Easy Mini Altimiter

[SuperNova-Rocketry]

Hello,
I'm using the Easy mini in one of my HPR projects as its able to get the job done. I was testing it and it says the altitude is 380 ish meters inside of my house when I am at 220 meters of elevation, I will test it again outside to see if it was just inside my house but the reason i'm concerned is that my main is set to deploy at 250 meters (the rocket is only going to 1 km). Do you think that I should set it higher so that it doesn't end up never deploying, or to test it on the launch site and add 200 meters to what it reads on the site?

Tell me what you think

 

[Lonnie Utah]

What base temperature and barometric pressure do you have it set to? Remember all simple altimeters operate off the ideal gas law pv=nrt. Changes in temperature or barometric pressure from when the altimeter was originally calibrated will make your altimeter change elevation. Ideally you should calibrate your altimeter every time you use it.

 

[cerving]

Do you have the heater or A/C on? That messes up the STP pressure-altitude model so your indoor reading is likely to be significantly off compared to the outdoor reading (assuming you give them sufficient settling time).

Rocketry altimeters are not weather stations, the temperature sensors in those digital baro chips aren't very responsive, and there usually isn't a correction for known ASL barometric pressure. It does not really matter... AGL altitude is computed using the RELATIVE change in pressure, and the short period of time from launch to apogee is unlikely to be seriously affected by temperature differences anyway. I've done comparison tests with different Eggtimer and third-party altimeters in actual flights, they all tend to be very close to one another (< 0.5%, typically).

 

[cls]

The altimeter will fire the main while descending through 250 meters above the launch site. It remembers the altitude just before launch.

It's all relative, absolute measurements don't matter.

 

[SuperNova-Rocketry]

What base temperature and barometric pressure do you have it set to? Remember all simple altimeters operate off the ideal gas law pv=nrt. Changes in temperature or barometric pressure from when the altimeter was originally calibrated will make your altimeter change elevation. Ideally you should calibrate your altimeter every time you use it.

I see, How do I recalibrate it?

 

[SuperNova-Rocketry]

The altimeter will fire the main while descending through 250 meters above the launch site. It remembers the altitude just before launch.

It's all relative, absolute measurements don't matter.

Thats good, is that definitely on the EasyMini? I wouldnt want to find out it wasnt when its barelling down at 30m/s

 

[SuperNova-Rocketry]

Do you have the heater or A/C on? That messes up the STP pressure-altitude model so your indoor reading is likely to be significantly off compared to the outdoor reading (assuming you give them sufficient settling time).

Rocketry altimeters are not weather stations, the temperature sensors in those digital baro chips aren't very responsive, and there usually isn't a correction for known ASL barometric pressure. It does not really matter... AGL altitude is computed using the RELATIVE change in pressure, and the short period of time from launch to apogee is unlikely to be seriously affected by temperature differences anyway. I've done comparison tests with different Eggtimer and third-party altimeters in actual flights, they all tend to be very close to one another (< 0.5%, typically).

I see, thank you, Ill check outside when I have time this weekend

 

[kramer714]

Let me start with I don't know what correction factors the easy mini use. Having said that there is actual altitude and density altitude. Im a pilot, the altimeter is just a barometer scaled in feet (meters) not pressure. There are 3 different altitudes, Actual Altitude, pressure altitude and density altitude. All of these are based on what is called standard temperature pressure STP

Pressure Altitude: the altitude displayed on the altimeter when the altimeter is set to 1013.25 mb or 29.92 inHg. standard pressure

Density Altitude: the pressure altitude corrected for non-standard temperature. This is the altitude in the standard atmospheric model corresponding to the current ambient air density.

Standard Temperature and Pressure STP is 29.92 and 15C or 59F The altimeter, without correction would only read the accurate altitude on an STP day, change the temperature or barometric pressure and the altimeter will be off. The pilot corrects the altitude by putting in the ACTUAL altitude, and or by correcting the atmospheric pressure. .

Just checked, Santa Ana airport (near me) is at an altitude of 17m above sea level, the pressure 1021.9 and the temperature is 16.4C, actually pretty close to STP, but correcting for pretty close gives;

Actual Altitude 17M
Pressure Altitude -47M What the altimeter would read WITHOUT correction
Density Altitude -22M What the altimeter would read with ONLY the temperature change calculated in

Obviously the airport isnt 47m below sea level, but that is what the altimeter thinks. When you come to land the airport gives you the 'altimeter setting' or the correction needed a that airport at that time.

So, the good news, The altimeter reads a change in altitude for the events, 250M is still 250M above where the altimeter thinks it is when you turn it on.

if you want to test this, find the barometric pressure at the closest airpotr and check the temperature, use the calculator at https://aerotoolbox.com/density-altitude/ Density Pressure


Side Note if the pilots mess this up, or the tower gives the wrong info (see link) to the pilots it can end bad.

5 feet from crashing A320

 

[cls]

Thats good, is that definitely on the EasyMini? I wouldnt want to find out it wasnt when its barelling down at 30m/s

Yes. That's how it works. Other people have used it successfully. Don't overthink this.

 

[SuperNova-Rocketry]

Yes. That's how it works. Other people have used it successfully. Don't overthink this.

Okay thanks
Ill check the altitude displayed outside tommorow morning anyway and see if it needs calibrating

 

[Handeman]

Okay thanks
Ill check the altitude displayed outside tommorow morning anyway and see if it needs calibrating

If you want an AGL number, just provide a way to zero the altitude readout when the altimeter is powered up. It doesn't matter what altitude it is calibrated for, it needs to reference the altitude it's at when on the pad as zero and everything else is based off that zero.

 

[cls]

Okay thanks
Ill check the altitude displayed outside tommorow morning anyway and see if it needs calibrating

no no no no no! it doesn't need calibrating! that's the entire point here. I guess you do not understand it, so just repeat to yourself over and over: "the altimeter does not need calibrating." it's fine, let it do its work.

if you really need a calibrated real altitude, get the USGS topographic map and read off the contour lines. PS: at any moment, your barometric altitude will not exactly match! neither will any GPS altitude match, for different reasons.

 

[SuperNova-Rocketry]

no no no no no! it doesn't need calibrating! that's the entire point here. I guess you do not understand it, so just repeat to yourself over and over: "the altimeter does not need calibrating." it's fine, let it do its work.

if you really need a calibrated real altitude, get the USGS topographic map and read off the contour lines. PS: at any moment, your barometric altitude will not exactly match! neither will any GPS altitude match, for different reasons.

Ohh, i’m sorry I was under the misconception that it needed calibrating due to an earlier message. Thank you for clarifying

 

[SDramstad]

It will work just fine straight out of the box. Dont mess with it.