Temperature Issue

Reinhard said:

I can't provide an answer to the question regarding the behavior of BMP180 below 300mBar, but the reason why the Perfectflite altimeters are specified to 100kft is that they actually use a different sensor. The pictures show what appears to be an MS5607 or MS5611 from Measurement Specialties (formerly Intersema). Both the old plastic version as well as the more recent plastic version can be seen. Those also match the description by Perfectflite (10mBar ~ 100kft, 24bit ADC). The BMP180 has a noticeable different package and can therefore be ruled out, unless the altimeters have since been redesigned.

Generally, every altimeter manufacturer that specifies its products to work at 100kft (PerfectFlite, Altusmetrum, Featherweight *), Black Magic Missileworks *), Entacore **), Altimax) appears to be using Meas Spec sensors.
*) Different sensor in different package, either MS5540 or MS5803
**) Speculative; the pictures of the AIM Extra that I found, don't show the underside with the sensor

Reinhard

Click to expand...

There are different altitude/pressure formulas in different atmospheric regimes. For example, in the troposphere, one assumes a constant temperature lapse (decline) rate with altitude. For much of the stratosphere, the temperature doesn't decrease with altitude - that's another formula. Then it actually rises with altitude. (The one formula in the chip spec is the tropospheric formula.)

I have heard that motors have variation in delivered impulse of up to 20%. I also have observed sim-variation of up to 40% based on trajectory (but that usually accounts for lower than expected flights, not higher). Any reason to think the flight was "less arc-y" than the sim?

The temperature reported by the altimeter is the temperature of the chip on the board, which won't change much during a short flight in a payload bay.

The chip temperature is used within the sensor (or by the circuit designer if you're using Bosch, they make you do it yourself) to compensate the sensor for its own pressure error at various temperatures. That temperature is not used, nor useful, for figuring out how to better convert that true pressure to true altitude. Instead, altimeter manufactures just assume standard conditions. There will be error--roughly 10%-- at commonly-found extremes of hot and cold. You can read Norm Diedzic's nice article on this math in Apogee's newsletter #257.

Bosch (and some others) only certify the accuracy of their altimeters in a certain range. There is some specs-manship going on, like in other industries; if you read the fine print of some sensors with datasheets that indicate 100K+ feet range you may find that their error specs only apply at lower altitudes. In one sense, all pressure sensors can be used to 300K feet--it won't hurt them--but the error band is really wide the further up you go. Like any system, they are designed to work well at the most common altitudes they'll see. At some point, there are just fewer and fewer molecules hitting the sensor, and so what you are reading has too much noise in it (as a percentage) to be very useful.

To all who are curious, here's a video of the launch:

[video=youtube;gsRx6q60VE4]https://www.youtube.com/watch?v=gsRx6q60VE4[/video]

John Beans said:

...The temperature reported by the altimeter is the temperature of the chip on the board, which won't change much during a short flight in a payload bay.

The chip temperature is used within the sensor (or by the circuit designer if you're using Bosch, they make you do it yourself) to compensate the sensor for its own pressure error at various temperatures. That temperature is not used, nor useful, for figuring out how to better convert that true pressure to true altitude. Instead, altimeter manufactures just assume standard conditions. There will be error--roughly 10%-- at commonly-found extremes of hot and cold. You can read Norm Diedzic's nice article on this math in Apogee's newsletter #257.

Bosch (and some others) only certify the accuracy of their altimeters in a certain range. There is some specs-manship going on, like in other industries; if you read the fine print of some sensors with datasheets that indicate 100K+ feet range you may find that their error specs only apply at lower altitudes. In one sense, all pressure sensors can be used to 300K feet--it won't hurt them--but the error band is really wide the further up you go. Like any system, they are designed to work well at the most common altitudes they'll see. At some point, there are just fewer and fewer molecules hitting the sensor, and so what you are reading has too much noise in it (as a percentage) to be very useful.

Click to expand...

And that's why we only rate Eggtimer altimeters to 30K... that's Bosch's spec for the BMP180. It's also in the firmware, although we're probably going to begrudgingly extend that out to about 60K based on some feedback from users that they're willing to trade accuracy (and resolution) for functionality. There aren't many places where you can fly above 30K, so it really doesn't matter much to most sport flyers, but like John says there's a certain amount of gamesmanship involved.

Reinhard said:

I can't provide an answer to the question regarding the behavior of BMP180 below 300mBar, but the reason why the Perfectflite altimeters are specified to 100kft is that they actually use a different sensor. The pictures show what appears to be an MS5607 or MS5611 from Measurement Specialties (formerly Intersema). Both the old plastic version as well as the more recent plastic version can be seen. Those also match the description by Perfectflite (10mBar ~ 100kft, 24bit ADC). The BMP180 has a noticeable different package and can therefore be ruled out, unless the altimeters have since been redesigned.

Generally, every altimeter manufacturer that specifies its products to work at 100kft (PerfectFlite, Altusmetrum, Featherweight *), Black Magic Missileworks *), Entacore **), Altimax) appears to be using Meas Spec sensors.
*) Different sensor in different package, either MS5540 or MS5803
**) Speculative; the pictures of the AIM Extra that I found, don't show the underside with the sensor

Reinhard

Click to expand...

It's totally possible I mixed up the PF sensor since when I was check it out I was building the NAR S&T altimeter calibration chamber using a PerfectFlite Pnut as a reference device, and at work was checking FAA altimeter requirements for drone aircraft registration and my company is using the BoschBMP180 as the barometric altimeter in our drone.

The current generation of temperature compensated integrated digital altimeter chips use standard atmosphere based algorithms and employ the same basic architectural structures and protocols, but the exact hardware and algorithms implementation can differ.

Market these sensors are designed for are industrial, environmental and automotive control, cell phones, weather stations, and picket altimeters. Since no place on earth is above 30 kft or below -1 kft the specifications for these newer devices are typically a resolution of +/- 1 foot. Any significantly higher altitudes have coarser resolution. The specifications for the PerfectFlite units are: 1’ up to 38,000’MSL; < 2’ to 52,000’MSL; and < 5’ to 72,000’MSL which reflects the exponential falloff of atmospheric pressure with altitude. That's why the daasheets state the sensors have a reduces accuracy outside their rated range.

Larry Curcio said:

There are different altitude/pressure formulas in different atmospheric regimes. For example, in the troposphere, one assumes a constant temperature lapse (decline) rate with altitude. For much of the stratosphere, the temperature doesn't decrease with altitude - that's another formula. Then it actually rises with altitude. (The one formula in the chip spec is the tropospheric formula.)

Click to expand...

https://www.meas-spec.com/product/pressure/MS5611-01BA03.aspx# is the datasheet for the MSI MS5611-01BA03 High resolution pressure sensor module (10cm altitude resolution). It is a low power sensor employing a Temperature and Pressure sensor with Integrated digital pressure sensor uP with a 24 bit &#916;&#931; ADC. The manual clearly shows operational range to 10 hPa = 1 mb = 100 kft. The methods for the conversion of measured pressure to temperature corrected pressure altitude is explained in this data sheet. Each sensor is factory calibrated for pressure and temperature response and the raw measurements are corrected by applying a custom curve fit developed for each sensor. I have no idea how "accurate" it is, however if all similar devices use the same methods, it is accurate enough for most purposes/

As I have stated before, if you want the most accurate absolute measurement of altitude at a reasonable cost, below 30 kft use a well calibrated pressure altitude barometric altimeter, and use a GPS unit for measurements above 30 kft.

Bob

Bob,

Hadn't meant to argue with you, and I'm not questioning the accuracy of the electronics. Objectively, however, the given formula is inappropriate much above 30K feet. It's based on a temperature lapse rate that doesn't hold above that altitude, and the inaccuracy will be significant long before 100K feet. So... I guess we differ.

The data sheet, BTW, doesn't say that the instrument uses that formula, or that all electronic systems incorporating the instrument will have to use the formula. It just says that the altitude *can be calculated* that way.

I'm certainly with you on your preference for GPS above 30K feet, even though GPS altitude also has its quirks.

Best Regards,
-Larry

Pressure reading can be reasonably accurate. If you want the best estimate of real altitude you can use balloon soundings.
https://weather.uwyo.edu/upperair/sounding.html

Pick a sounding near where you launch. Fortunately its doesn't vary that much if your reasonably close (within a few hundred miles).