Topic: Smartphone Barometer Calibration

[Buick]

One assumption I'm making by using an app that employs the pressure sensor that my ~5yo iphone contains is that the sensor is accurate and precise. Another is that the reported altimeter settings from the nearby ASOS are also accurate and precise. Since both sensors essentially always precisely agree, I think those assumptions are probably correct.

[Buick]

I'm also assuming that both the sensor in the iphone and the asos sensor are more accurate and precise than the one in the vp2 console.

[CW2274]

More accurate? I know the ASOS is. More precise? The VP2 and Vue console (at least with my ancient USB 6.0.3 I still use) will resolve to the foot (ie 30.001 inHg), as did the ASOS altimeter I used at work.

[Buick]

I think I'm reporting accurate altimeter setting to CWOP.

[Mattk]

There a lots of assumptions being made here without any substance. Resolution and absolute/relative accuracy get lost in much of this type of discussion and comparatively tends to be misleading.

So who can state what sensor is actually in an IPhone? And which sensors in which model IPhones? The best from Bosh (which are in the latest IPhones) have an absolute accuracy of +/- 1hPa so I fail to accept how anybody can state their IPhone sensor is better than the one in the VP2 console.

Way too many assumptions and the assumptions are more expectations of what some want to believe

[Buick]

Why do you suppose the pressure that my app indicates is equal to the altimeter setting at the nearby ASOS?

[Buick]

...and, thus, allows me to calibrate my console barometer to report values that are so similar to the altimeter setting values reported by the nearby ASOS.

[Buick]

You didn't read carefully. My assumptions were based on inference, not expectations.

[ocala]

Thanks for the thread. My pressure had drifted so I reset it to two ASOS stations that I am right in the middle of.
Crazy thing was the barometer app I use matched exactly both ASOS stations.
I'll let you guys get back to the bloodshed.

[gszlag]

Should we use a local airport as a barometer calibration tool?

The manuals of our weather stations and some weather-related books suggest using a close-by airport to do the initial set up and calibration of your barometer.

Perhaps their reasoning is to calibrate our barometers with a convenient and free calibration tool.

However, using an airport as a reference calibration tool is not ideal. According to David Burch's, The Barometer Manual, a minimum of four or five airports should be used for calibration purposes. For calibration purposes, one airport is not enough, he says. Stephen Burt of the Weather Observer's Manual echoes that viewpoint, but adds another rather strict provision: your weather station should be surrounded by close multiple METARS (synoptic stations) i.e. NSEW of your location.

Unlike our personal weather stations, keep in mind that airport weather stations (AWOS or ASOS) calculate their station pressures. Absolute pressure is not published and is unknown. Sensor elevation is not published and unknown. For personal weather stations, absolute pressure (station pressure) is a measured (actual value). Sensor elevation is known (Dr. Burch likes to use Google Earth to determine elevation).

Rather than using several airports to calibrate your barometer, the best way to calibrate is by using another reference barometer. All you need to is to adjust your absolute pressure to match the same reading (side-by-side) with a reference sensor of known accuracy. Known accuracy is key.

Can you use a smartphone as a calibration tool? Phones, even top-of-the line ones, have no assurance of being calibrated by the phone manufacturer. Therefore, using barometer phone apps will be as good as their phones reported absolute pressure. They might be,accurate or they might not be accurate. We don't know unless we check. Phones calculate elevation poorly so depending on how the app works, it is better to determine your elevation manually from another source like Google Earth.

Almost all barometers drift over time (sometimes quite quickly) and accuracy suffers. Few of us can afford to send our barometers to a calibration lab for yearly barometer calibration, as annual costs start around $200 USD (and up) for this service. However, we can obtain a low cost, accurate barometer that has been checked for accuracy in a calibration lab. This unit can be used as our calibrated reference barometer.

Note: I am referring to the Starpath UB baro.

Using a calibrated reference barometer side-by-side with your barometer is the only way to be sure that your barometer's readings (or phone readings) are accurate. If you know your absolute pressure has been calibrated properly, then you can calculate (or have your console) calculate an accurate sea level pressure like Altimeter setting.

[Jim_S]

I calibrated my station's barometer 4 or 5 years ago using elevation and fine tuning based on information from 3 nearby airports. I've spot checked it regularly since then by comparing readings with the airports. My readings sometimes match, sometimes are a little high or low but are always close (+/- 0.01 inHG).
Early this summer I decided to do a more complete check and recorded 24 hours worth of data and compared it to the airports. If I was seeing a consistent trend of the readings being either high or low I'd consider recalibrating but as it is I don't see any sign of drift.

Blue = My station, Red = Airport 5 miles W, Green = Airport about 7 miles SE, Yellow = Airport about 14 miles N.

Can you use a smartphone as a calibration tool? Phones, even top-of-the line ones, have no assurance of being calibrated by the phone manufacturer. Therefore, using barometer phone apps will be as good as their phones reported absolute pressure. They might be,accurate or they might not be accurate. We don't know unless we check.

Have you checked yours? You have the Starpath USB and I assume a smartphone of some kind. I'd be curious to see your results and anyone else that wants to share.

[gszlag]

I live somewhat remote - no cell signal here. Doubtful, Android Lollipop (last used several or more years ago) can be classified as a smartphone. I use an Android tablet to run apps but no barometer in my mobile device.

I think it all boils down to the barometric chip. Whether the chip is in a phone or weather console doesn't matter. If you buy 10 phones, they could have 10 different absolute values. Same if you bought 10 consoles. Which of the 10 has the true pressure? The phone app probably calculates an absolute pressure value/station pressure/QFE by comparing to a reference sea level pressure (Altimeter). Maybe a close-by METAR?Nothing wrong with that, I still recommend that procedure if an airport is all you have for a reference. But as mentioned, airports calculates pressures differently than your weather station but yes, you can still use it for a reference - but it is not the best or easiest way.

Since you are at a very low elevation, pressure calculations are far more accurate close to sea level than say, 1000 meters or more above sea level.

Alternatively, you may want to compare your app and console with the NWS online Altimeter calculator. All you need to enter is your elevation and current station pressure and see if the calculated Altimeter value matches the airport, your console and your app.

But if you are only .01 inHg different than your airport -we are splitting hairs. You are essentially already calibrated.

[gszlag]

To address Buick's other questions:

- Gladstone/CWOP is an average of your neighbour's calibrated and uncalibrated barometers. Therefore, I would not use it for calibration/comparison purposes.

- as one approaches sea level elevation, all pressures get closer and closer to be the same. So yes, Altimeter and SLP (sea level pressure) will be close most of the time - farther in winter, closer in summer.

- at sea level, Station pressure = Altimeter = SLP. I checked Bellingham International last evening and Altimeter and SLP were about 0.5 mb apart. Keep in mind that SLP changes  when temperature changes so the 0.5 mb spread may change a bit as well.

- to see all three pressures, you can use mesowest to to see CWOP stations and your airport (KBLI).

- according to CWOP guidelines if you are within +/- 1 mb accuracy, you are considered to be calibrated.

A manufacturer's barometer printed  data sheet specification is irrelevant if the weather station owner hasn't checked and verified readings out-of-the box. Think of it as a "potential" accuracy specification. You have to check and calibrate if necessary. Barometer accuracy drifts over time. Calibration or re-calibration is an on-going process - at least annually. More frequently is better.

- you are at a very low elevation - maybe around 37 meters would be my guess? KBLI is at 45 meters.

- you mentioned SLP. If you have the new Davis console (6313), you can calculate Altimeter setting only.

[CW2274]

However, using an airport as a reference calibration tool is not ideal. According to David Burch's, The Barometer Manual, a minimum of four or five airports should be used for calibration purposes. For calibration purposes, one airport is not enough, he says. Stephen Burt of the Weather Observer's Manual echoes that viewpoint, but adds another rather strict provision: your weather station should be surrounded by close multiple METARS (synoptic stations) i.e. NSEW of your location..

Not only ridiculous but impractical. Airports are not exactly close together, and pressures can vary enough to cause a novice to become even more confused. I have four ASOS in my neck of the woods, and pressures never match. Like you say, get a reference barometer, if not, pick the closest airport and fine tune from there.

I know I harp on this, but it's true.. if you're that worried about the most accurate pressure reading in your own backyard, buy a Vue console and be done with it. Makes this entire thread irrelevant.

[Buick]

Thanks for the thread. My pressure had drifted so I reset it to two ASOS stations that I am right in the middle of.
Crazy thing was the barometer app I use matched exactly both ASOS stations.
I'll let you guys get back to the bloodshed.

Yep. I don't calibrate using the nearby ASOS, I calibrate using my phone. The fact that the phone's sensor consistently reports the same values as the reported altimeter setting of the nearby ASOS has a non-zero chance of being a coincidence - but I'd wager that it's pretty dang close.

[Mattk]

So what is calibrating the so called phone pressure?

[Buick]

What is causing the so called phone pressure to mirror the ASOS altimeter setting? It may be that I use an app that's well suited to my use case and that I input the information needed to make it work properly.

[Buick]

This, of course, would be a very different story if a significant average difference in altimeter setting between two near-sea level locations, 2mi apart, and with a ~20' difference in elevation was the normal, expected situation.