Hi,
I am working on an altimeter for a model rocket. I am using a barometer and temp sensor and I am hoping to convert those readings into altitude. I started reading articles and forum archives trying to find a formula to calculate altitude.
The easiest way is to directly convert pressure into altitude. But that has some error as the cold and humidity can mess with the readings.
A trickier way to to use the barometric formula that incorporates temp into the calculation. But after reading forum posts I realized that you need the current pressure at sea level. It was recommended that you check the sea level pressure near the location you plan to launch. But I am too lazy to do that and because most commercially available model rocket altimeters are very accurate and don't have you input the sea level pressure I think that there is a better way.
I was unable to find a solution to this and was wondering if anyone could help
Thanks!
How long will your rocket be airborne? Temperature sensors take several dozens of seconds to stabilize to an accurate new reading. It seems to me, at that rate, you would be obtaining the temperature that is several miles behind the rocket.
Not really. Almost all reports of the "sea level pressure" calculate that value by knowing the location elevation, and the location pressure. You can do that too, if you know the altitude of the launch site.
Thanks for the reply!
I would like to use this altimeter on multiple rockets so it difficult to tell how long each one will be airborne. However couldn't I start recording temp on the ground so it could stabilize its readings on the launch pad?
I will say no. Use the air pressure at the launchpad as zero level. Then You calculate height above ground.
How do You intend to use the knowledge about the height? Do You need to blow the rocket at a certain altitude in order to keep air traffic safe?
Mostly just to see how high the rocket goes, however I think I can divide altitude by time to get velocity.
This data wouldn't be used to control any in flight events!
Okey. Get up on Mount Everest and throw it with Your hand. It will reach some 8800 meters.... Joking!
Time for the shot until it reaches the top depends on the integrated force of the rocket motor. I doubt that will give any reliable results.
As @anon57585045 says, temperature is out of question due to need for constant temperature for long time to give good data. Air pilots learn that temperature decreases by one C per 100 meter...
Use barometric sensors that looks like being faster.
Will data be stored on an SD card or sent as telemetry data to ground?
The effect of the assumed sea level temperature on the altitude calculation is not large. The pressure sensor already takes the local temperature into account.
The standard that I recall is 1"/1000' or for every inch of baro pressure reduction equates to 1000 feet altitude gain. Do you want readings AGL (Above Ground Level) or MSL (above Mean Sea level)?
When temperature is involved it equates to pressure altitude which relates to air-breathing engine performance and airspeed indications for pitot-static airspeed indicators.
My recommendation is:
Use AGL and convert after the flight for MSL if needed.
Check to verify the exact air pressure lapse rate instead of 1"/1000' in case there is significant difference.
Don't worry about temperature because for aviation purposes it does not figure in.
It's basic physics, taught during the first 9 years in school I think.
According to Newton, 500 years ago: Force equals mass times accelereration, F = m*a. Consequently a (acceleration) equals Force divided by mass.
The Force, and consequently the acceleration, likely varies during the shot so integration, like a summing up for the force during each very small time interval is needed.
No Woodoo, no army or sensors, changes that.
Wouldn't they just take the pressure before launch as the ground level pressure? Then calculate altitude above ground level. If you want to know your altitude above sea level, you can add the launch altitude in later.