I put this question to the manufacturer, Dave West of Winged Shadow Systems. His reply is a little technical, but I think you'll be able to follow it. I really appreciate him going through all this, as I'm sure it may be important to your project too. He writes:
"It is understandable that users want a simple single number for accuracy. Unfortunately, for barometric-pressure-based altimetry the issue is anything but simple. There are several factors involved. First there is the question of the instrument’s ability to accurately measure pressure. This includes factors such as sensing resolution, linearity, temperature drift, calibration, and sampling errors. For the How High this “instrument accuracy” is better than 1% + 5 feet. But that is not the whole story. We could use this number as a part of the specification -- but it is a little misleading. As you stated, most users want to know "How close a reading of 3,000 ft is to 3,000 ft?" and applying only the instrument accuracy doesn't come close to answering that question.
There is the larger issue of the barometric altitude model. Every barometric altimeter uses a model of the atmosphere to convert pressure readings into altitude. These model atmospheres, like the International Standard Atmosphere (ISA) used in the How High, consist of a set of equations that represent the way air pressure changes with altitude. Unfortunately, the real-world does not always match the model. Think of barometric pressure as a measurement of the weight of all the air in a column from the ground to space. The weight of the air varies with density which is influenced by temperature and other weather conditions. The How High compensates for overall changes in the total barometric pressure using a post-flight ground reference pressure reading.
But the column of air might differ from the standard model in its density at various altitudes. So, even an instrument that can measure pressure with absolute accuracy is limited by the real atmosphere. How big is this error? It’s hard to say. In full-scale airplanes errors of a few hundred feet are common. Fortunately, compared to full-scale aircraft, we fly relatively close to the ground and our flights are relatively short so our error sources are smaller. From atmospheric lapse-rate data and our experience, I’ve never seen a case where a 30 minute model airplane flight to about 1000 feet would give more than about a 20 foot error. For most model flight conditions it would usually be much less, likely under 10 ft. Of course, model rocket flights are much shorter (good) but generally go to higher than model airplanes (bad). On average, I'd expect similar error levels.
Unfortunately, I've seen model altimeters claiming absurdly tight accuracy numbers. I prefer to believe that they are not intentionally misleading there customers, but instead simply don't have a true understanding of the inherent limitations of pressure-based altimetry.
Some units report absolute MSL altitude and do not correct for elevation. While that may seem a simple matter of subtracting the field elevation, the inaccuracy of such units is much larger. Such units do not compensate for local barometric pressure but assume a "standard" pressure of 29.92 "Hg. Since each .01 "Hg represents about 9 feet of altitude, on a day with local barometer reading of, say 30.10 "Hg, the altitude will be off by a whopping 162 feet. In contrast the How High makes a ground reference pressure reading at the end of the flight when you activate the display. It then uses the ratio of in-flight minimum pressure to this reference pressure to calculate altitude. This way it automatically compensates not only for field elevation but also the local barometric pressure. This method is superior to fixes reference systems and systems that make reference readings at power-up when thermal drift and warm-up can create inaccuracies.
A third issue is mounting. The How High must be located so that it can sense the pressure of the surrounding air – the static air pressure. In rockets, the payload section can be subject to false pressures during flight. Since the How High looks for the lowest pressure to determine peak altitude, false high pressures (caused by ram air or even the ejection charge) will not affect readings. However, low pressure (due to suction effects possibly caused by airflow) can give falsely high readings. In practice, the mounting doesn't seem to be too fussy and results are better than I at first expected. Still, mounting can affect the accuracy of the reading in a way that is hard to quantify. [Interestingly, some folks have been placing the How High SP in the main body tube, instead of using a payload section, and reported good results. While the ejection pressure won't affect the reading, recovery wadding must be carefully used to insure against "meltdown".]
I know I've been long winded and this discussion is not as satisfying as just saying “Your altitude is accurate to XX feet”. However, the situation is complex. I would be very leery of any altimeter manufacturer claiming a specific altitude accuracy number.
Best regards,
Dave West
