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 have some disadvantages, and thus cannot replace a well designed marine barometer—namely they use a lot of power and so cannot be left on permanently, and they do not store pressure history, at least in apps we have seen, and they do have to be calibrated as they are not as accurate (uncalibrated) as we want for marine weather analysis.  Nevertheless, out of the box they could still be the most accurate barometer on the boat.

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 to set your phone to the right pressure.

We have a link to phones with barometers in the list of resources at, 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.