Barometer calibration - the role of temperature (Part IV)Note: Although some of the following methods could be used for other weather stations, these methods are meant for Fine Offset manufactured weather stations or consoles branded as Ambient Weather, Ecowitt or other Fine Offset clones. For more information refer to Part I, Part II, Part III (below).
Barometer calibration - the role of temperature (Part I)
https://www.wxforum.net/index.php?topic=42761.msg436327;topicseen#msg436327Barometer calibration - the role of temperature (Part II)
https://www.wxforum.net/index.php?topic=42761.msg436328#msg436328Barometer calibration - the role of temperature (Part III)
https://www.wxforum.net/index.php?topic=42761.msg436331#msg436331Summary of calibration methods:1. Compare with reference barometerThe easiest and most accurate method to calibrate a barometric sensor is to compare your current raw station pressure reading side-by-side with a reference barometric sensor. The reference barometer need not to be an expensive one – just one that is more accurate and precise. ABS in the display console is adjusted to match the reference barometer. For the Ecowitt GW1000/1100 sensor calibration adjustment is done by adjusting the ABS offset until you match the readings of the reference barometer.
2. Compare with METAR QNH/AltimeterMost countries in the world report QNH in their METAR reports. In these countries, METAR SLP values are either rare or are not calculated and may not be available. To calibrate using QNH/Altimeter in the display console set REL = QNH or Altimeter. In the GW1000/1100 firmware set a REL offset value until you match METAR QNH/Altimeter. Calibrating ABS/Station pressure/QFE or calculating an altimeter elevation offset is a bit trickier. The elevation offset can be calculated from a Altimeter setting calculator or QNH to QFE calculator.
QNH/Altimeter uses fixed standard ISA temperatures. If outside temperatures are different from the fixed ISA temperatures – QNH/Altimeter readings will be in error. Altimeter errors increase in magnitude with increased elevation resulting in larger discrepancies at higher elevations*.
Example of a ISA temperature: What is the ISA temperature at an elevation of 191 meters AMSL? Using an ISA calculator., the temperature is calculated to be 13.76°C @ 191 meters AMSL
* see Table A2 (scroll down a bit)
https://davidburchnavigation.blogspot.com/2013/10/air-temperature-dependence-of-sea-level.html for pressure corrections to QNH/Altimeter (in millibars) due to temperature and elevation. The far right column represents fixed ISA temperatures (rounded off to the nearest degree) at various elevations. The ISA temperature starts at 15°C/59°F at sea level (zero meters) and declines with increasing elevation. The horizontal top row are 12 hour average temperatures.
3. Compare with METAR SLPAdjust REL reading to match with a reference METAR SLP (if available). Mean sea level pressure can be calculated using a fixed elevation offset added to station pressure. This value is unique to your elevation (above sea level) and needs to be calculated once only at initial setup of your Fine Offset weather station. You can optionally add a fixed temperature correction/offset based on annual or seasonal temperature averages.
Since we are reducing station pressure down to sea level (elevation) we can use 1013.25 mb as our fixed point of reference in order to calculate a fixed offset. Additionally, the reference ISA temperature of 15°C at mean sea level should be used or depending on the calculator, the fixed ISA temperature for your particular location/elevation can also be used.
4. Weather software method:OK. WeeWX is not a calibration method but it will help calculate a more accurate SLP.
Fine Offset weather stations can not calculate SLP (temp corrected) directly but weather software can be employed (such as WeeWX ) that can dynamically calculate and graph SLP for you. Optionally, WeeWX can be configured/modified to display Altimeter and/or station pressure.
5. Spreadsheet method:Some online weather services allow you to export METAR data from an official station close to you which you can import to your own spreadsheet. You can compare METAR SLP to METAR Station pressure and calculate a fixed elevation offset for the METAR station. Create a column that calculates SLP minus station pressure minus elevation offset. Basically you want to see how much of the SLP reading incorporates a temperature correction. Likewise, you can easily import your own weather data from ambientweather.net, ecowitt.net, pwsweather.com, etc. to make comparisons with your METAR station – create deltas or graph the results. Ultimately, you could fine tune your fixed offset SLP weather station to a variable offset METAR within an acceptable range.
6. Other considerationsAt my location, morning temperature inversions are very common so my temps can easily be several degrees(Celsius) cooler than the METAR station I am comparing to. My SLP value will be much higher so there is no use to compare SLP values under that scenario until later in the day when the air mixes, temperature increases and I see SLP falling to match the METAR..an example of why we might ignore METAR at times and concentrate on the calibration of our weather stations for our own local climate. Temperature matters.
CAUTION: Anytime you are comparing your station barometric readings with a METAR station you must be confident that your station and the METAR station you are comparing to are in the same pressure zone.
Glossary
ABS = Absolute Pressure = Station Pressure
REL = Relative Pressure – can be set to SLP or QNH or Altimeter
ISA = International Standard Atmosphere
MSLP/SLP = station pressure reduced to mean sea level elevation (temperature corrected)
---
