I’ve been watching the forum for some time and it occurs to me that there are a fair number of people who are confused about how the altitude and barometric functions of their ABC watches work. This seems particularly evident when they are trying to calibrate the pressure or the altitude and then cross check the readings or they are confused as to why the readings “drift”. Sometimes it’s a real issue with the watch but the majority of the time it’s a lack of understanding of how ABC’s work and the relationship between altitude and barometric pressure. Many of the more experienced members of the forum have answered these questions in the various posts but I thought it would be helpful to put an explanation of the issues/explanations in one spot. This is not intended to be a scientific explanation, but a simplified version so people will have a better understanding of how to use their ABC watch.
First, there’s a good detailed article on the subject of atmospheric pressure and relationship to altitude on Wikipedia here:
so forgive me if I lift from this article a lot. I’ve tailored this to ABC watch users but it works for altimeters and barometers also. Let’s get started.
Barometric Pressure - Barometric or Atmospheric pressure is sometimes defined as the force per unit area exerted against a surface by the weight of air above that surface at any given point in the Earth’s atmosphere. Basically we live at the bottom of a giant ocean of air and the weight of all that air above us exerts pressure on us. This is what the pressure sensor in an ABC watch is measuring.
This pressure varies across the surface of the earth. The most basic reason it can vary is because of a change in altitude. At sea level the average pressure is 1013.25 millibars (mbar) or hectopascals (hPa) or 29.921 inches of mercury (inHg). At 18,000 feet, the pressure would be roughly half of this. The definition of these measurements isn’t important. The point is that there is a direct relationship between altitude and air pressure. The higher you go, the less air there is above you, and therefore the less pressure is exerted on you.
Unfortunately, it gets more complicated than that. The atmospheric pressure also changes for reasons not associated with just altitude. There are multiple reasons for this (the Earth even has a twice daily global atmospheric pressure tide) but suffice it to say these high pressure zones and low pressure zones are very important to the climate and weather (which is why you see them mentioned so often on the weather forecasts).
Current Absolute Ambient Pressure vs. Mean Sea Level Pressure – We’ve discussed that high pressure zones and low pressure zones or important to weather forecasting. This is why there are so many weather stations across the globe that measure barometric pressure. The problem is that these stations are all at different altitudes. We know that altitude also affects the pressure. So how do you know if the weather station is measuring a low pressure zone or is just a low reading because it is at a high altitude? This is where the difference between Current Absolute Ambient Pressure and Mean Sea Level Pressure comes in. Let’s say you’ve got a weather station at 75m or 246ft above sea level. Its pressure sensor is reading 996 mbar or 29.43 inHg. This is referred to as the Absolute (or Current or Ambient) Pressure or the real pressure being felt at that weather station. If you know the altitude (in this case 75m) you can then use a formula to compute what the pressure would be if that station were theoretically dropped to sea level instead of 75m. This is referred to as the Mean Sea Level Pressure (and in this case it would be corrected to 1006mbar on 29.71 inHg Mean Sea Level Pressure). This number can now be used by forecasters to determine the pressure or mass of atmosphere in that area versus other surrounding areas. When you see an atmospheric or barometric pressure referred to in weather reports, it's almost always in Mean Sea Level Pressure.
Putting it all together in an ABC watch – Hopefully we now have a basic understanding of the relationship between Altitude, Current Absolute Pressure and Mean Sea Level Pressure. There is a formula that describes this relationship, however, it's pretty complicated so the simple way to summarize it is as follows: there is a direct relationship between Altitude and Current Absolute Pressure and Mean Sea Level Pressure. If you know two of the above factors, you can figure out the third with a version of the above formula. ABC watches always know one of the three factors because its pressure sensor is always measuring Current Absolute Pressure (by the way it does need to know temperature for this measurement which is the real reason ABC watches have a temperature sensor. There are complaints that the temperature sensor is affected by body heat so you don’t get an ambient temperature reading, but the sensor is really there to measure temperature at the pressure sensor).
Since your ABC watch knows Current Absolute Pressure, if you then calibrate it by inputting the Mean Sea Level Pressure, it can then figure out the Altitude with the above formula. Or if you input the Altitude, it can figure out the Mean Sea Level Pressure. In real life, it’s easier to get an accurate altitude reference for calibration (either from Google Earth, markers, etc.) than to get an accurate Mean Sea Level Pressure reading (unless you’re very close to a weather station). This is why most people recommend calibrating to a known altitude. Note that some watches don’t even let you calibrate to Mean Sea Level Pressure, only to Altitude.
Once calibrated, the watch can now work in two ways. On Altitude reading mode or Barometer lock, it assumes the Mean Sea Level Pressure is not changing, so if its pressure sensor is sensing that the Current Absolute Pressure is changing, it must be because you are moving and your altitude is changing. On Pressure reading mode or Altitude lock, it assumes your altitude is not changing, so if the pressure sensor is sensing a change in Current Absolute Pressure, it must mean that Mean Sea Level Pressure is also changing i.e. a high pressure zone or low pressure zone is moving in. ABC's vary as to whether you need to select Barometer or Altitude lock manually or the watch does it automatically. For watches that have an automatic function it usually assumes that a very fast change in Current Absolute Pressure is due to an altitude change and a very slow change in Current Absolute Pressure is a Mean Sea Level Pressure change. An ABC watch’s ability to tell if it is going through an Altitude change or a Mean Sea Level Pressure change is limited which is why these readings will “drift” over time unless calibrated. Some ABC's don't bother with a lock and you just have to live with the more drift and calibrate often. Some ABC's won't display Mean Sea Level Pressure but just Current Absolute Pressure.
Is calibrating to Altitude or Mean Sea Level Pressure important? It depends. If you’re using your watch for altitude, then somewhat yes because the watch needs to either start with the right Altitude reference or have a good Mean Sea Level Pressure reference to give you a good altitude number. If your using the watch for weather forecasting, then no. An accurate pressure number isn’t as important as the trend or change in pressure. If the pressure is rising, the weather is probably going to get better. If the pressure is falling, it’s probably going to get worse. It the pressure is staying the same, the weather is likely to stay the same. This is an oversimplification but it explains why it’s the trends that are important, not the number, and why good ABC watches will have a good trend graph capability.
Why do some watches let you calibrate Absolute Current Ambient Pressure separately? – Pressure sensors in ABC watches are like any other instruments and can sometimes lose calibration. That is they are not reading the Absolute Current Pressure correctly. Some watches let you correct for this and recalibrate the sensor. This is tricky business and you’d better have a good reference to calibrate to (and know what you’re doing). Whenever doing any pressure calibration, it’s important to know which pressure you’re adjusting. If you mistakenly input a Mean Sea Level pressure when you are supposed to input an Absolute Current Pressure, you are going to get some real screwy numbers.
As I said, I’ve taken some liberties with the science to make the explanation easier to understand. I think ABC watches are great tools but it’s important to understand the basics of the science to understand their limitations and use them effectively. Hopefully this helps.