Interesting, but I'd have to agree. If something is only good to 34k, and it's flown to 40k, can it be trusted? Good thing I never fly more than 1500' in day 🙂

Edward

It's a tough call, but Curt goofed. Now if he brought calibration data that showed that particular baro altimeter has good data at those altitudes and I were Tripoli's contest director, I would accept it. I agree that since the GPS data isn't what the witnesses signed off on (if that part of the story is correct), he should re-fly it if he wants to use that data for the record.

It's a tough call, but Curt goofed. Now if he brought calibration data that showed that particular baro altimeter has good data at those altitudes and I were Tripoli's contest director, I would accept it.

I was told that Jim Wilkerson did precisely that for his N boost... and it was still denied. I witnessed his boost at BALLS a few years ago. It was a monster blast, bullet straight, mid 40's, and it was denied.

Knowing these guys, I'm betting there is more to the story here that we haven't yet heard. It is a small world. I'd bet CVD knows Jim Wilkerson, and I'd bet he knew of Jim's record being denied.

The Rocketry Planet article claimed that only the G-Whizz unit is OK over 34K MSL. What about the Missile Works and PefectFlite high altitude products? Jim claims 40K, and PF claims 45K. Adrian, I understand your unit is also OK to those altitudes? Adept also has a 50K unit....

Further to THAT, Curt has the L record, well over 31K AGL, but that would be over 35K MSL. I wonder which altimeter he used for THAT boost?

The Rocketry Planet article claimed that only the G-Whizz unit is OK over 34K MSL. What about the Missile Works and PefectFlite high altitude products? Jim claims 40K, and PF claims 45K. Adrian, I understand your unit is also OK to those altitudes? Adept also has a 50K unit....

I think that comment about the G-wiz unit was just Tom Rouse thinking off of the top of his head in an email that was posted in the article. He has stated tha the Parrot altimters are certified for Tripoli records. The Parrot's baro sensor is spec'd for sea level to vacuum. When I calibrate them I can set them up to basically zoom in on the 0-30kft range for better resolution or look at the whole range, from 0 to 100+ kft. At 100 kft the atmosphere is so thin (1% sea level) and the pressure changes so slowly with altitude that the readings will be inherently less precise, but it's still a valid reading within the sensor's measurement range.

Given Tom's history of rejecting altitude claims, I'm waiting to see what happens when someone claims a "serious" record on a Parrot. (J or above). There have been a lot of rejections on the basis of electronics and as far as I know, no true certification program. He doesn't test them to see if they're OK, he just goes on manufacturer's claims and the specs on the baro sensor. As evidenced by the rejected N record, he seems to be somewhat arbitrary about it - though since I don't know him nor do I know all the rest of the background on that story other than what John Wilke has told me, there may be other issues involved.

Warren

I felt really bad for Jim W a few years ago. He apparently had his altimeter checked in an FAA certified chamber, and he had a gorgeous boost. It was the same day as Gene N's 90K+ boost, IIRC. It would be a real bummer to fly to 40K+ and have it rejected if you had done the legwork to stay ahead of that.

Adrian, can a Parrot lie on edge or on its back and record accurately on the baro sensor? Will the accelerometer data just be gibberish? in other words, could I fly one flat on it's back and just use the baro component?

The baro sensor sensor would put out good data, but it wouldn't get recorded because the Parrot uses the accel to detect the launch. Either end up is fine (new feature) but sideways wouldn't work. How many Gs are you expecting around liftoff?

You know, it's almost time that all altitude records start again from scratch.

Maybe it's just me, but it seems that there’s way too much discussion about competing altimeters rather than competing rockets. I mean after all this is a rocket forum. I wish it were as simple as every altimeter using the same calculation for altitude. Forget about true height MSL, just worry about the difference from liftoff to apogee. Forget about temperature variances in the vertical column of air, or changes to US or international SAM. Everyone should be forced to pick a formula - and preferably a product line, i.e. Freescale Semiconductor - and go with it. Maybe we could even eliminate the 2% rule with more standardization.

Other things to consider:
-Consumer GPS is not accurate, and it's not very fast – don’t even think about it.
-Pico's been thrown out
-You just can't use electronics outside their specifications

Adrian, I love you're altimeter - and I really want a dual deployment 18mm altimeter.

But if I fly a rocket against someone else's - and it’s for any kind of contest, club record, or Tripoli record - I want if to be an apples-to-apples comparison. It seems to me that a shorter, smaller diameter rocket flown on a hot summer day should fly higher.

Just my rants for the day....

But if I fly a rocket against someone else's - and it’s for any kind of contest, club record, or Tripoli record - I want if to be an apples-to-apples comparison. It seems to me that a shorter, smaller diameter rocket flown on a hot summer day should fly higher.

Well said.

And a shorter, smaller diameter rocket flown on a hotter day will fly higher. But that's not what any of the current altimeters will tell you, as compared to what they would tell you for the same rocket flying on a colder day.

I look forward to the day when GPS gets better, smaller, and cheaper, so that we can all use that for our altimeters and we don't have to learn how to convert pressure to altitude.

In the meantime, I'd like to see an independent certification program for altimeters, and I'm advocating for an improvment to the standard methods for pressure/altitude conversion so that you can use whichever altimeter you want and be able to count on it giving results that are accurate within 2-4%

I've long felt that none of the commercial altimeters on the market are either accurate or precise. This is largely a matter of the economics of the marketplace to be sure. As I see it, there are several problems.

1 - the tolerances and variability in the baro sensors used are quite wide. Manufacturers can have quite wide differences between individual examples of the exact same part. On top of that add the issues we've been discussing caused by the SAM and ambient air temperature. On this basis alone I tend to doubt the accuracy of ANY altitude record based purely on barometric methods. The temperature sensitivity of all strain gauge baro sensors make all of them suspect without corresponding temperature data for both the sensor and ambient air.

2 - the code used to process the raw data from the baro sensors is essentially a trade secret for every manufacturer. All a flyer has to go on is their word that they implement the SAM and who knows what shortcuts they've taken along the way. A good example being Robert DeHate's altimeters which perform no temperature compensation.

3 - It probably would be better to call these devices deployment controllers rather than altimeters as the altitudes reported are incidental to detecting apogee and proximity to the altitude they were initialized at. Basing a record on a device with 200' of slop in reported altitude or perhaps worse when given the tolerances and variation in baro sensors seems foolish to me if one is truly trying to determine an absolute altitude.

An independent third party SHOULD be certifying altimeters used for altitude competition - Tripoli should have been doing it when they started certifying world altitude records for rockets but they plainly didn't. The difficulties are great - the need for an absolute pressure chamber that allows fine resolution and repeatability means a very expensive instrument is required to perform the certification. Also, I wouldn't be surprised to find substantial variation in reported altitude between examples of the exact same model altimeter.

Much of this comes down to the fact that these baro sensors are consumer grade parts, not calibrated lab standards. If manufacturers accurately calibrated altimeters the way lab equipment is - traceable to NIST standards or similar, we'd be paying $500 or more instead of a $100.

I'm open to the debate on this - what could be done to produce accurate and precise altimeters for hobby rocketry use?

I hate to keep disagreeing, but hopefully I can do it without being disagreeable:

Measuring delta-pressure with 1%-2% accuracy is very do-able with today's altimeters without any price increase.

A 0.5% accuracy pressure reference gauge with a calibration to a 0.025% NIST-traceable standard costs under $300. A microcontroller with a 13-bit A/D and a built-in temperature sensor costs about $4. Pressure accuracy under 1% is basically free if a manufacturer takes the time to do a calibration at all.

Even better are some new digital sensors that cost about $15 in quantity and output serial pressure and temperature data with 15-bit resolution and are spec'd to 150 Pa absolute accuracy (0.1% absolute accuracy!) over a wide temperature range. And typical errors are signficantly less. It's a very low power, small device that reduces parts count and eliminates the time-costly calibrations, and the only downsides are a 30kft equivalent max altitude and a relatively slow update rate of about 7 Hz for the 15-bit resolution.

The problem with baro altimeters is clearly not the ability to measure pressure accurately. All manufacturers should easily be able to pass a certification test with the bar set to 2% absolute accuracy over a reasonable temperature range.

The only inherent problem with baro altimeters is in converting pressure to altitude. And here, the manufacturers have taken the easy way out by just operating as if the standard atmosphere model is the best -or only- way to convert measured delta-P to delta-h. If the SAM were just randomly wrong with weather systems, air inversions and the like, that would be one thing. But the fact is that there is a straightforward means to make a 10% accurate altitude sensor and make it a 2%-3% accurate altitude sensor, but almost nobody even knows that there is a fixable problem.

If all the altimeters reliably reported altitude to 2%-3% accuracy, there would be no reason to use optical tracking for rockets above A or B impulse.

Adrian, I gladly defer to your knowledge and experience on this - I have no problem being taken to task - my last serious look at baro sensors was 5 or so years ago and parts weren't nearly as good as they are now.

My main overriding issue comes down to this: An altimeter HAS to be an appliance - what it reports is what it is. Post-processing data offboard is almost certainly a non-starter for most flyers. What the altimeter reports should be what the altitude is. It sure would be nice if altimeter manufacturers reported their actual accuracy - 0-25K +/- x' or accurate to X%. I don't think I've seen any manufacturer state accuracy specs.

Warren

Post-processing data offboard is almost certainly a non-starter for most flyers.

I think 10% accuracy and a successful deployment is fine for most flyers. But if you're filling out a record form to Tripoli, then A) you're already not "most flyers" and B) the form already asks for the ambient temperature anyway.

I'm just proposing that we should:
1. Educate the folks who are interested in altitude accuracy about how to get the most accurate information out of their altimeter and

2. Use best practices when it comes to records and contests. And that means at least using a look-up table correction for altitude based on the ambient temperature at the time of the launch. And optionally, looking up the closest weather balloon data. Ideally, there should be independent certification of pressure accuracy, with the cost shared by Tripoli/NAR and the altimeter manufacturers who want to get certified. That last part may become a moot point if the new digital sensors are adopted more widely, since they have true factory calibration and sensor temperature compensation built-in.

All a flyer has to go on is their word that they implement the SAM and who knows what shortcuts they've taken along the way. A good example being Robert DeHate's altimeters which perform no temperature compensation.

By the way, I'm kind of curious why Robert DeHate's altimeters got singled out on this, because I haven't seen evidence that any altimeter compensates for the altimeter temperature, other than the Parrot. Maybe somebody noticed that his pressure readings were particularly sensitive to temperature? I wonder if the other ones have been checked, also.

I don't know the full story, just have seen posts specifically pointing out Robert's altimeters as being unsuitable for any Tripoli altitudes due to a lack of temp compensation. I have been under the impression that Missileworks and Perfectflight altimeters did do temp compensation, but I have no specific place I can point to that says so and perhaps I got that idea purely by inference from the fact that they are acceptable for Tripoli altitude records.

By the way, I want a lot better than 10% - I want something like 20' resolution up to 20K, maybe 50' resolution to 40K and 100' resolution out to 80K. Doesn't seem too much to ask although I know damn well baro sensors aren't the way to go about getting it. A combination of GPS with WAIS should keep you within 20m for elevation, but getting that grade of GPS is a tough go from my understanding.

Warren