Baro Sensor Works!
It’s amazing how often a problem that seems unsolvable while you’re working on it ends up having an easy solution after you put it aside for a while.
Several months ago I wrote about a problem with a Measurement Specialties barometric pressure sensor. I had come to the conclusion that either the sensor was faulty, or I had damaged it during installation on the board. I kept putting off desoldering it, partly because it’s a challenging part to solder, and partly because I only had two spares, and they’re expensive; I didn’t want it to be faulty.
Well, late last night I got the bug to look at it again. The data sheet shows the calculations that need to be made to get a calibrated result, and shows “typical values” for each of the six factory calibration parameters, uncalibrated pressure and temperature measurements, and each step of the process. It never dawned on me that those values might all be part of one measurement and calculation operation.
So I wrote a small app on the Mac that used the same calculation code that was on the sensor board, but put in the example values instead. Sure enough, the result I got did not match, and I started looking into the intermediate results. I noticed one of those was exactly double the example value, and that got me looking at the implementation of the equation. Looking very closely at the data sheet, I started re-writing the equations. Turns out, the code I had found on their site was incorrect, and the code I wrote based solely on the data sheet worked correctly.
For reference, here is C/C++ code that works. mC1 through mC6 are the calibration parameters from the device ROM. mRawTemperature and mRawPressure are the raw sensor readings. mTemperature and mPressure are the final, calibrated result. Temperature is in degrees Celcius * 100, so you have to divide the result by 100 to get the temperature. Pressure is in millibars * 100, so do a similar division to get mb.
This sensor should prove to be very accurate, and will give us the balloon’s pressure altitude, as well as the temperature inside the insulated payload box. It’s only real drawback is a lower pressure limit of 10 mbar, corresponding to an altitude of about 26 km (~85 kft). We’re hoping to go past 30 km (~100 kft). Hopefully the GPS will be a good backup.
Decimal Days and Metric Time
I always thought it would be cool if we counted time in units that were multiples of ten of each other. This is known as decimal time. There’d be 100 seconds in a minute, 100 minutes in an hour, and ten hours in day. A recent Twitter conversation got me thinking about this again.
The problem with this idea is that it requires the second to be redefined. To be sure, there are other problems, too, like how do you convince six billion people to change their notion of what seconds, minutes, and hours are?
But changing the length of a second means changing a lot of scientific constants, and that’s a pretty serious undertaking. It would be better to start with the existing unit of a second (which, incidentally, is a metric unit), and build on top of that.
So, let’s put aside the actual labels we’ll give each of these units, and let’s keep a second a second. Now let’s put 100 seconds in a minute, 100 minutes in an hour, and that gives us 8.64 hours in a day. That’s not a very nice, round number. How important is it that it be a “nice” number, though?
On Earth, the average day is 86,400 seconds long. On Mars, the average day (sol) is 88,775 seconds. In our decimal time units, that would be 8.86 hours. On Earth’s moon, a day is 2,551,443 seconds long, or 255.1 hours (are you starting to see the advantage of decimal time?). Obviously, as our species spreads out to other celestial bodies, having an even number of hours in the day everywhere will impossible, because not all days will be the same duration.
So, let’s compare some other durations:
| Duration | Traditional Units | Decimal Time |
|---|---|---|
| Second | 1 s | 1 s |
| Traditional Minute | 1 m | 0.6 m |
| Traditional Hour | 1 h | 0.36 h |
| Day | 24 h | 8.64 h |
| Shower | 15 m | 0.36 h |
| Typical Work Day | 8 h | 2.88 h |
| Lunch | 1 h | 0.36 h |
| Movie | 2 h | 0.72 h |
Yuck! The problem with decimal time based on the existing second is that there are no conveniently-sized units for most day-to-day human activity.
The quarter-hour, or 15 minutes, is 900 seconds. That’s nearly 1000 seconds, so perhaps the kilosecond would be a convenient unit. Ten kiloseconds would be a little over 2 h 45 m traditional, so maybe we’re on to something here.
| Duration | Traditional Units | Kilseconds |
|---|---|---|
| Kilosecond | 0.27 h, 16.6 m | 1 ks |
| Traditional Hour | 1 h | 3.6 ks |
| Decimal Hour | 2.78 h | 10 ks |
| Day | 24 h | 86.4 ks |
| Shower | 15 m | 1 ks |
| Typical Work Day | 8 h | 29 ks |
| Lunch | 1 h | 4 ks |
| Movie | 2 h | 7 ks |
I’ve rounded the kilosecond times for the activities, because their durations aren’t very precise to begin with. It seems like the kilosecond could be a fairly convenient unit, after all.
Now we just need to find good names for these decimal units. Seconds are fine, but the rest need new names that are easy to say, abbreviate acceptably to unit labels, and don’t sound cheesy (the old Battlestar Galactica used “centons,” and I never did figure out how much time that represented).
We might also ponder how one writes decimal time. Traditionally, in the U.S. and other parts of the world (but definitely not all!), a time of day (or duration) is written as double-digit numerals separated by colons: 12:37:58. Decimal time can be written much more simply, as decimal hours in the day: 4.548. Now, imagine you want to add (in traditional units) 4 minutes and 22 seconds to 12:37:58. Try it. It sucks. But adding 262 seconds to 4.548 decimal hours is much easier: 4.548 h + 0.0262 h = 4.574 h.
It may seem weird, but if you grow up with these units, and everything around you uses them, they’d be obvious, and the units we use today would seem strange.
Sources
Metric time. (2011, February 17). In Wikipedia, The Free Encyclopedia. Retrieved 05:51, March 27, 2011, from http://en.wikipedia.org/w/index.php?title=Metric_time&oldid=414432996
Timekeeping on Mars. (2011, January 15). In Wikipedia, The Free Encyclopedia. Retrieved 05:52, March 27, 2011, from http://en.wikipedia.org/w/index.php?title=Timekeeping_on_Mars&oldid=408111863.
Lunar day. (2011, March 17). In Wikipedia, The Free Encyclopedia. Retrieved 05:53, March 27, 2011, from http://en.wikipedia.org/w/index.php?title=Lunar_day&oldid=419235268.