The Altimeter One is a very small and easy-to-use altimeter for model rockets, airplanes, kites and even birding.
Jolly Logic's Altimeter Two is similar, but has specific features for model rockets only, and is a bit more sophisticated and a bit more expensive.
I bought mine for $50 (Altimeter One) and $70 (Altimeter Two). This was money well spent as these altimeters have enhanced my enjoyment of rocketry a lot. No more guessing about how high the rocket went, or relying on RockSim or other programs that are only aproximations. What you get with these altimeters is the cold, hard truth whether you like it or not. These models are so small and so light they are useful for low-power rocketry as well as higher powers. The altimeters are small enough to fit inside a BT-20 body tube or payload, and they only weigh about 7 grams - or about 1/4 oz. The daylight-readable LCD is super-simple to use, you don't have to download anything or count beeps to get answers.
With the numbers provided by these two devices, you can gauge performance of various motors and measure flying in various winds without having to rely on rough "guestimates". You can have real measurements of the differences of various motor types and brands and explore how weight affects altitudes. You can have "Guess the Altitude" games and even have altitude contests.
The altimeters are claimed to be able to measure altitude to the foot up to 9999 feet and within 10 feet up to 29000 feet (over 5 miles!). It is not possible to download any of the data to a computer. These devices just don't have the circuitry necessary. Fine with me, that would just make the altimeters heavier and more expensive and more complicated! Battery power is claimed to last for 24 hours or more on a full charge. I know that I've been able to get in a whole day of flying without having to recharge these devices.
A single button allows you to control the altimeter. When the unit is off, pressing the button will turn it on. When it is on, pressing it will turn it off. To clear the data and prepare for the next flight, press and hold the button until "0000" appears and then release it. The unit is then cleared and armed for the next flight. If you turn the unit off or it goes off by itself with the battery save feature, the data is not lost. Simply turn the unit back on and the 'One' will show the apogee altitude. The 'Two' will alternate between the altitude and the speed.
To access some other functions in these units, simply hold the button down while the unit is on and watch the LCD. When the feature is displayed that you want, release the button. On the 'Two' the first choice that appears is "data", so when you release it you see other readings such as acceleration, descent time, etc.
Both units allow you to switch between two different units - English or Metric. English feet gives you more altitude precision than is available with meters.
Both of these altimeters, the 'One' and the 'Two', use a special internal lithium rechargeable battery and are easily recharged. You don't need to worry about power as long as you charge these units the night or morning before flying. Charging generally takes an hour or two. They are plugged into a USB port (like on a computer) to receive power and a red light indicates it is charging. When it is done charging you will see a green light. It is that simple.
I imagine you "might" be able to charge them on a standard 12V-to-USB adaptor such as provided with many cell phone or MP3 mobile charging devices, if for example like me you forgot to charge it the night before. As a matter of fact, I did just that to my first Altimeter One and was able to use it for the day but I had to wait about an hour to top it off before flying. I say "might" because in the next month when I went to charge the 'One' up the night before - it wouldn't take a charge and no lights would lite - the unit was dead. I suspect but can't confirm that the car charger may have damaged the 'One'. In my case, I took the unit apart, removed the battery and recharged it using a precision micro-amp current source. The altimeter circuit worked fine, just the charging circuit failed. This allowed me to "limp-by" using it for a few more months until I ordered a replacement.
I have a club buddy who had the 'One' and he claimed both red and green lights lit and it wouldn't work either, but I don't know what he used for a charger. So be aware that you use these products at your own risk, and if they stop working that's the end of it as Jolly Logic does not repair or warrant these products. That may change if the company grows and can later provide such a service. I personally think that the data I get from flying is worth the risk.
My first experience was with the Altimeter One which uses a precision air pressure sensor to determine altitude. This can be used on model airplanes, kites or even large birds. As luck would have, the Altimeter Two, with more features, was introduced just after I ordered the Altimeter One, so of course I had to get the 'Two'.
The Altimeter Two has the same air pressure sensor of the One, but adds a three-axis accelerometer which allows, in addition to peak altitude, many other measurements including speed, motor burn time, coast time to apogee, apogee to ejection time, average acceleration, peak acceleration, descent speed, and overall flight time.
For only about $20 dollars more, I would recommend getting the 'Two' which provides much more information. But the 'One' is a better deal if money is tight and altitude is all you are looking to measure. A small hole and clip are provided, so the easiest way to use these is to clip this unit onto the screw-eye of your nosecone, but you must provide some small static air vent holes in the body tube to allow the outside air pressure into the altimeter for making measurements.; I have tried it without static vent holes and it still worked, but whether the reading was valid or not is always going to be a question. Jolly Logic claims that they will work fine when clipped with the parachute, but again - they don't provide any warranty on these devices (which is totally understandable considering the way they are used). You must accept that you are using these at your own financial risk.
A better way, and the way I normally use them, is to fly them in the payload bay of a rocket that has a payload. Since most of my rockets didn't have a payload, I made small additional payload bays for my rockets by using a bulkhead and a section of body tube below the nosecone. This provides much more protection to the altimeters, since they will not be shaken violently on ejection and are completely protected from ejection gasses and particles. They are also better protected from striking the ground, pavement, or rocks directly upon landing. I had a buddy of mine loose his when his clip failed and it fell out of the sky from several thousand feet never to be seen again - you can see it on his video of the flight.
Adding a payload section for these is quite easy and the cost of a little balsa and cardboard is much cheaper than replacing the altimeter. (Note: I am preparing an article about adding payload bays to low-power rockets which should be published soon on this very website.)
The One is the more affordable of the two units, but is only able to measure altitude. It can't be more simple. Turn it on, clear the reading, and fly it. When the flight is over, the highest altitude is shown on the LCD. Then either turn it off or clear it to fly it again.
The 'One' will memorize the air pressure when you clear it. Assuming that you are standing on the ground where you will launch it, it calls that zero altitude. When it begins to sense a rapid drop in pressure of about 50 feet or so, it will make rapid measurements and remember the lowest air pressure reading that it measured and convert that into altitude (it assumes a "standard" atmosphere which may not always be the case, but that's how airplanes, helicopters, blimps and just about anything controlled by the FAA measures altitude). I believe that it also measures temperature and compensates for the difference in temperatures - I'll need to confim that though. All I do know is that these devices are very accurate (repeatable) and precise (able to distinguish fine differences). They have been accepted by one of our big organizations for competition, but duh...I can't remember which, maybe the NAR?
Since the 'One' senses the launch with an air pressure drop, it may be succeptable to momentary gusts of wind, but that hasn't happened to me ever. I have been told that using three equal-sized static vent holes around the rocket at 120-degrees is more imune to false triggering than four holes spaced 90-degrees. This is from a club member that has experience with a number of other altimeters. My experience suggests that the Altimeter One is very good at not falsely triggering a reading with light wind gusts.
The Two is the more advanced model for a slightly higher cost. But the extra cost gives you much more data to work with. I imagine the most popular would be speed. That's always a satisfying number to get. My personal favorite after altitude and speed is the coast-to-apogee time. With this information, I will get valueable information on whether my delay time was too short or too long or OK as is.
While it's very difficult to see from the ground, via the Altimeter Two, I've noticed several of my rockets deployed before reaching apogee, and apogee always follows shortly thereafter as the greatly increased drag tends to stop the rocket within a few feet while over-stressing the recovery system. Other rockets - we've all seen this - seem to coast horizontal or even downwards forever before ejection, but it's hard to judge exactly how much too far. Some rockets seem to speed along horizontally, slowing down some and only loose a few feet of altitude, where others seem to do the same but actually loose 30 or 50 feet or more.
The 'Two' has a three-axis accelerometer inside, so instead of measuring air pressure drop to determine liftoff, it senses a sudden change in motion. I suspect that's how it determines ejection time, also. This makes it immune to wind gusts, but also makes it sensitive to shaking and jarring before launch. I had many flights with invalid data because of shaking the rocket too much after arming it and before launch. Once I learned this however, I clear and pack the altimeter last and handle it not quite so roughly before launch, and I've never had this problem again.
I actually learned about that by sending an email to the support folks at Jolly Logic from their website. To my amazement I received a reply whithin maybe one hour! The false-triggering problem was solved and I was happy.
I can describe how much I like these products by telling you that even though my first Altimeter One died on me after one day of launches, I went and got another one. In fact, I actually bought TWO Altimeter Twos, one as a backup just in case I lost one or it died on me. I simply don't want to fly rockets with out these on board. That's pretty much my endorsement - but then I'm a nerd rocket scientist and I like numbers. I'm building a spreadsheet full of performance numbers and graphs for all my rockets.