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In the quest for semi-accurate Pressure Data, I am baffled by where the Meteobridge gets its Sealevel pressure from. It does not get it from Relative Pressure (OberverIP) and it must get it from some extrapolation from either Absolute or Relative. The value for Altimeter setting (CWOP) seems to be close but I have never had a confirmation as to the exact formula used. If anyone has any thoughts on the origin and what algorithm pumps out the Sealevel, please enlighten me.

In addition and seeing that the software allows for calculations in the Template side, can the Template be used to create a new Variable that could be used as an new sensor value, and in this case a Sea Level Pressure value, and thus be published accordingly. For those users that are at nominal altitude, I realize that this might be splitting hairs, but if you are in the mile high club, the published values or fixed offsets are worthless.
If one goes to the trouble and pride of having a published PWS, might has well have some meaningful data, and as its stand now, the SLP data is way off.
The simple solution would be if all weather data collector would align themselves with CWOP and use Altimeter Setting or Station Pressure with altitude info, and plug their own algorithms across the board, but I don't see that happening soon. Different countries cannot agree on which SLP algorithm to use, as it stands.

I wondered the same thing during setup and I came to the conclusion that MB reads the ObserverIP absolute pressure value and applies an altitude calculation using the altitude setup in MB to convert to sea level pressure.

With that in mind, calibration instructions from Ambient are sending you in the wrong direction (ad they only have you calibrate the relative pressure). Here is what I did and what gave me excellent error stats on barometer values for CWOP/MADIS:

• First determine the altitude of your indoor sensor (which measures the pressure; not your outdoor integrated sensor suite). Altitude above sea level (ft)
• Next lookup how much of a pressure correction this would result in by going to http://www.calctool.org/CALC/phys/default/pres_at_alt
  • First set altitude to 0 ft and read pressure. (I see 29.9213)
  • Then set altitude to your altitude from the first step and read pressure. For my station at 50 ft I see 29.8562
  • Compute difference. For me 29.9213 - 29.8562 = 0.0651
• Now apply this difference as the calibration correction for ObserverIP relative pressure (I rounded to 0.07 for ObserverIP which takes only 2 decimals). Note that this is only so the live data page on the Observer will show something relative pressure as correct for sea level (if absolute pressure is correctly calibrated, which we have not yet done at this point). The relative pressure does not appear to be used by MeteoBridge. It will also help with the next step.
• Lookup the sea level pressure for a nearby airport. Do make sure you use sea level pressure and not altimeter value (there is a slight difference).
• Compute with what the ObserverIP live data page reports for sea level pressure. Compute the difference.
• Combine this difference with the value already in the calibration field for "Absolute Pressure".

Using the above procedure you essentially calibrate the ObserverIP absolute pressure value so that its relative value (as compensated for altitude) will correctly report sea level pressure. If you now use that same altitude in the setup for MeteoBridge you should be getting good results because MeteoBridge applies its own calculation to reflect altitude and correct to sea level (for sending data to CWOP it calculates altimeter value as needed). I have found that you may need to tweak the calibration value by 0.01 in some cases.

Dolfs,
Thanks for your comments. You are part of the lucky ones that are in the "low" altitudes. As I mentioned I am in the mile high club and fixed offsets do not work. The Altimeter setting is not a linear correction, and that becomes apparent if you were at higher elevation. The question I had is how Meteobridge calculates its SeaLevel Pressure. Mesowest which gets its data from CWOP/MADIS does calculate sea level for you on the fly, and its formula is quite complex. I was hoping to use the Template functions to re-calculate the published SeaLevel using the station pressure, using my own formulas and feed it to WU or other, by replacing that value in Meteobridge. The CWOP Meteobridge Altimeter Setting Pressure feed is pretty close but I have no confirmation on the exact formula used as there are some simplified variations out there. I have reached out to Boris, but I am sure he has more important things to worry about.
There is little documentation about what you can do with the templates and the data points.

Yeah. My bad for not reading your post thoroughly, but I can confirm it only uses the absolute pressure.

Having a specification of how MB makes the calculation, might help, but only if it uses an appropriate formula for your case. On the other hand, MB would seem to have everything needed to use a complicate formula including humidity, temp, etc. to make the calculation.