Digital Barometer Accuracy Evaluation - December 2016
I own four relatively common digital barometers:
For the month of December 2016, once a day, I compared the local pressure as measured by the four digtal barometers with the local pressure I measured with my Henry J Green Fortin mercury barometer. The raw data are summarized in the table below:
- 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.
- A Dracal Technologies USB Precision Barometer, 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.
- A Novalynx Model 355 digital barometer. I bought mine used and don't know what it originally sold for.
For the month of December 2016, once a day, I compared the local pressure as measured by the four digtal barometers with the local pressure I measured with my Henry J Green Fortin mercury barometer. The raw data are summarized in the table below:
For each digital barometer, four statistics were calcualted:
- 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 plotted 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 - December 2016 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. All four of the barometers had similar standard deviations of the errors.
The ASI barometer had the lowest average error, lowest standard deviation of errors, and significantly lower drift. It could have performed better, but the firmware only allows entering a calibration offset to 0.01 in Hg. An extra digital would have been useful. Here's what the ASI data looks like:
The ASI barometer had the lowest average error, lowest standard deviation of errors, and significantly lower drift. It could have performed better, but the firmware only allows entering a calibration offset to 0.01 in Hg. An extra digital would have been useful. Here's what the ASI data looks like:
The Dracal USB barometer came in second, which is interesting considering it costs one-eighth of the price of the ASI instrument. However the drift is significantly higher than for the ASI device. The Dracal barometer has the disadvantage of requiring a computer to operate. This can be an advantage if you want to datalog, though a restart of the computer stops the datalogging which must then be restarted to a new file. This can be a problem if Windows auto-updates. The Dracal error data is shown in the following plot:
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.
Comparison of these results with the aneroid barometer accuracy analysis found here shows that a top quality aneroid barometer has similar average accuracy when compared with these digital barometers, but that the ASI barometer shows a significantly lower drift.
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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.
Comparison of these results with the aneroid barometer accuracy analysis found here shows that a top quality aneroid barometer has similar average accuracy when compared with these digital barometers, but that the ASI barometer shows a significantly lower drift.
Go to Home Page.
