Topic: Calibrating relative (corrected sea level) pressure

[elisatems]

I knew it, I knew it, I knew it.    Seeing your user name, or whatever it's called, I wondered, but said man anyway. A thousand apologies!

FYI, Mooney drivers are nuts. Just sayin'...

I'm very glad you have the wherewithal to avoid potential icing conditions, as many, especially here in AZ, think that can't happen to them. Wrong. Every winter during colder storms I'd have to bail out "dinks" that had no de-icing, well, other than carb heat. Doesn't do much for weight and drag. PIC lesson learned.

No worries!

"Bail out..."? Sounds like you must work in ATC... if so, thank you for all that you do for us pilots! But yes, I can imagine how pilots who live somewhere it's usually warm like AZ might get complacent and think that ice can never happen to them, but... ice is where you find it, and during the winter months you can find it just about anywhere. Here in the northeast it's a constant presence, I don't think anyone gets complacent about ice (at least, I hope not). Even so, most people have a story to tell.

Once, soon after moving out here, I was flying down to Nashua, NH (KASH) in December. I filed IFR on a day when it was clear up in VT, but there was a stratocu deck just below the MEA right around where I'd enter NH. There was an Airmet Zulu out, and a few PIREPs for icing. As I came up to the layer a little voice told me to cancel and duck under it, but it didn't look very thick, I was above it (though barely), and I decided to just tell ATC I didn't want to linger inside it and please just give me a clean let-down when it was time to bring me down. I couldn't have taken more than 30 seconds to get through it, and I didn't notice any buildup on any surfaces, no degradation of performance, landed without incident... but as soon as I got out of the plane I could clearly see about a quarter inch of ice on the wings in the process of melting off - VERY slowly because it was just above freezing on the ground. Lesson learned!!

BTW, just because you don't have a glass cockpit, doesn't mean it's obsolete. Kinda like when everyone complains about Davis's consoles being archaic looking, which they are, but still does the job flawlessly.

Oh I know! I have no complaints about my panel, in fact it has some instruments that used to be considered "aftermarket glass"... a Sandel 3308, and a GMX-200. My GPS is a CNX-80 a.k.a. GNS480, so WAAS-capable, a must-have today. It's just not state of the art but that's okay, like you say it does the job flawlessly.

[CW2274]

I knew it, I knew it, I knew it.    Seeing your user name, or whatever it's called, I wondered, but said man anyway. A thousand apologies!

FYI, Mooney drivers are nuts. Just sayin'...

I'm very glad you have the wherewithal to avoid potential icing conditions, as many, especially here in AZ, think that can't happen to them. Wrong. Every winter during colder storms I'd have to bail out "dinks" that had no de-icing, well, other than carb heat. Doesn't do much for weight and drag. PIC lesson learned.

a little voice told me to cancel and duck under it, but it didn't look very thick, I was above it (though barely), and I decided to just tell ATC I didn't want to linger inside it and please just give me a clean let-down when it was time to bring me down. I couldn't have taken more than 30 seconds to get through it, and I didn't notice any buildup on any surfaces, no degradation of performance, landed without incident... but as soon as I got out of the plane I could clearly see about a quarter inch of ice on the wings in the process of melting off - VERY slowly because it was just above freezing on the ground. Lesson learned!!

Ah yes, hindsight being 20/20, canceling IFR to get below the deck would had definitely been the way to go. You can always pick up a "pop up" IFR later if need as well. No idea what the MVA's are like where you fly, but never hesitate to ask for lower and/or a vector out of the weather if available. Many times we'll issue an altitude above what is actually available just for the sake of ease of radio transmission, like saying six thousand when actually five thousand seven hundred is available. Can make all the difference in the world in flight conditions. Glad your descent was on your terms and not the ice's!

[Randall Kayfes]

"Current" airport data is not necessarily needed, recent data should do you just fine - let me explain. All you need is a recent data point logged at the the same time as one of your data log times. Even if it is only logged once an hour at the airport. Go back an hour in your logs and you can still calculate the differential between the two readings. As long as the log times matchup you can see the difference. Looking at it that way you can compare several readings for differential and obtain a higher level of accuracy. That could be helpful in widening your choices of calibration data.

BTW here in Southern Arizona with summers averaging 100°F and Winters in the 60's our barometers are nothing but a pain to keep accurate.

[CW2274]

BTW here in Southern Arizona with summers averaging 100°F and Winters in the 60's our barometers are nothing but a pain to keep accurate.

I never adjust my pressure. No need to do so.

[Randall Kayfes]

Greetings, yes that Cessna with retractable landing gear looks killer. From there though CW (ATC wow what a broad skillset you must hold to do that job), you said on our Davis VP2's you set the elevation to zero on 29.92 day and then let it go sounds great to me as adjusting twice a year is a pain. I just want to make sure that is what you said? I can't afford another console (Vue)

[CW2274]

Greetings, yes that Cessna with retractable landing gear looks killer. From there though CW (ATC wow what a broad skillset you must hold to do that job), you said on our Davis VP2's you set the elevation to zero on 29.92 day and then let it go sounds great to me as adjusting twice a year is a pain. I just want to make sure that is what you said? I can't afford another console (Vue)

Yeah, with the VP2, that's exactly what I did and lived with it being too high in low pressure and too low in high pressure. Hence the Vue console and the altimeter setting being available. It's a complete pressure headache cure. I'd save up. BTW, it doesn't have to be exactly 29.92, just the in the ballpark with benign WX conditions.

[Randall Kayfes]

CW, thank you, I will have to give that a go. BTW if I get a Vue console (currently I am using an Envoy) does it pass on the solar and UV data and will it handle my sonic wind sensor.

[CW2274]

BTW if I get a Vue console (currently I am using an Envoy) does it pass on the solar and UV data and will it handle my sonic wind sensor.

Solar/UV, I don't think so...the wind sensor shouldn't matter.

[PaulMy]

Hi Randall,

BTW if I get a Vue console (currently I am using an Envoy) does it pass on the solar and UV data and will it handle my sonic wind sensor.

I have the VP2 Plus ISS and Envoy/USB logger, and also a Vue console with WiFiLogger sending to WL.com and to my CumulusMX and Solar and UV is logged and sent to WL.com https://www.weatherlink.com/bulletin/a3a3c5e7-b8bd-4910-ba3a-f6ad6e1f09d1


Sorry, I don't have the sonic wind sensor so can't comment on that.


Enjoy,
Paul

[mcrossley]

Sorry, I don't have the sonic wind sensor so can't comment on that.

It's just a wind sensor, no different from any other data wise.

[Randall Kayfes]

So here is the real question I did not make very clear in the beginning:

The Vue does not include the solar UV suite like my VP2+ does. So, if I use a Vue Console (sans the Vue ISS) and the Vue Console holds/interrogates my data logger and then reports to my WL PC and my WL PC reports to WL Live will the Vue console know enough to push the solar/UV data to my WL PC even though the console will not display it.

[PaulMy]

Hi Randall,

and my WL PC reports to WL Live

I presume you mean "reports to WL.com"

The Vue does not include the solar UV suite like my VP2+ does. So, if I use a Vue Console (sans the Vue ISS) and the Vue Console holds/interrogates my data logger and then reports to my WL PC and my WL PC reports to WL Live will the Vue console know enough to push the solar/UV data to my WL PC even though the console will not display it.

I don't have the same configuraion as you (Vue to WLPC) as mine is Vue console with IP logger to WL.com.  But the View console does receive the solar and UV from the VP2 ISS Plus and sends it to WL.com.  I can view all the Vue console data through the IP and that includes Solar and UV. 
I presume the Vue console will do the same to WLPC.  Hopefully someone with the same configuration as you mention can confirm that.


Enjoy,
Paul

[Randall Kayfes]

That answers my questions. Thank you Paul

[Buick]

I have a VP2 and am sending data to CWOP (GW1826) via WiFiLogger. Gladstonefamily indicates that my barometer is reading 1.6mb high despite being calibrated to match SLP of the nearest airport (BLI). The ASOS/AWOS is ~2mi from my station. Is is correct that the SLP at my station should match that at BLI rather than what the Gladstonefamily analysis indicates?

Thanks!

[CW2274]

Forget Gladstone. Compare here.

https://mesowest.utah.edu/cgi-bin/droman/meso_base.cgi?stn=KBLI&unit=0&time=LOCAL&year1=2022&month1=5&day1=24&hour1=13&hours=24&graph=0&past=0

https://mesowest.utah.edu/cgi-bin/droman/meso_base.cgi?stn=G1826&unit=0&time=LOCAL&year1=2022&month1=5&day1=24&hour1=13&hours=24&graph=0&past=0

BTW, little surprised you're sending obs every minute. Not that I care, but I thought that was a no-no. Every five was max. Anyway...

[Buick]

I'll change update frequency to every 5min shortly. Is it normal for SLP and altimeter to be the same? Also, I see that something was flagged as suspect but I can't seem to figure out what. Any advice?

[CW2274]

Is it normal for SLP and altimeter to be the same?

 Also, I see that something was flagged as suspect but I can't seem to figure out what. Any advice?

You're very close to sea level, so your SLP and altimeter are more prone to be close together, temperature being a consideration.

As far as being flagged, for some weird reason your average wind speed was 4mph, while you gust was 3mph. Haven't seen that before. It may well have to do with your one minute update getting screwy with the wind and how it's extrapolated, but that's a guess. 

[Buick]

Interesting. I can see why that would have been flagged. I've changed update frequency to every 5 min now so hopefully there's no recurrence of erroneous data.

[gszlag]

..I s it normal for SLP and altimeter to be the same?...

I am assuming that you are uploading Altimeter to CWOP?
CW2274 is correct... Altimeter = SLP at sea level elvation. You are 38 meters
elevation so you are pretty close although there still could be a difference due to additional temperature and perhaps humidity corrections.
Your Altimeter should be matching your airport's Altimeter reading too.
You can use this online calculator to verify Altimeter values and run comparisons with your station and KBLI:
https://www.weather.gov/epz/wxcalc_altimetersetting

[Buick]

Appreciate the responses! MesoWest shows my altimeter matching BLI so I think I've got it dialed in. I did some more tinkering using the barometer in my phone and several apps (to check for consistency) into which one can input a reference elevation and temperature to convert the station pressure to SLP. I'm now reading ~.6mb (averaged over the preceding 8 hours) lower than BLI. Since the SLP calculation requires information about temperature and humidity and there are differences in the temperature and humidity between my site and BLI (eg at 0753PDT BLI reported 54F, 100%rh, 54F dewpoint while my station reported 51.7F, 95%rh, 50.3F dewpoint), does the difference in SLP between my site and BLI make sense?

[gszlag]

Appreciate the responses! MesoWest shows my altimeter matching BLI so I think I've got it dialed in. I did some more tinkering using the barometer in my phone and several apps (to check for consistency) into which one can input a reference elevation and temperature to convert the station pressure to SLP. I'm now reading ~.6mb (averaged over the preceding 8 hours) lower than BLI. Since the SLP calculation requires information about temperature and humidity and there are differences in the temperature and humidity between my site and BLI (eg at 0753PDT BLI reported 54F, 100%rh, 54F dewpoint while my station reported 51.7F, 95%rh, 50.3F dewpoint), does the difference in SLP between my site and BLI make sense?

For SLP you would need to calculate the 12 hour average/mean temp for both you and KBLI. (current temp + 12 hour ago temp ) divided by 2.

If they are different then you will not match as the temperature offsets will be different for each location. Humidity has a minimal impact maybe 0.1 mb or so.

Your console has the ability to calculate SLP but the Davis consoles use an algorithm that is a close match to a ASOS/AWOS but it is not exactly the same. We can also say that the algorithm used by AWOS approximates the WMO mathematical pressure reduction formulas but are not exact either.

You mentioned that you are matching Altimeter - probably a lot closer than SLP. This tells us that your barometric sensor's station pressure is indeed calibrated properly. Your weather station is doing its job!

You can get more accurate results - maybe by buying a Vaisala barometer but if you are calibrated well within 1.0 mb SLP - you are calibrated.

[Buick]

Sweet - thanks for your help!

[CW2274]

Appreciate the responses! MesoWest shows my altimeter matching BLI so I think I've got it dialed in. I did some more tinkering using the barometer in my phone and several apps (to check for consistency) into which one can input a reference elevation and temperature to convert the station pressure to SLP. I'm now reading ~.6mb (averaged over the preceding 8 hours) lower than BLI. Since the SLP calculation requires information about temperature and humidity and there are differences in the temperature and humidity between my site and BLI (eg at 0753PDT BLI reported 54F, 100%rh, 54F dewpoint while my station reported 51.7F, 95%rh, 50.3F dewpoint), does the difference in SLP between my site and BLI make sense?

Your console has the ability to calculate SLP but the Davis consoles use an algorithm that is a close match to a ASOS/AWOS but it is not exactly the same.

For the OP's clarification, the VP2 console will do SLP, but not the altimeter setting. You would need a Vue console for that if you want a true comparison with the ASOS/AWOS altimeter.  You can get close with the VP2, but it's not apples to apples, especially in higher and lower pressures. I recently talked one member here into buying a Vue console for just that purpose.

[Buick]

You're right - there does appear to be a discrepancy between the altimeter setting that WiFiLogger is reporting to CWOP and the value I get when I plug my station pressure into the altimeter calculator but it only amounts to 0.01inHg...at least in the current conditions. Over the past 18hr or so the mean difference in altimeter setting between my station and BLI is very small - less than 0.01inHg - so I'm satisfied for the time being. I'm already planning on adding additional sensors to my current setup (Rainwise 111, Davis 7714 and 6415) as well as WeatherLink Live to handle the new data so I'll probably have to wait on a Vue console but it's tentatively on my list.

[CW2274]

If you want the VP2 to reflect the altimeter as close as possible, wait til the ASOS is "around" 29.92 inHg in benign-ish conditions (not windy from pressure gradient) and when the obs comes out, which is usually just before the hour. Also set the console elevation to zero ft. It works well til it doesn't.