Author Topic: WS-2902A WS-2000 ObserverIP How to Calibrate for Sea Level Pressure or Altimeter  (Read 155 times)

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Offline galfert

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Here is how I calibrate my Ambient console pressure.

It is best to do this on a clear day when the pressure is close to 1013 hpa or 29.92 inHg and you want to do this between 11am and 1pm when the pressure is most level (not rising nor falling) for the day. But this is something you don't have to wait for. You can do initial calibration with whatever weather you have now or whatever time it is now and then fine tune another day.

First thing to do is switch your console to display hpa instead of inHg. It will be more precise during the calibration steps and after calibration if you still prefer inHg then you can just switch units back to inhg, and everything will stay calibrated. So stick to hpa for calibration, you'll see why later.

The first thing we need to calculate is the offset based on your elevation. This is referred to as the relative pressure offset. Basically you are telling the console your elevation indirectly by entering in this offset. Some other brand manufactures ask you for the elevation in feet or meters which is simpler than what we have to do. So with Ambient you have more work to do, and you have to do the offset calculation manually in order to provide it to the console.

I use this website to figure out the offset:
https://keisan.casio.com/exec/system/1224575267

Lets use the example that your elevation is 57 feet. Enter in your elevation into that website calculator in meters. 57 feet = 17.4 meters. That website requires all units to be entered in metric units. Enter in your stations absolute pressure to that website, even though it isn't calibrated yet, it doesn't really matter. We just need a number. Actually any pressure you enter will work because at this point we are only interested in finding out the offset. If our pressure is 29.78 then convert 29.78 inHg = 1008.5 hpa, because again that web calculator needs metric.
I use this website to convert between inHg and hpa.
https://www.convertunits.com/from/hpa/to/inhg

Lastly in the Keisan SLP website just leave the temperature at 15 C because we aren't really interested in finding out true Sea Level Pressure right now ...we just want the offset that will be carried across to all temperatures in your console and 15 C (59F) is standard ISA temperature. Now click the Execute button for the results. The results will show you 1010.58 hpa for SLP. That number doesn't mean anything right now. What matters is the difference between your absolute station pressure 1008.5 hpa and the calculated SLP result 1010.6 hpa (I rounded to tenths), and the answer is +2.1 hpa difference. So +2.1 hpa is the offset (difference) between your consoles absolute and relative pressure. In other words +2.1 hpa represents the elevation difference between 57 feet and see level (0 ft).

In your console enter in the +2.1 offset for relative pressure by moving up or down the relative pressure to be this much more than the current absolute pressure. We aren't calibrated yet and so it doesn't matter what absolute nor what relative ends up showing at this point. For now we are only interested in having it so that the difference between absolute and relative is +2.1 hpa. This difference amount between absolute and relative will never change. Unless you move your console to the second floor of your home and then you will need to correct for 57 + whatever more the elevation is at the second floor. Make sure that your 57 feet that you are using for elevation is not ground level (as in the floor) but rather where your console is now on a table or shelf. So that 57 feet should correspond to the elevation of the console. So if it is 3 feet off the ground on a counter and your ground level is 57 feet, then your real elevation for the console is 60 feet (remember to convert to metric for entering in to the web calculator). So now go back and redo the calculation with the proper elevation of your pressure sensor. Some people mistake the elevation to that of the outdoor sensor. That isn't where the pressure sensor is located. The pressure sensor is in the console of the WS-2902A, or in the separate indoor module in the case of the WS-2000 and in the case of the ObseverIP.

Now that the offset is entered in your device you can check out what the console is reporting as the relative pressure. Compare that to your local airport. Figure out how much your relative pressure is different than your local airport. You can calibrate to the airport's METAR Altimeter pressure or to the METAR Sea Level Pressure. I would recommend calibrating to the airport's Sea Level Pressure since you will be reporting to Weather Underground and not to CWOP with your station. As Weather Underground uses SLP and CWOP uses Altimeter. But the choice is yours if you still prefer your station to be set to Altimeter instead of SLP. For example lets say that your console is showing relative = 1009.4 and the airport is at 1010.1 (for ALT or SLP whichever you are targeting), then you need to adjust by that difference. BUT DO NOT change the relative offset of 2.1 hpa that we already set. You bring up or down your relative pressure to match the airport by moving up or down your absolute (station) pressure by the desired amount. In my example you would bump up the absolute by 0.7 hpa  (1010.1 - 1009.4 = 0.7). The thing to realize is that you are changing absolute and so you need to know how much to move up or down and you aren't matching absolute to the airport. By way of changing absolute you effectively are changing relative (since there is a fixed offset that we put in).

That is it! Well almost. Now you need to watch it and fine tune on a better day if the weather or time of day was not perfect. Remember the fine tuning is done by moving up or down the absolute station pressure as that moves the relative up or down too by the same amount. That is why it is an offset and it matches or corresponds to your elevation. Never change the offset because your elevation doesn't change unless you move the console like I mentioned upstairs or to a taller or lower shelf or table.

Lastly you can switch back to inHg after all these steps. But you'll want to keep it at hpa for a while to monitor and fine tune. The reason for doing it all with hpa rather than inHg is because 0.01 inHg = 0.34 hpa. So for every .01 inHg that you move up or down then you are moving up or down by 0.34 hpa. So you can be more precise if you can move up or down by 0.1 hpa which equals 0.00295 inHg and that is a level of precision that you can't adjust with our equipment using inHg directly. There is nothing inherently wrong with inHg if Ambient had given us an extra decimal place and then inHg reported at .001 inHg decimal places would be more precise than 0.1 hpa (as it represents 0.034 hpa). But calibrating at 0.001 inHg would be pointless because there is nothing reporting inHg to that level of precision (common sources) to compare to and calibrate with. Realize that internally the conversion is happening to a much greater degree of precision and then things get rounded before you see them on the display. If you tried to calibrate using inHg you would find that you would be constantly needing to fine tune up or down and chasing an adjustment that would just seem impossible. Do it with using hpa and you'll remove that frustration. But knowing this you'll realize that if you kept it showing hpa instead of inHg that you are seeing that much more precise of a pressure. Also on the WS-2902A display the barometer trend +- numbers next to the current pressure would be more meaningful as those + and - numbers are referring to hpa. I'm referring to this little scale:

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If all that is too complicated there is another way of doing it. You make the offset zero or basically absolute is made to match and be the same as relative. It is like telling the console that you are at sea level. Then you calibrate the absolute to match the airport's Altimeter or Sea Level Pressure (whichever you are targeting) and then you don't have to do any of the work of figuring out the offset. You've removed the offset by keeping relative equal to absolute. But then you'll never know what your real absolute station pressure is as you will only really be able to see your adjusted pressure, but some would argue that it is not that meaningful to need real station pressure. I still prefer to do it right. I don't know if there are any repercussions from taking this approach in the different reporting services if any of them need station pressure. But even if that isn't the case now it may change. But this is still an interesting option to consider even if you don't do it because I think it helps the understanding of the concepts of what we are doing.

UPDATE: I just realized that if you want to calibrate to Altimeter instead of Sea Level Pressure then the offset calculated above may not be right and be a bit off. As we used a formula to calculate the offset for Sea Level not an offset for Altimeter. There is an easy fix though. Just look at the airport's METAR and see how much the Altimeter differs from Sea Level pressure and factor in that correction to your final offset. Basically you still use your given elevation but then change the resulting offset by however much more or less the METAR Altimeter is than SLP.
« Last Edit: October 14, 2018, 07:05:06 AM by galfert »
WS-2902A | ObserverIP | WeatherBridge (Meteobridge)
WU: KFLWINTE111  |  PWSweather: KFLWINTE111
CWOP: FW3708  |  AWEKAS: 14814
Tele-Pole flag pole is here (not installed yet)

Offline cjtamu

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Thank you, sir.

Offline galfert

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For those of you with an ObserverIP you are basically doing the same thing in the calibration screen.

If you intend to report to CWOP with a Meteobridge , the Meteobridge will calculate Altimeter from your absolute station pressure on your ObserverIP. I haven't done it yet but it probably is the same thing for any other software (WeeWx or Weather Display ..etc). Which is why I say calibrate to Sea Level Pressure and then those other software programs that send to CWOP will do the Altimeter calculation for the purpose of sending to CWOP. Unless of course you prefer to always just see Altimeter and never care about Sea Level Pressure.
« Last Edit: October 13, 2018, 04:22:57 PM by galfert »
WS-2902A | ObserverIP | WeatherBridge (Meteobridge)
WU: KFLWINTE111  |  PWSweather: KFLWINTE111
CWOP: FW3708  |  AWEKAS: 14814
Tele-Pole flag pole is here (not installed yet)