Topic: Barometer Accuracy

[Jim_S]

Does Acurite have any information on how accurate their barometer (pressure sensors) are? As an example when I look at the specs for my 5 in 1 it gives me quite detailed information on the humidity sensor but the specs for the Access don't mention barometric pressure at all. Same for the Smart hub and the displays I've looked at.

[nincehelser]

Does Acurite have any information on how accurate their barometer (pressure sensors) are? As an example when I look at the specs for my 5 in 1 it gives me quite detailed information on the humidity sensor but the specs for the Access don't mention barometric pressure at all. Same for the Smart hub and the displays I've looked at.

I don't think they've ever published or disclosed exactly what baro sensors they use in the displays, SmartHUB, or Access.

In the SmartHUB and some displays, this seems to be the sensor used: https://datasheetspdf.com/pdf-file/748092/HOPERF/HP03S/1

For the Access, John Z and I think it might be this, but I don't think it's been verified: https://www.te.com/commerce/DocumentDelivery/DDEController?Action=srchrtrv&DocNm=MS5637-02BA03&DocType=Data+Sheet&DocLang=English

[Jim_S]

@nincehelser

Thanks!

[DoctorKnow]

All I know is that when I calibrate the barometers, they stay accurate. They do not tend to drift or become slow to respond to changes. Once a year I will check them and apply very fine adjustments if at all.

[Jim_S]

All I know is that when I calibrate the barometers, they stay accurate. They do not tend to drift or become slow to respond to changes. Once a year I will check them and apply very fine adjustments if at all.

Would you explain how you calibrate them? I have two displays and an Access (I also have a smart hub but in a few days that will be moot).

[DoctorKnow]

Some displays are not able to be calibrated. The Atlas display can be adjusted, just choose that option in the settings.

The access is calibrated through my acurite dashboard. Settings - Devices - Edit (just above pic of Access) - Scroll down to the Barometric press. section - bubble for "Station Pressure" - Calibrate - scroll up to "Save" (green box) click it. Wait 15 min.

[Jim_S]

Some displays are not able to be calibrated. The Atlas display can be adjusted, just choose that option in the settings.

The access is calibrated through my acurite dashboard. Settings - Devices - Edit (just above pic of Access) - Scroll down to the Barometric press. section - bubble for "Station Pressure" - Calibrate - scroll up to "Save" (green box) click it. Wait 15 min.

Thanks! I have two 5 in 1 displays so I may be out of luck there.

Right now my Access is set to Adjusted Pressure based (I assume) on the altitude I entered. Are you saying you're using an offset for station pressure to adjust to SLP? I wish the dashboard displayed station pressure and adjusted pressure with the ability to correct the station pressure.

[DoctorKnow]

Some displays are not able to be calibrated. The Atlas display can be adjusted, just choose that option in the settings.

The access is calibrated through my acurite dashboard. Settings - Devices - Edit (just above pic of Access) - Scroll down to the Barometric press. section - bubble for "Station Pressure" - Calibrate - scroll up to "Save" (green box) click it. Wait 15 min.

Are you saying you're using an offset for station pressure to adjust to SLP? I wish the dashboard displayed station pressure and adjusted pressure with the ability to correct the station pressure.

Yes. I set the pressure to be correct on Wunderground. The dashboard pressure ends up being about .09 or so too low as a result. I am not interested in the dashboard being right, I just want what I am sending out to be what it should be.

[Jim_S]

Yes. I set the pressure to be correct on Wunderground. The dashboard pressure ends up being about .09 or so too low as a result. I am not interested in the dashboard being right, I just want what I am sending out to be what it should be.

Thanks! I definitely agree that the information sent to WU should be the best we can provide. What do you use as a reference when calibrating your barometer?

Also (I asked this in another thread but I'll ask here too) do you use "altimeter" or "sea level pressure" (at my location altimeter is about .1" lower thatn SLP)? I would have thought SLP would be correct but as near as I can tell altimeter is what the NWS uses.

[nincehelser]

Yes. I set the pressure to be correct on Wunderground. The dashboard pressure ends up being about .09 or so too low as a result. I am not interested in the dashboard being right, I just want what I am sending out to be what it should be.

Thanks! I definitely agree that the information sent to WU should be the best we can provide. What do you use as a reference when calibrating your barometer?

Also (I asked this in another thread but I'll ask here too) do you use "altimeter" or "sea level pressure" (at my location altimeter is about .1" lower thatn SLP)? I would have thought SLP would be correct but as near as I can tell altimeter is what the NWS uses.

Altimeter (or "adjusted" in myAcurite parlance) is what you should use.  It's the most common way people express pressure data. 

[galfert]

I disagree that Altimeter is what you "should" use. I say use whatever you want for your station and your software. It's your stuff. But if you want to know what the online services ask for that you send them ...then the answer is they all ask for Sea Level Pressure. Only CWOP asks for Altimeter. But you can have both. If you run the correct software then you can leave your console to show you whatever it was designed to show (most are Sea Level Pressure, exception being Davis Vue console as the only one I've heard of that was designed to do both). Then even though your console has Sea Level Pressure the software that you use to augment your station will be smart enough to do a separate calculation and send Altimeter to just CWOP. That is how the Meteobridge, WiFi Logger, Weather Display and other software works.

Then by sending Altimeter to CWOP you'll have your station data on CWOP to compare yourself to other services that use Altimeter (some METARs that are only Altimeter or NWS stations, TV and Radio media). But in the case that you want to compare your station to other stations that use Sea Level Pressure then you'll also have that too (if you report to WU, PWSweather, WeatherCloud, AWEKAS, WOW, as they all ask for Sea Level Pressure). Does that mean that everyone does this? No. Some people ignore this and send Altimeter to those services. Some know what they are doing others don't, and yet others don't care.

[nincehelser]

(*sigh*)

Most systems don't calculate SLP.  Most don't run extra software.  I've currently 3 brands of stations in my backyard and none of them do SLP, but they all do altimeter.

That's why everyone accepts altimeter.  That's why CWOP spelled it out to eliminate confusion.

We all know you're hung up on SLP.  Go for it.  But please stop implying that the rest of us don't know what we are doing.

[Jim_S]

I disagree that Altimeter is what you "should" use. I say use whatever you want for your station and your software. It's your stuff.

Fair enough so I'll explain more. I'm pretending I'm a National Weather Service office and I have a (super accurate) barometer pressure sensor that reports local station pressure. I want to convert that to what the NWS would report as local barometric pressure in a weather report (and report that to WU). From what I've seen so far that is "altimeter" not SLP. I could be wrong which is why I asked. As far as online reporting (WU etc.), I can't imagine they're looking for something different than the NWS.

[CW2274]

I disagree that Altimeter is what you "should" use. I say use whatever you want for your station and your software. It's your stuff.

Fair enough so I'll explain more. I'm pretending I'm a National Weather Service office and I have a (super accurate) barometer pressure sensor that reports local station pressure. I want to convert that to what the NWS would report as local barometric pressure in a weather report (and report that to WU). From what I've seen so far that is "altimeter" not SLP. I could be wrong which is why I asked. As far as online reporting (WU etc.), I can't imagine they're looking for something different than the NWS.

The NWS uses data from ASOS/AWOS's from local airports for their "official" readings for that particular area and this is what the public sees. The pressure given is the altimeter setting.

[galfert]

I disagree that Altimeter is what you "should" use. I say use whatever you want for your station and your software. It's your stuff.

Fair enough so I'll explain more. I'm pretending I'm a National Weather Service office and I have a (super accurate) barometer pressure sensor that reports local station pressure.

Then you need to pretend with your own weather software or your own website. If you are going to use someone else's online system (WU) then you should use their system as they requested.

I want to convert that to what the NWS would report as local barometric pressure in a weather report (and report that to WU). From what I've seen so far that is "altimeter" not SLP. I could be wrong which is why I asked. As far as online reporting (WU etc.), I can't imagine they're looking for something different than the NWS.

Weather Underground requests Sea Level Pressure.

Here is Weather Undergrounds API documentation. You'll notice they define Pressure as: Mean Sea Level Pressure, the equivalent pressure reading at sea level recorded at this station.
https://goo.gl/ZUoqvC

In Weather Underground's glossary Mean Sea Level Pressure is defined. Missing from the glossary is any reference to Altimeter.
https://www.wunderground.com/glossary/

Why? Because if you are a meteorologist and you want to compare weather conditions at one location to another then you use Sea Level Pressure. If you are a pilot or an airport and you don't want to crash while landing planes then you use Altimeter. The difference between Sea Level Pressure and Altimeter is that Sea Level Pressure takes the last 12 hours of temperature into account into the calculation, where Altimeter does not and uses standard temperature of 59 deg F always. So Altimeter does not take into account different weather conditions.

Why is Altimeter used for planes? Because it allows the pilot to set their Altimeter equipment to match the airport that they are approaching regardless of the difference in weather conditions at the current position from the conditions at the airport. That way when the plane lands it will zero out and match the airport's Altimeter.

But back to your statement that NWS uses Altimeter. Yes they use Altimeter but they also use Sea Level Pressure. So they have both. Here is the NWS report on local airport in Boulder Co weather reporting data and you can see Station Pressure (uncorrected Absolute Pressure), Sea Level Pressure, and Altimeter Pressure:
https://www.wrh.noaa.gov/mesowest/getobext.php?wfo=lox&sid=KBDU&num=48

So if you want to be like NWS then you need to have both SLP and ALT. I have both. But notice how the NWS does not call Altimeter a "pressure" they call it Altimeter Setting. Then in reference to Sea Level it is "Pressure."
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Most ASOS METARs also show both Altimeter and Sea Level Pressure. Sometimes with special broadcasts they only show Altimeter.

[galfert]

Now a small local airport like Boulder Airport KBDU may not ever report SLP on their METAR. But the NWS weather data does have their SLP as you saw from my previous link.

But KDEN Denver International Airport's METAR does show SLP (along with the obligatory Altimeter that airports and pilots need):
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[Jim_S]

Then you need to pretend with your own weather software or your own website. If you are going to use someone else's online system (WU) then you should use their system as they requested.

Sorry, I'm really not trying to be argumentative. I just want to learn and understand, and provide correct/accurate data to WU (since that's the only site Acurite reports to).

But back to your statement that NWS uses Altimeter. Yes they use Altimeter but they also use Sea Level Pressure.

Yes, I've seen that they report both in some places but when I look at their local weather reports, ones aimed at the general public, (not pilots or meteorologists) they appear to me to be using "altimeter" for local barometric pressure not SLP.

In Weather Underground's glossary Mean Sea Level Pressure is defined. Missing from the glossary is any reference to Altimeter.
https://www.wunderground.com/glossary/

Here is a the quote from the glossary:

Mean sea-level pressure is a pressure value obtained by the theoretical reduction or increase of barometric pressure to sea-level. The calculation corrects for the altitude difference from the pressure reading at the station elevation, to what it would otherwise be at sea-level. Land elevation affects the pressure reading at the surface. For example, a station on a hill may read a lower pressure than it would read if the station was at the same point and there was no hill. High pressure and low pressure systems are based on mean sea-level pressure to keep them comparable at any geographic location. Thus, surface pressure is different that mean sea-level pressure as it has not yet been corrected for the difference in altitude from sea-level.

True "Mean Sea Level Pressure" require compensating for altitude, and temperature (last 12 hours), and latitude of the station (because the earth isn't a perfect sphere). While WU may say "mean sea level pressure" I think what their glossary describes more closely matches the definition of "altimeter".

Ultimately I don't see how my system (Acurite Access) can accurately calculate true SLP. It knows my altitude but it doesn't know the temperature at the sensor (now or for the last 12 hours) or my latitude. But, again, I'm just trying to learn and understand, so perhaps I'm missing something simple and obvious.

[CW2274]

Then you need to pretend with your own weather software or your own website. If you are going to use someone else's online system (WU) then you should use their system as they requested.

Sorry, I'm really not trying to be argumentative. I just want to learn and understand, and provide correct/accurate data to WU (since that's the only site Acurite reports to).

But back to your statement that NWS uses Altimeter. Yes they use Altimeter but they also use Sea Level Pressure.

Yes, I've seen that they report both in some places but when I look at their local weather reports, ones aimed at the general public, (not pilots or meteorologists) they appear to me to be using "altimeter" for local barometric pressure not SLP.

Firstly, pilots do use the altimeter (that's it's prime function), and yes, that's what the NWS displays for normal public usage. SLP is almost always referred to in millibars and the altimeter in inHg. NWS could certainly put out SLP if they wanted, but very few would understand it outside of mets because it then requires a conversion to inHg for the average Joe. If millibars of SLP were to be displayed to the public in the States, very few would understand what the hell they were talking about because the altimeter has always been used.

[galfert]

Ultimately I don't see how my system (Acurite Access) can accurately calculate true SLP. It knows my altitude but it doesn't know the temperature at the sensor (now or for the last 12 hours) or my latitude. But, again, I'm just trying to learn and understand, so perhaps I'm missing something simple and obvious.

Why do you assume that your console is not capable of calculating and displaying Sea Level Pressure? The console does know the temperature at the barometric sensor. The pressure inside your house is the same pressure outside of your house. So the location is the same. Most consoles that display pressure need some time to adjust. Partly what they are doing is figuring out the trend in temperature.

True "Mean Sea Level Pressure" require compensating for altitude, and temperature (last 12 hours), and latitude of the station (because the earth isn't a perfect sphere). While WU may say "mean sea level pressure" I think what their glossary describes more closely matches the definition of "altimeter".

No, it exactly describes SLP, which is that for SLP you need to include in the calculation the average of the last 12 hours in temperature. Which is why SLP is more accurately referred to as MSLP. Conversely Altimeter always uses 59 deg F in the calculation. MSLP is more indicative of true weather conditions. SLP and ALT both compensate for elevation, but only MSLP also compensates for weather.

As CW2274 said SLP is given in milibars (mb) which is the same as hectoPascal (hPa). I believe the reason why it is not done in inHg is because it is used by meteorologists which are scientists and science mostly uses metric.

[galfert]

NOAA's NWS map has a tick box to turn on SLP of METAR locations. There is no tick box to show ALT.

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[Jim_S]

First, let me thank everyone for sharing their knowledge and experience!

Firstly, pilots do use the altimeter (that's it's prime function), and yes, that's what the NWS displays for normal public usage. SLP is almost always referred to in millibars and the altimeter in inHg. NWS could certainly put out SLP if they wanted, but very few would understand it outside of mets because it then requires a conversion to inHg for the average Joe. If millibars of SLP were to be displayed to the public in the States, very few would understand what the hell they were talking about because the altimeter has always been used.

Yes, I know pilots use altimeter. I didn't mean to infer otherwise and I apologize if my post wasn't clear. What I was trying to say is that the National Weather Service uses altimeter for barometric pressure in their weather reports aimed at the general population (as well as aviation reports intended for pilots) however in the reports intended for the public they refer to it as 'Barometer'.

[Jim_S]

Why do you assume that your console is not capable of calculating and displaying Sea Level Pressure? The console does know the temperature at the barometric sensor. The pressure inside your house is the same pressure outside of your house. So the location is the same. Most consoles that display pressure need some time to adjust. Partly what they are doing is figuring out the trend in temperature.

Mostly I'm guessing, I haven't found any information on how Acurite calculates "adjusted" pressure. All the setup calls for is the altitude of the station. That's not enough information to calculate true SLP. If there is a temperature sensor in the Access (the device that sends barometric pressure to WU) it isn't displayed anywhere I can find.
I have a barometer app for my iPad and this is what it says about calculating sea level pressure.

Warning: The barometric height formula / altimetry contains a lot of factors and can only be exact if you include information such as temperature and a good model of air density. This application uses a simplified formula and assumes an atmosphere with equal air density at every altitude...

My guess is that Acurite (and most other, low cost, consumer weather stations) also use a simplified formula. But it's only a guess. I think Davis publishes their formula (seems like I've seen it discussed here in the forum) does anyone else?

[CW2274]

First, let me thank everyone for sharing their knowledge and experience!

Firstly, pilots do use the altimeter (that's it's prime function), and yes, that's what the NWS displays for normal public usage. SLP is almost always referred to in millibars and the altimeter in inHg. NWS could certainly put out SLP if they wanted, but very few would understand it outside of mets because it then requires a conversion to inHg for the average Joe. If millibars of SLP were to be displayed to the public in the States, very few would understand what the hell they were talking about because the altimeter has always been used.

however in the reports intended for the public they refer to it as 'Barometer'.

Semantics. Just as the folks that don't understand pressure in "millibars", won't understand "altimeter" either.

[nincehelser]

Why do you assume that your console is not capable of calculating and displaying Sea Level Pressure? The console does know the temperature at the barometric sensor. The pressure inside your house is the same pressure outside of your house. So the location is the same. Most consoles that display pressure need some time to adjust. Partly what they are doing is figuring out the trend in temperature.

Mostly I'm guessing, I haven't found any information on how Acurite calculates "adjusted" pressure. All the setup calls for is the altitude of the station. That's not enough information to calculate true SLP. If there is a temperature sensor in the Access (the device that sends barometric pressure to WU) it isn't displayed anywhere I can find.
I have a barometer app for my iPad and this is what it says about calculating sea level pressure.

Warning: The barometric height formula / altimetry contains a lot of factors and can only be exact if you include information such as temperature and a good model of air density. This application uses a simplified formula and assumes an atmosphere with equal air density at every altitude...

My guess is that Acurite (and most other, low cost, consumer weather stations) also use a simplified formula. But it's only a guess. I think Davis publishes their formula (seems like I've seen it discussed here in the forum) does anyone else?

It would be technically possible for the Access to calculate SLP.  You would have to use the outside temperature, not the temperature of the baro sensor.  myAcurite already knows your latitude and could possibly push it down to the Access.

However, the majority of stations out there do not calculate SLP.  It's not in common use with the general population.  Altimeter is the CWOP pressure standard because it is the simplest pressure reduction format that most CWOP stations can reliably deliver.

Which pressure reduction method isn't even stated in the wunderground upload protocol.  It's been ambiguous from the get-go, and some stations even send station/absolute pressure instead.

Also, many use "SLP" or "MSL" as just meaning the pressure has been reduced to sea level, without any regard to the method.  I've found the only way to know what someone really means is to ask them about the details, such as if temperature and/or latitude is considered in the reduction.

[CW2274]

Why do you assume that your console is not capable of calculating and displaying Sea Level Pressure? The console does know the temperature at the barometric sensor. The pressure inside your house is the same pressure outside of your house. So the location is the same. Most consoles that display pressure need some time to adjust. Partly what they are doing is figuring out the trend in temperature.

Mostly I'm guessing, I haven't found any information on how Acurite calculates "adjusted" pressure. All the setup calls for is the altitude of the station. That's not enough information to calculate true SLP. If there is a temperature sensor in the Access (the device that sends barometric pressure to WU) it isn't displayed anywhere I can find.
I have a barometer app for my iPad and this is what it says about calculating sea level pressure.

Warning: The barometric height formula / altimetry contains a lot of factors and can only be exact if you include information such as temperature and a good model of air density. This application uses a simplified formula and assumes an atmosphere with equal air density at every altitude...

My guess is that Acurite (and most other, low cost, consumer weather stations) also use a simplified formula. But it's only a guess. I think Davis publishes their formula (seems like I've seen it discussed here in the forum) does anyone else?

Also, many use "SLP" or "MSL"

Actually, MSL is Mean Sea Level, which is nothing more than one's height above or below, nothing pressure related. Some folks say ASL, for Above.