Thursday, November 19, 2015

First pass at cell phone barometer calibration.

Most modern cell phones have a barometer in them that could be a valuable aid to your navigation.  They tend to draw extra power if we try to record pressure history with them, but we can always turn them on, read the pressure and write it in the log book, and turn it off—ie, the way mariners have recorded pressure history for several hundred years.  The only difference now is these phone barometers are generally pretty accurate, whereas many barometers on boats are not very accurate. As noted below, we have easy and free ways to check their calibration online and we should do so. But even without that, chances are, right out of the box they could still be the most accurate barometer on the boat. More than adequate apps are free; in fact the more complex expensive ones are not as functional in many cases. If you have not done so, it will pay to download a free one and play with it.

So it was inevitable that we start a study of their accuracy and indeed we report here a first  step in that direction... thanks in large part to Erik Kristen who brought a few over to the office this afternoon to test.

Below are the results for a Galaxy S5, iPhone 6+, and a Samsung Note 2.






The actual pressures displayed were at best accurate to within in 0.8 mb and the worse to within 2.2 mb. But the bigger question is are these off by the same amount at all pressures we might run into, and the answer is yes, that seems to be the case in the ones we tested, which is not really a surprise.

In short, they are pretty good. They were each linear over the range covered: best at ± 0.1 and the worst ± 0.2, though we should not conclude anything from this first study on relative models. The S5 would not work below 950 mb, but that was likely an issue with the particular app we were using and not the phone barometer.  A different app would most likely go to lower pressures.

The best app is the simplest one. We just need the sensor pressure and a way to offset it. Many apps include confusing ways to set the pressure to local airport values, or they use GPS elevation, or smarter ones could use the National Elevation data set based on Lat-Lon, but in any case these schemes do not work. The errors in these corrections are larger than the inherent sensor error. We want just plain station pressure read from the sensor, and we have to know it is not sea level pressure till we are on the boat.

The best iPhone app I have found is called "hPa," which is the abbreviation for hecto Pascal, a pressure unit the same as a millibar. Maybe the real name is "barometer." It is in the iPhone App Store.

More news on this as we learn more, but the first pass at testing them looks very promising. You can always use the free resource at www.starpath.com/barometers to set your phone to the right pressure.

We have a link to phones with barometers in the list of resources at www.starpath.com/cyc, but you have to do a bit of research on older phones. For example, the baro phone link lists Moto X (Gen 2, 2014)  as having a barometer, and indeed many of the original spec sheets listed one, but it seems this phone does not have a barometer in it... at least one that can be accessed by any 3rd party apps, which are required to read the pressure.


7 comments:

  1. Could you clarify the name of the iPhone app?

    It doesn't show up I'm the App Store.

    Thanks

    ReplyDelete
    Replies
    1. It is written by Peter Breiting in Germany. The company is plaincode.com but strangely they do not list the app on their web site. The name could also be "barometer." It seems plain code means also plain name.

      I have written to them to ask about it. I found it in the iPhone app store searching on "barometer"... about the 6th option presented. We will have an android phone here shortly and then I will look for some counterpart for Android. Again, the beauty of this app is its simplicity. Just the sensor pressure and a way to calibrate it. In particular it does not try to find MSLP, which frankly never works, and the errors this process generates are almost always larger than the native sensor error... which can be corrected for with our free online service.

      Delete
    2. Just checked this app, and they now have added some reference pressure link, which we have to ignore. It is too bad we do not have some programmer with the courage to just show the pressure and allow for an offset and add the statement that "There are no other features, because none of these bells and whistles work well enough to be of any value."

      Delete
  2. Just picked up a Moto X Gen 1 for its barometer, any app recommendations? I've tried a few but as you said they get confusing with the automatic adjustments they try to provide.

    I've also found that Metar pressure data is rounded so that's not great. Some sites like Meteociel show one decimal but I don't know where they get it from...

    ReplyDelete
    Replies
    1. This comment has been removed by the author.

      Delete
    2. We have an Android tablet, but no device with a barometer, so not sure on this. Guideline to me is simpler the better, ie we want station pressure alone (ie raw sensor pressure), no corrections for elevation or sea level etc, which are typically all wrong. and ideally a way to make a permanent offset to the sensor reading, as needed to fine tune the accuracy. Not sure about your metar note. we link to air port pressures at www.starpath.com/barometers and they give pressure to the tenth of a mb.sp

      Delete
  3. Indeed, TACOMA has it for instance with an SLK behind an RMK but European airports don't seem to included RMKs unfortunately, like Heathrow : http://aviationweather.gov/metar/data?ids=egll&format=raw&date=0&hours=0

    ReplyDelete