantage that it requires a computer with a USB port to operate or log data.
There is, of course, error in the mercury barometer measurements. The accuracy of my mercury barometer data is discussed here.
Both the ASI and Weems & Plath barometers have the capability of calculating and displaying sea level pressure. However, neither barometer has access to the outdoor temperature and humidity, so the adjustment to sea level is presumably performed at standard conditions. I prefer to leave these instruments set to local pressure and perform the adjustment to sea level manually.
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Both the ASI and Weems & Plath barometers have the capability of calculating and displaying sea level pressure. However, neither barometer has access to the outdoor temperature and humidity, so the adjustment to sea level is presumably performed at standard conditions. I prefer to leave these instruments set to local pressure and perform the adjustment to sea level manually.
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