The pressure sensor for all Davis Vantage systems is located in the console (or equivalent device such as Weatherlink Live) and not in the ISS or other outdoor sensors. The reason is that pressure is normally exactly the same indoors and outdoors and locating the sensor in the console (usually at a reasonably consistent temperature indoors) allows more stable readings and a longer service life for the sensor. Faults and troubleshooting to do with the pressure reading are therefore dealt with in the console section.
However, correction of pressure to a sea-level value is one issue that is a constant source of confusion and we’ll deal with that here:
Pressure correction to sea-level
It is well-known that air pressure decreases with height and is does so in a fairly sensitive manner, decreasing at sea level by approximately 1mb (0.03”) for every 27 feet (~8m) of altitude. All that a pressure sensor can do, however, is to measure the pressure in its immediate surroundings and at whatever height it finds itself. In any hilly or mountainous terrain this would make it almost impossible to compare pressure readings from stations at different locations. There is therefore a convention that actual pressure readings are always converted (automatically) to the estimated pressure at sea level thereby allowing valid comparisons between stations.and hence allowing the familiar pressure patterns of highs and lows to be visualised.
Setting up a new console
When you set up a new console, one of the key settings you enter is the elevation/altitude of the console’s location. The more accurately you can do this (at least within a meter or two or a few feet) then the more accurate will be the final sea level pressure reading. The console automatically uses the height setting to correct the measured pressure back down to sea level.
The second initial step that it’s prudent to take is to calibrate the pressure reading. This is best done on a day with steady pressure readings, eg during a period of high pressure. You need to find a reliable nearby site such as an airfield with a frequently-updated online pressure reading and calibrate your console to that reading following the calibration instructions in your manual.. Assuming that your height has been accurately entered then you you should find that the existing reading agrees with the reference reading to within around 2-3mb but it should then be fine-tuned to match the reference reading exactly. Thereafter your console should track the reference site reasonably well, but for best results it’s advisable to check against the reference reading from time to time and tweak your console calibration if required.
Sea level correction issues
The issue with pressure comparisons arises because there is more than one way of correcting pressure readings to sea level and the confusion is caused by not being clear about which method is actually used or required in a certain context. Davis use two correction algorithms in different models:
Altimeter method: This assumes that pressure always decreases with height according to a relatively simple formula, no matter where you are or what the weather conditions and depending only on height. In truth, this is not a very accurate approach because the calculation should also take account of the recent air temperature and various other factors. But it is the method widely used in aviation and so is an important reference method. In aviation use this method generates QNH values.
Standard method This method uses a more sophisticated algorithm that takes into account the air temperature over the past 12 hours and the dew point temperature. The result should be a more accurate estimate of the sea-level pressure. In aviation use this method generates QFF values (though this designation seems to be rarely used in practice).
Details of the algorithm used can be found in the Davis Application Note #28 ‘Derived variables in Davis weather products’. There is also an extension discussion of four different pressure correction methods in ‘The Weather Observer’s Handbook’ by Stephen Burt (Cambridge University Press 2012).
The Altimeter method generates ‘Altimeter pressure’ values, while the Standard method generates what are termed ‘Barometric pressure’ values. (This latter ‘barometric pressure’ terminology is, in my opinion, rather unhelpful because arguably all pressure readings are of barometric pressure. However, this distinction between Altimeter and Barometric pressures is firmly embedded in common usage in the weather and aviation communities, especially in the US, and so we have to be resigned to its continuing use.)
While the standard method really ought to be preferred to the Altimeter approach because of its greater accuracy, the waters have been muddied because the online US weather network CWOP mandates that pressure readings should be uploaded as Altimeter values (and failure to do so incorrectly labels the pressure readings as inaccurate). This seems to have caused Davis to change tack in how they correct pressure to seal level. So:
VP2 and Envoy consoles:use the ‘Standard/Barometric pressure’ method only;
Weatherlink Live: and 6313 console currently offer only the Altimeter option;
Vue consoles: offer the user a choice of using either Standard/Barometric pressure or Altimeter values;
This isn’t quite the full picture in that the Weatherlink logger API also provides a value for the absolute pressure reading (ie as actually measured by the console and before correction). in the LOOP2 data record. So this absolute pressure could in principle be used by software to calculate sea level pressure using any algorithm that the software author chooses, although this further option doesn’t seem to have been widely implemented..
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