Topic: Calibrating relative (corrected sea level) pressure

[Jim_S]

Altimeter is what Wunderground and ambientwether.net is expecting.

If that's true then my only problem is how to calculate my REL - ABS offset. Do I use 15 C at station elevation or at MSL? It would seem that I should be using ISA, and that's not 15 C except at MSL. But the video linked to by the MADIS QC page clearly uses 15 C at station elevation - the narrator says to leave the temperature alone, and the calculator by default uses 15 C at whatever elevation you put in.

In your OP you said, "My readings are consistently 0.7 to 0.8 mb lower". If they're that consistent my suggestion would be to adjust your readings to match so they're within +/- 0.1. Watch them for a week or two and see if it stays consistent. If it does you should be good.

[CW2274]

Then why does the video linked to on the MADIS QC page have you calculate your instrument's offset using standard sea level pressure of 1013.25 hPa? Or is that really standard altimeter pressure?

Yes, it really is standard altimeter pressure.

No. That is standard atmospheric pressure measured in hPa/mb. In inHg, it's 29.92. Has nothing to do with the method of measurement.

[elisatems]

In your OP you said, "My readings are consistently 0.7 to 0.8 mb lower". If they're that consistent my suggestion would be to adjust your readings to match so they're within +/- 0.1. Watch them for a week or two and see if it stays consistent. If it does you should be good.

Adjust which readings? REL, ABS, or both?

What I gather from that video is that you have to adjust both. I really think that if there's a problem, it's with the way I'm calculating my offset. I just calibrated my PWS the way I described in post #37, except I used my ISA temperature, 13.39 C in the calculator. Now my reported REL pressure is 0.5 mb *higher* than the MADIS calculated altimeter pressure. Next I'll try using 15 C as directed in the video and see what MADIS tells me...

[elisatems]

The question really comes down to, how do you convert from ABS to altimeter pressure? With neither temperature does the keisan calculator give the same offset that MADIS is using to calculate altimeter pressure. With 15 C at station elevation my reported readings are still 0.5 mb higher than what MADIS calculates.

[CW2274]

The question really comes down to, how do you convert from ABS to altimeter pressure?

Like I said, buy a VP2 with a Vue console. Just sayin'...

[elisatems]

In your OP you said, "My readings are consistently 0.7 to 0.8 mb lower". If they're that consistent my suggestion would be to adjust your readings to match so they're within +/- 0.1. Watch them for a week or two and see if it stays consistent. If it does you should be good.

BTW they seemed to be consistently 0.7 mb lower when I wrote the OP, but earlier today they were within about 0.2 mb. So that tells me that MADIS calculation of altimeter pressure can't be equivalent to a simple offset, and my unit will never reliably match what MADIS expects to within +/- 0.1 mb. From the CWOP manual you linked to, +/- 1 mb is within spec for CWOP. Can a simple calculated offset approximate that accuracy over a wide temperature range (here in VT, about 60 C)?

[elisatems]

The question really comes down to, how do you convert from ABS to altimeter pressure?

Like I said, buy a VP2 with a Vue console. Just sayin'...

From the prices I've seen on Davis equipment, that's a little more money than I'm willing to sink into this at the moment. I'd rather learn how to do it with what I have (assuming that's possible - the CWOP people seem to think it is).

[CW2274]

Yes. I admire your fortitude. Guarantee I'd try to.

Hopefully you didn't shop Davis stuff at Davis themselves. It can be had a good 40% less than that.

[elisatems]

Yes. I admire your fortitude. Guarantee I'd try to.

Hopefully you didn't shop Davis stuff at Davis themselves. It can be had a good 40% less than that.

I think it was weathershack.com.

Anyway, it was a PITA to buy and pound the mounting pole that I'm using into the ground and then install the sensor array on it. If I exchange it for something else, chances are I'd have to go through that all over again... and now the ground is frozen anyway.

[Jim_S]

Adjust which readings? REL, ABS, or both?

I get that we're weather geeks and that it's cool to adjust the absolute pressure so it's dead on accurate but is it really necessary? If we adjust the relative pressure to match the nearby reference stations that will account for any inaccuracy in the absolute pressure. It's the relative pressure that your station will be compared to everyone else's.

BTW they seemed to be consistently 0.7 mb lower when I wrote the OP, but earlier today they were within about 0.2 mb. So that tells me that MADIS calculation of altimeter pressure can't be equivalent to a simple offset, and my unit will never reliably match what MADIS expects to within +/- 0.1 mb. From the CWOP manual you linked to, +/- 1 mb is within spec for CWOP.

Could it be that the readings are old? That the pressure is changing rapidly? Without seeing the actual data I can't really say what's going on. Is your data online so we can look at it? What's the link?

[elisatems]

So here's something I've noticed. The difference between my readings and the MADIS calculated altitude pressure varies between 0.2 mb and 0.5 mb randomly - sometimes switching back and forth with each successive reading. My guess is it's roundoff error, it can't be the algorithm itself since the parameters are all basically the same except for minute differences in pressure.

[elisatems]

Adjust which readings? REL, ABS, or both?

I get that we're weather geeks and that it's cool to adjust the absolute pressure so it's dead on accurate but is it really necessary? If we adjust the relative pressure to match the nearby reference stations that will account for any inaccuracy in the absolute pressure. It's the relative pressure that your station will be compared to everyone else's.

But doesn't MADIS also compare my relative pressure to the altimeter pressure they calculate from my reported absolute pressure and the station elevation? That comparison reflects the algorithms used to convert absolute to relative: do they give the same answer to within some margin of error?

BTW they seemed to be consistently 0.7 mb lower when I wrote the OP, but earlier today they were within about 0.2 mb. So that tells me that MADIS calculation of altimeter pressure can't be equivalent to a simple offset, and my unit will never reliably match what MADIS expects to within +/- 0.1 mb. From the CWOP manual you linked to, +/- 1 mb is within spec for CWOP.

Could it be that the readings are old? That the pressure is changing rapidly? Without seeing the actual data I can't really say what's going on. Is your data online so we can look at it? What's the link?

No the readings are current. My CWOP ID is GW0979, or KVTNORTH90 on wunderground. And no, the pressure is fairly stable right now.

[gszlag]

Altimeter is what Wunderground and ambientwether.net is expecting.

If that's true then my only problem is how to calculate my REL - ABS offset. Do I use 15 C at station elevation or at MSL? It would seem that I should be using ISA, and that's not 15 C except at MSL. But the video linked to by the MADIS QC page clearly uses 15 C at station elevation - the narrator says to leave the temperature alone, and the calculator by default uses 15 C at whatever elevation you put in.

Despite what CWOP tells you, if you use 15C at your elevation that is incorrect. You want to set 15C at sea level.
When you set the elevation to 0m in the keisan T & P at destination calculator you effectively changed it to a ISA calculator so that is where you calculated your ISA temperature and ISA pressure from. In retrospect, I should have recommended the ISA calculator right off the bat. It is more intuitive.

If you want to use CWOP you have to calibrate your weather station to Altimeter. Altimeter is calibrated using strictly ISA.standard conditions.  For a 250m elevation:

ISA Pressure @ 0m =1013.25
ISA Temperature @ 0m =15C
ISA Temperature @ 250m= 13.38C

You still can use the keisan calculator that CWOP suggested - just change the 15C to 13.38.
It's all the same equation - just re-arranged.

Picturers are worth..

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[elisatems]

Yes, based on the NOAA definition of altimeter pressure, I came to the same conclusion: I must use ISA temperature at MY elevation, which is equivalent to using 15 C at sea level in the T and P at destination calculator and using 13.39 C* in the P from altitude calculator. According to the definition, altimeter uses a completely standard temperature profile, which means ISA at any altitude, or 15 C at MSL and the standard lapse rate.

Actually the resulting offset corrections for my elevation differ by a minuscule amount, 29.28 mb vs 29.45, but still, might as well do it correctly. Either way, there will still be at least a few tenths of a hPa difference between my reported pressure and what MADIS calculates, and I'm not sure why. MADIS calculates altimeter from MY reported ABS and station elevation so the two should agree to within the precision of whatever the internal numerical representation is, modulo any roundoff error. I'm using hPa to 2 decimal places and rounding to the nearest tenth, since that is the finest resolution I can achieve on my 2902C console. I have no idea what MADIS is doing.

*I rechecked my station elevation (console) and it's actually 248 m not 250. The map shows a 245 m plateau at street level where I'm located, and I'm on that plateau about 10 ft or 3 meters above ground level. That's why 13.39 C for ISA instead of 13.38.

[elisatems]

I should add, though I'm rounding hPa to 1 decimal place, another source of error is that the mesonet obs reports altimeter setting in in Hg to 2 decimal places, which I then convert to hPa. From there I use full internal calculator precision which I think is 9 decimal places on a HP-35s. 0.01 in Hg is coarser resolution than 0.1 hPa so that's another possible source of error... but that should have nothing to do with the comparison between my reported altimeter pressure and MADIS's calculated altimeter pressure, since that uses whatever my reported ABS pressure is. I suppose it's possible that my console reports ABS with greater precision than it displays for me. I just don't know.

[elisatems]

So here's a typical MADIS report on my station, after calibrating REL to altitude pressure using ISA. (ABS was calibrated using the latest 5 minute altimeter report from KMPV, converted to hPa, minus my calculated offset):

Last WX Update    12/22/2021 09:08:33 EST (54s ago)
Temperature    29.8 °F
Humidity    89%
Barometer (REL)    29.59 inHg (1002 mb)
Barometer (ABS)    28.72 inHg (972.6 mb)
Pressure (Altimeter)    29.58 inHg (1001.3 mb) (Calculated from ABS and Elevation)
Wind    253° (WSW) at 1.3 mph, gusting 2.2 mph
Rain today    0.00 in
Rain rate    0.00 in/hr
Solar Radiation    27.54 W/m²

According to that, my REL pressure is 0.7 hPa high. It was closer to MADIS when I was using 15 C at 248 m to calculate the offset. What's the difference between MADIS's algorithm and what I'm doing? For the record, my calculated offset comes to 29.45 hPa (29.46 if I use 814 ft = 248.1 m, which is my station elevation in MADIS).

[gszlag]

So here's a typical MADIS report on my station, after calibrating REL to altitude pressure using ISA. (ABS was calibrated using the latest 5 minute altimeter report from KMPV, converted to hPa, minus my calculated offset):

Last WX Update    12/22/2021 09:08:33 EST (54s ago)
Temperature    29.8 °F
Humidity    89%
Barometer (REL)    29.59 inHg (1002 mb)
Barometer (ABS)    28.72 inHg (972.6 mb)
Pressure (Altimeter)    29.58 inHg (1001.3 mb) (Calculated from ABS and Elevation)
Wind    253° (WSW) at 1.3 mph, gusting 2.2 mph
Rain today    0.00 in
Rain rate    0.00 in/hr
Solar Radiation    27.54 W/m²

According to that, my REL pressure is 0.7 hPa high. It was closer to MADIS when I was using 15 C at 248 m to calculate the offset. What's the difference between MADIS's algorithm and what I'm doing? For the record, my calculated offset comes to 29.45 hPa (29.46 if I use 814 ft = 248.1 m, which is my station elevation in MADIS).

Is your barometric sensor at 248.1 elevation? or 250? I believe the sensor is in your console so use your console elevation.
For North America, MADIS probably uses this Altimeter calculation:
https://www.weather.gov/epz/wxcalc_altimetersetting
If you are consistently < 1.0hPa you are efectively calibrated for the equipment you have. Remember, you a using a fixed linear offset to match a non-linear pressure.It is an estimate. If you need better precison you need weather software like WeeWX that uses the actual formulas.

[elisatems]

Yes, I answered my own question using information from here:

https://www.weather.gov/epz/wxcalc_altimetersetting (same as the link you gave)

That formula is apparently what MADIS is using and, as you say, using a fixed offset is only an estimate. The effective "offset" at my elevation (248 m, as I said, after carefully scanning the vicinity of my location on the map and adding 3 m because I'm on the 2nd floor - I gave MADIS 814 ft because they want your elevation in integer feet, so they have my elevation as 248.1 m) varies from about 28.9 mb to 29.9 mb over a roughly 40 mb range of station pressures. At the current pressure, it's closer to 28.8 mb whereas I'm using 29.5 (because of the 0.1 mb resolution), which accounts for the 0.7 mb discrepancy.

That said, maybe I should calibrate my equipment when the pressure at my site is "about average", say an altimeter setting of about 29.92 in Hg. That way I'll minimize the error in my reported altimeter setting relative to MADIS.

Or... instead of using the estimated offset from the keisan calculators, would it make sense to simply calculate an offset from an average station pressure (using the formula) and use that to calibrate my station? Or should I wait for an "average" day and then calibrate using the estimated offset?

Edit: actually, I think I'm already using an offset calculated at an "average" pressure, by using ISA conditions (temp and pressure) in the keisan calculator. Right now I'm reporting high relative to MADIS because the pressure is lower than "average" (ISA standard); when it's higher than "average", I should be reporting low.

But to set my ABS, I think I should wait for an "average" day, or else take the console to the airport and set it there using reported station pressure.

At least, I *think* that's right. If I have it wrong, please correct me.

[CW2274]

That said, maybe I should calibrate my equipment when the pressure at my site is "about average", say an altimeter setting of about 29.92 in Hg. That way I'll minimize the error in my reported altimeter setting relative to MADIS.

Right now I'm reporting high relative to MADIS because the pressure is lower than "average" (ISA standard); when it's higher than "average", I should be reporting low.

Before I bought my Vue console this is just how I did my VP2 console because it's not altimeter capable as is the Vue. I set my elevation to zero (I'm actually at 2421' MSL), wait for benign weather and pressure around 29.92". That way it would track the altimeter quite nicely til getting into higher and lower pressures where the discrepancy would become greater. Higher, I would be too low. Lower, too high. It was better than nothing. For this reason is exactly why I bought the Vue. I now have two.

[elisatems]

That said, maybe I should calibrate my equipment when the pressure at my site is "about average", say an altimeter setting of about 29.92 in Hg. That way I'll minimize the error in my reported altimeter setting relative to MADIS.

Right now I'm reporting high relative to MADIS because the pressure is lower than "average" (ISA standard); when it's higher than "average", I should be reporting low.

Before I bought my Vue console this is just how I did my VP2 console because it's not altimeter capable as is the Vue. I set my elevation to zero (I'm actually at 2421' MSL), wait for benign weather and pressure around 29.92". That way it would track the altimeter quite nicely til getting into higher and lower pressures where the discrepancy would become greater. Higher, I would be too low. Lower, too high. It was better than nothing. For this reason is exactly why I bought the Vue. I now have two.

Let me guess: the Vue console is only compatible with Davis equipment, right? Can't be made to work with Ambient? For me that's a non-starter right now, am just not willing to sink $600 + for a top of the line weather station (aviation is expensive already, you know... )

But right, the greater the difference between the actual pressure and the one you use to calibrate your offset, the greater the discrepancy is going to be. But what I'm wondering is whether it might be just as good to take the console right to the source (KMPV) and set ABS then and there. It will still only track the altimeter setting really well near 29.92", but at least I'll have station (absolute) pressure accurately calibrated without waiting for a 29.92" day. At least, that's what I'm thinking...

[CW2274]

That said, maybe I should calibrate my equipment when the pressure at my site is "about average", say an altimeter setting of about 29.92 in Hg. That way I'll minimize the error in my reported altimeter setting relative to MADIS.

Right now I'm reporting high relative to MADIS because the pressure is lower than "average" (ISA standard); when it's higher than "average", I should be reporting low.

Before I bought my Vue console this is just how I did my VP2 console because it's not altimeter capable as is the Vue. I set my elevation to zero (I'm actually at 2421' MSL), wait for benign weather and pressure around 29.92". That way it would track the altimeter quite nicely til getting into higher and lower pressures where the discrepancy would become greater. Higher, I would be too low. Lower, too high. It was better than nothing. For this reason is exactly why I bought the Vue. I now have two.

Let me guess: the Vue console is only compatible with Davis equipment, right? Can't be made to work with Ambient?

 it might be just as good to take the console right to the source (KMPV) and set ABS then and there.

Yeah, you're outta luck there and that's also what I suggested yesterday. Certainly wouldn't hurt to try.

BTW, if I may ask, what do you fly?

[elisatems]

BTW, if I may ask, what do you fly?

A 1976 Cessna Cardinal RG. I've owned it for about 11 years now, never tire of flying it, though I wish I had more time to do that (and also wish that the state would plow the parking lot at the hangar more often in the winter so that I could access the plane when I want - most of the time there's over 2 feet of snow on it and the taxiways are too icy to drive - or taxi - safely on).

[CW2274]

BTW, if I may ask, what do you fly?

A 1976 Cessna Cardinal RG.

OMG....the most beautiful Cessna ever built! I remember when they first came out, the styling was a little "aggressive" for some. Being an RG makes even that much sexier! You, are a lucky man!

[elisatems]

BTW, if I may ask, what do you fly?

A 1976 Cessna Cardinal RG.

OMG....the most beautiful Cessna ever built! I remember when they first came out, the styling was a little "aggressive" for some. Being an RG makes even that much sexier! You, are a lucky man!

Haha! You're half right... I'm lucky but I'm not a man. 

Anyway yes, it's a beautiful plane, very sleek and aerodynamic-looking. Not to put Cardinals down too much, but some of those looks are deceiving. Mooney gets much better performance out of the same engine. The 177RG is definitely not the fastest 4-place RG around, even from Cessna, and there are times I wish I'd bought a faster plane. What it has going for it is stability: it's a superb instrument platform, and I have a pretty good, if rather dated, panel for IFR.

When I first bought it, I lived in Michigan, where the MEAs are low enough that I could find days even in the winter when it was safe to go out and shoot approaches in actual IMC for practice. Here they're so high that the clouds are pretty much off-limits from October through April or even May unless your bird is certified for Flight Into Known Icing (FIKI)... and mine definitely isn't.

[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.

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.