Topic: Calibrating relative (corrected sea level) pressure

[CW2274]

The only thing worse than not being able to enter ones elevation in a PWS is not having a battery backup. Ed can say what he wants.

[elisatems]

Can't cha take ten out?

Sure, but why would that be better than just setting ABS and using the computed offset back home? How much does the offset depend on other factors like temperature?

In fact, why not take ABS from the airport, correct it for the elevation difference using the actual temperature instead of ISA, and set ABS here at my station?

[CW2274]

Honestly, I don't know. Just seeing what might stick. Like I said, I never deal with this so I'm probably out in left field more than not.

[elisatems]

So here's something I don't understand. The latest SLP from KMPV is 1016.3 hPa. The airport elevation is 1165 ft MSL or 355 meters. If I go to the keisan T and P at dest calculator and plug in 355 meters and -5 C (current temperature there) for both current location and destination, I get a station pressure of 971.54 hPa. That converts to 28.696 inHg. But the reported station pressure is 28.73. Did I do this wrong?

[CW2274]

That converts to 28.696 inHg. But the reported station pressure is 28.73. Did I do this wrong?

All I can tell you is that's only 34' difference. Well within the spec for your PWS.

[elisatems]

That converts to 28.696 inHg. But the reported station pressure is 28.73. Did I do this wrong?

All I can tell you is that's only 34' difference. Well within the spec for your PWS.

Yeah I'm sure that's true. I'd just like to understand how they're calculating SLP, that's all.

[CW2274]

You can always call your local WFO for some professional guidance.

[elisatems]

I didn't think of doing that. Maybe I should try it, thanks.

[Jim_S]

Yeah I'm sure that's true. I'd just like to understand how they're calculating SLP, that's all.

Looking at your post you used the current temperature (-5 deg C). The weather service SLP calculation uses the mean temperature over the last 12 hrs. That could account for the discrepancy.

My main question is why are you calibrating to SLP? I thought CWOP stations were supposed to report altimeter not SLP.
Here is a link to a CWOP manual that may be useful.

[elisatems]

Yeah I'm sure that's true. I'd just like to understand how they're calculating SLP, that's all.

Looking at your post you used the current temperature (-5 deg C). The weather service SLP calculation uses the mean temperature over the last 12 hrs. That could account for the discrepancy.

I'll bet that explains the discrepancy. The average temperature over the last 12 hours was close to 0 C, and I determined that using 0 C in the calculator would get me very close to the reported station pressure.

My main question is why are you calibrating to SLP? I thought CWOP stations were supposed to report altimeter not SLP.
Here is a link to a CWOP manual that may be useful.

Well that may be a showstopper. I certainly don't want to be reporting altimeter on ambientweather and wunderground, and the WS-2902C can only report one pressure. I'm not sure it's even possible to calibrate the unit to report altimeter as it can only handle a fixed offset from ABS. If there's no workaround, I probably should just cancel my request for a MADIS ID then, if that's possible.

[sky_watcher]

Yeah I'm sure that's true. I'd just like to understand how they're calculating SLP, that's all.

Looking at your post you used the current temperature (-5 deg C). The weather service SLP calculation uses the mean temperature over the last 12 hrs. That could account for the discrepancy.

My main question is why are you calibrating to SLP? I thought CWOP stations were supposed to report altimeter not SLP.
Here is a link to a CWOP manual that may be useful.

The CWOP handbook referenced says "Altimeter (QNH) Pressure - Altimeter pressure is a corrected pressure value that includes the pressure between the station’s elevation and sea level." That is the observed pressure adjusted to Seal Level pressure.

From Australian Bureau of Meteorology "The acronym QNH is one of the Q(uestion) code names developed, circa 1909, for use in morse code. To concisely ask for atmospheric pressure at mean sea level (MSL), the operator would transmit the letters QNH. This was understood to mean "I have a question. What is the atmospheric pressure at Nil Height", i.e. at mean sea level."

I don't think there is a problem here as QNH is a global term.

[CW2274]

Yeah I'm sure that's true. I'd just like to understand how they're calculating SLP, that's all.

Looking at your post you used the current temperature (-5 deg C). The weather service SLP calculation uses the mean temperature over the last 12 hrs. That could account for the discrepancy.

I probably should just cancel my request for a MADIS ID

Not to be flip, and I suggest if you have the means, return/sell your Ambient and get a Davis VP2 with a Vue console. Yes, more money, but it is an incremental step up. With a Davis PWS, you can enter elevation and all theses pressure hoops are eliminated. Not only, the Vue console has the reduction algorithm for the altimeter setting, which is exactly what I use. Merely something you may want to consider.

[elisatems]

Yeah I'm sure that's true. I'd just like to understand how they're calculating SLP, that's all.

Looking at your post you used the current temperature (-5 deg C). The weather service SLP calculation uses the mean temperature over the last 12 hrs. That could account for the discrepancy.

My main question is why are you calibrating to SLP? I thought CWOP stations were supposed to report altimeter not SLP.
Here is a link to a CWOP manual that may be useful.

My main question is why are you calibrating to SLP? I thought CWOP stations were supposed to report altimeter not SLP.
Here is a link to a CWOP manual that may be useful.

I looked more carefully at the MADIS QC page for my station. Here is what it says:

Please ensure your station's sensors are working properly and that your sensors are correctly calibrated!
Specifically, double check your barometer calibration (both absolute and relative) and make certain it's correct
We use your absolute barometer reading with your station elevation to determine altimeter pressure value sent to CWOP!

The way I read that, it doesn't say that my station should report Altimeter for relative pressure. It says that it uses my station's reported ABS pressure and converts it to altimeter and then sends it to CWOP. They also link to a video that shows you how to calibrate your ABS and REL pressure on an Ambient 2902B PWS. Basically, the procedure is:

1. Get your location and *console* elevation as accurately as possible.

2. Determine your offset between SLP and ABS using the Keisan Atmospheric Pressure from Altitude Calculator at https://keisan.casio.com/exec/system/1224579725 (NOT the T and P at dest calculator)

3. Set your REL pressure using the calculated offset.

4. Then get a current local METAR and use the calculated offset to determine your true ABS pressure and set your ABS to that value. That will also adjust your REL pressure since the unit remembers the offset (this I gather, kind of reading between the lines).

Now in the last step it isn't clear whether you're supposed to get the SLP or the altimeter setting from the METAR, but the offset you get from the keisan calculator is clearly between absolute and SLP, so using altimeter wouldn't make sense to me.

But the offset is a little different* than you get with the T and P at destination calculator, because now you are using 15 C at station elevation instead of at sea level. So I wonder which is actually correct.

*In my case, 29.52 vs. 29.68 hPa.

[CW2274]

Yeah I'm sure that's true. I'd just like to understand how they're calculating SLP, that's all.

Looking at your post you used the current temperature (-5 deg C). The weather service SLP calculation uses the mean temperature over the last 12 hrs. That could account for the discrepancy.

My main question is why are you calibrating to SLP? I thought CWOP stations were supposed to report altimeter not SLP.
Here is a link to a CWOP manual that may be useful.

I don't think there is a problem here as QNH is a global term.

Terminology is not the issue here, sending SLP when the altimeter setting is required is.

[sky_watcher]

Yeah I'm sure that's true. I'd just like to understand how they're calculating SLP, that's all.

Looking at your post you used the current temperature (-5 deg C). The weather service SLP calculation uses the mean temperature over the last 12 hrs. That could account for the discrepancy.

My main question is why are you calibrating to SLP? I thought CWOP stations were supposed to report altimeter not SLP.
Here is a link to a CWOP manual that may be useful.

I don't think there is a problem here as QNH is a global term.

Terminology is not the issue here, sending SLP when the altimeter setting is required is.

Okay, I must be missing your point. The CWOP book says to report the sea level pressure (local pressure plus the difference in pressure to sea level). The QNH reading is sea level pressure. What are you defining "altitude setting" as, if not sea level pressure?

[CW2274]

Yeah I'm sure that's true. I'd just like to understand how they're calculating SLP, that's all.

Looking at your post you used the current temperature (-5 deg C). The weather service SLP calculation uses the mean temperature over the last 12 hrs. That could account for the discrepancy.

My main question is why are you calibrating to SLP? I thought CWOP stations were supposed to report altimeter not SLP.
Here is a link to a CWOP manual that may be useful.

I don't think there is a problem here as QNH is a global term.

Terminology is not the issue here, sending SLP when the altimeter setting is required is.

The QNH reading is sea level pressure. What are you defining "altitude setting" as, if not sea level pressure?

No, QNH is the altimeter setting. Without looking it up, I believe QFE is SLP.

[Jim_S]

Well that may be a showstopper. I certainly don't want to be reporting altimeter on ambientweather and wunderground, and the WS-2902C can only report one pressure. I'm not sure it's even possible to calibrate the unit to report altimeter as it can only handle a fixed offset from ABS. If there's no workaround, I probably should just cancel my request for a MADIS ID then, if that's possible.

I don't think this is a showstopper, "altimeter" (using the NWS definition) is essentially a fixed offset correction to sea level based on a fixed average temperature. It's pretty much what every personal, consumer grade, weather station reports. A few Davis station can calculate SLP, but your Ambient station doesn't factor in a 12 hour average temp and only calculates altimeter (which is good because that's what you need to be reporting).
Here is a link to the NWS page on pressure definitions: https://www.weather.gov/bou/pressure_definitions
quoting:
ALTIMETER SETTING: This is the pressure reading most commonly heard in radio and television broadcasts. It is not the true barometric pressure at a station. Instead it is the pressure "reduced" to mean sea level using the temperature profile of the "standard" atmosphere, which is representative of average conditions over the United States at 40 degrees north latitude.
I really wish the NWS would call it "barometer" or "relative pressure", anything but "altimeter" which makes people think it only applies to aviation.

[CW2274]

Well that may be a showstopper. I certainly don't want to be reporting altimeter on ambientweather and wunderground, and the WS-2902C can only report one pressure. I'm not sure it's even possible to calibrate the unit to report altimeter as it can only handle a fixed offset from ABS. If there's no workaround, I probably should just cancel my request for a MADIS ID then, if that's possible.

I don't think this is a showstopper, "altimeter" (using the NWS definition) is essentially a fixed offset correction to sea level based on a fixed average temperature. It's pretty much what every personal, consumer grade, weather station reports.

Exactly the opposite, no PWS reports the altimeter except a Vue console.

[elisatems]

Well that may be a showstopper. I certainly don't want to be reporting altimeter on ambientweather and wunderground, and the WS-2902C can only report one pressure. I'm not sure it's even possible to calibrate the unit to report altimeter as it can only handle a fixed offset from ABS. If there's no workaround, I probably should just cancel my request for a MADIS ID then, if that's possible.

I don't think this is a showstopper, "altimeter" (using the NWS definition) is essentially a fixed offset correction to sea level based on a fixed average temperature. It's pretty much what every personal, consumer grade, weather station reports. A few Davis station can calculate SLP, but your Ambient station doesn't factor in a 12 hour average temp and only calculates altimeter (which is good because that's what you need to be reporting).
Here is a link to the NWS page on pressure definitions: https://www.weather.gov/bou/pressure_definitions
quoting:
ALTIMETER SETTING: This is the pressure reading most commonly heard in radio and television broadcasts. It is not the true barometric pressure at a station. Instead it is the pressure "reduced" to mean sea level using the temperature profile of the "standard" atmosphere, which is representative of average conditions over the United States at 40 degrees north latitude.
I really wish the NWS would call it "barometer" or "relative pressure", anything but "altimeter" which makes people think it only applies to aviation.

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, I know that reported "barometric pressure" isn't the true barometric pressure, that's the absolute pressure. But SLP and altimeter are, to my understanding, two different ways of reducing absolute pressure to sea level. I was certainly not aware that reported pressures for lay public use were altimeter, I thought they were SLP.

If altimeter can be estimated using a fixed offset from absolute then it's not a showstopper, but like I said, when my unit sends to Wunderground and ambientweather.net i want it to send what is normally expected. Weather.gov says that really IS altimeter?? Now I'm thoroughly confused...

[sky_watcher]

Yeah I'm sure that's true. I'd just like to understand how they're calculating SLP, that's all.

Looking at your post you used the current temperature (-5 deg C). The weather service SLP calculation uses the mean temperature over the last 12 hrs. That could account for the discrepancy.

My main question is why are you calibrating to SLP? I thought CWOP stations were supposed to report altimeter not SLP.
Here is a link to a CWOP manual that may be useful.

I don't think there is a problem here as QNH is a global term.

Terminology is not the issue here, sending SLP when the altimeter setting is required is.

The QNH reading is sea level pressure. What are you defining "altitude setting" as, if not sea level pressure?

No, QNH is the altimeter setting. Without looking it up, I believe QFE is SLP.

I suggest you look at this. http://www.aviationchief.com/altimeter-settings.html

[CW2274]

Yeah I'm sure that's true. I'd just like to understand how they're calculating SLP, that's all.

Looking at your post you used the current temperature (-5 deg C). The weather service SLP calculation uses the mean temperature over the last 12 hrs. That could account for the discrepancy.

My main question is why are you calibrating to SLP? I thought CWOP stations were supposed to report altimeter not SLP.
Here is a link to a CWOP manual that may be useful.

I don't think there is a problem here as QNH is a global term.

Terminology is not the issue here, sending SLP when the altimeter setting is required is.

The QNH reading is sea level pressure. What are you defining "altitude setting" as, if not sea level pressure?

No, QNH is the altimeter setting. Without looking it up, I believe QFE is SLP.

I suggest you look at this. http://www.aviationchief.com/altimeter-settings.html

Okay. Just says what I said.

[elisatems]

Yeah I'm sure that's true. I'd just like to understand how they're calculating SLP, that's all.

Except QFE is the setting that causes the altimeter to read zero at field elevation, and that is not SLP. QNH is what you said, I believe.
Looking at your post you used the current temperature (-5 deg C). The weather service SLP calculation uses the mean temperature over the last 12 hrs. That could account for the discrepancy.

My main question is why are you calibrating to SLP? I thought CWOP stations were supposed to report altimeter not SLP.
Here is a link to a CWOP manual that may be useful.

I don't think there is a problem here as QNH is a global term.

Terminology is not the issue here, sending SLP when the altimeter setting is required is.

The QNH reading is sea level pressure. What are you defining "altitude setting" as, if not sea level pressure?

No, QNH is the altimeter setting. Without looking it up, I believe QFE is SLP.

I suggest you look at this. http://www.aviationchief.com/altimeter-settings.html

Okay. Just says what I said.

Except that QFE causes the altimeter to read zero at field elevation. That isn't SLP. QNH is what you said, I believe: with altimeter setting in the Kollsman window, the altimeter should read actual field elevation relative to MSL.

[Jim_S]

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. This is a terminology/nomenclature confusion. When the NWS says SLP they are referring to a very specific algorithm when (almost) everyone else says SLP they mean altimeter.

If altimeter can be estimated using a fixed offset from absolute then it's not a showstopper, but like I said, when my unit sends to Wunderground and ambientweather.net i want it to send what is normally expected. Weather.gov says that really IS altimeter?? Now I'm thoroughly confused...

Altimeter is what Wunderground and ambientwether.net is expecting.

[CW2274]

Yeah I'm sure that's true. I'd just like to understand how they're calculating SLP, that's all.

Except QFE is the setting that causes the altimeter to read zero at field elevation, and that is not SLP. QNH is what you said, I believe.
Looking at your post you used the current temperature (-5 deg C). The weather service SLP calculation uses the mean temperature over the last 12 hrs. That could account for the discrepancy.

My main question is why are you calibrating to SLP? I thought CWOP stations were supposed to report altimeter not SLP.
Here is a link to a CWOP manual that may be useful.

I don't think there is a problem here as QNH is a global term.

Terminology is not the issue here, sending SLP when the altimeter setting is required is.

The QNH reading is sea level pressure. What are you defining "altitude setting" as, if not sea level pressure?

No, QNH is the altimeter setting. Without looking it up, I believe QFE is SLP.

I suggest you look at this. http://www.aviationchief.com/altimeter-settings.html

Okay. Just says what I said.

That isn't SLP. QNH is what you said, I believe: with altimeter setting in the Kollsman window, the altimeter should read actual field elevation relative to MSL.

Whether SLP is or is not correct, I can't say. But yes to your Kollsman being field elevation when set with a current altimeter setting.

[elisatems]

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.