General

Readings of barometric pressure are typically accurate for extended periods and are rarely the source of any problems. But there are a couple of general points to make about pressure readings:

The pressure sensor is located on the main console circuit board for both Vue and VP2 stations – on the basis that air pressure is equal inside and outside any building (unless it’s hermetically sealed!) and so it’s simpler and more reliable to sense pressure at the console. Any pressure issue is therefore a console issue and nothing to do with the ISS or any other external transmitter.

Second, air pressure obviously varies with height/altitude and it’s the convention in weather reporting to adjust reported pressure to sea level (the value that’s referred to in aviation as QNH or QFF – see below for more detail) so that pressure values can sensibly be compared across large areas irrespective of the measurement altitude. So for any pressure reading made other than at sea level a calculation needs to be made to correct the reading to sea level. The console has no independent way of knowing its altitude and so, in practice, the procedure for setting up pressure readings at a new location is as follows:

1. Check that the height/altitude value in the console settings is correct. Remember that this needs to be the elevation of the console, not the ISS. Changing this height value will not alter the current pressure reading but will slightly alter the correction applied as the pressure subsequently changes higher or lower for any console sited significantly above sea level.

2. Find a reliable current reading for sea-level pressure (QNH) on the Internet for a nearby location such as an airfield and then follow the instructions in the console manual for entering a calibration pressure reading to the console. (In the UK, www.xcweather.co.uk/ is a good source, but there are many others around the world.)

Pressure-related issues

Faulty sensor readings

Although rare, very occasionally the pressure readings may give problems. Symptoms can range through complete loss of a pressure reading (eg the pressure display dashed out); a reading so high or low that it is obviously spurious; failure to update readings; and so on. And sometimes a failing pressure sensor may give intermittently good and bad readings.

The first troubleshooting step is to double-check that the height/elevation and pressure offset settings have not been inadvertently mis-set. As a check, try clearing any pressure offset (see the ‘Clearing Weather Variables’ section in the console manual) and also reset the elevation to some basic value like 10 feet or metres (depending on the units in use). Don’t set a negative or zero elevation, nor a very high value – only a simple value like 10 will provide a reliable check on pressure sensor operation. The correct elevation can be reset once any problem has been investigated (but don’t forget!).

It is then always worth fully rebooting the console to clear any potentially corrupted values in the console memory. Remember that it may take a minute or two for the correct pressure reading to reappear.

Any apparent pressure fault that can’t be corrected by recalibration or can’t be attributed to a height/correction issue is likely to be a sensor fault. The bad news is that the sensor is soldered on to the main console circuit board and typically requires a new circuit board to be fitted, the cost and inconvenience of which may not be much less than sourcing a new console, especially for a Vue (assuming that the unit is out of warranty). A suspected faulty sensor needs discussing with your local Davis dealer or service centre.

Deviations from expected sea-level (QNH/QFF) readings

The intrinsic accuracy of the barometric pressure readings for both Vue and VP2 stations is quoted as ±1mb or ±0.03“Hg and, once calibrated, most units will track other high-quality pressure readings in the locality pretty well and to within this nominal accuracy. There may be some long-term drift, in which case an occasional recalibration against a reference reading should fix any slight problem.

However, stations that are located at significant altitude (eg 1000-2000ft and above) may experience more frequent discrepancies against reference readings. This is not necessarily caused by any fault with the console but can arise because of the way that corrections are made from the measured local pressure back down to sea level. (The more elevated the location, the greater any sea-level correction will be, which is why this issue is usually only noticeable at stations located at significant altitude.)

The basic issue is that an assumption has to made about the composition (temperature, humidity, gradient etc – all factors affecting the correction calculation for the pressure) of the air column between the point of measurement and a notional point at sea level below. The nature of the this air column can never be known in detail and there is more than one way that the column can be assumed to behave, with various approximations being involved. Inevitably then, different approaches will give different values for sea level pressure and it may well be that the Davis console is using a different correction method from a reference source. This is not the place for a detailed discussion of the various correction methods (which are obviously very important in aviation for accurate altitude readings) but in brief the two main options are:

QNH: Pressure is reduced to sea level assuming the temperature profile of the International Standard Atmosphere. This gives an approximation to what is often referred to as the altimeter pressure;

QFF:Pressure is reduced to sea level taking the current measured temperature (and, potentially, humidity) at the station and assuming an isothermal atmosphere (sometimes referred to as the US NOAA method);

So, for example, discrepancies can arise if the Davis console is reporting QFF pressure while a reference source is citing QNH.

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