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Thermal Chart Recorder

Recorder set-up testing at 2000 WSMC (Slovakia)

Chart Plot from 1996 WSMC (Slovenia)


For picking thermals at the 1987 WSMC, I tried using a pre-printed grid chart to keep track of temperature by hand. Every minute, taking a reading from a digital thermometer. It ended up being more of a distraction than a help. So in winter & spring of 1988, I worked up a homemade chart recorder. The idea was not new for thermal picking, I had read some articles about F/F model airplane fliers who had done so, and even ran across plans for a couple of really simple ones. I really had to learn more about electronics though, since it would need Op-Amps to amplify the weak voltages being produced by a thermistor in a Wheatstone bridge (temperature) and a small DC motor acting as a generator (anemometer). A big hurdle also was passed when Art Rose provided a Servo tester circuit to use for driving the servos for the pens. Once I learned enough about how to use that servo circuit and the increased knowledge on Electronics (thanks to Forrest Mims' Radio Shack books), that led to another spin-off project, Sunguidance.

The first version of the Chart recorder used a wind-up clock to drive a drum with paper on it (one of the F/F model plane recorder articles had used that method). I would tape down three or four layers of paper, and once an hour peel off one layer so it did not over-write he previous layer. That was annoying enough, also the clock drive was pretty wimpy and fragile. But otherwise, the recorder worked out well for trying to find lift.

I quickly found that normal pens would not work. They needed too much pressure applied to work properly. A felt tip marking pen would work ,but it left a very thick trace and would dry out too quickly. I finally tried a BIC "Roller" pen, and it worked great. So, that is what I used ever since.

The anemometer was made up of a 3 volt DC motor that acts like a generator when turned. The one used was a good one that made more voltage for a given rotation rate than some others I had tried. The motor did not have a lot of friction, but in order to be able to get the anemometer to work well even in low winds, the anemometer used larger cups (Leftover vac-formed Little Joe II Apollo BPC cones) and set up on longer beams than one would usually expect for an anemometer.

A year or two later at a local HAM radio show, one of the vendors had old computer printer paper drive mechanisms for sale. I got one, then went about trying to work out how to run the thing at a reasonably slow speed (shooting for a rate of about 1" of paper movement per 5 minutes). The solution was to get a Stepper motor Experimenter's kit from All Electronics. That allowed being able to set the stepper motor to any desired speed, and even to run it backwards if need be to re-set the paper. However, the stepper motor did not have quite enough torque to run the paper drive reliably. So, a reduction gear was needed. I found a cheap wind-up toy train whose gearbox was enough, something around 9:1 ratio. So, in the end, the stepper motor worked out well for driving the paper. It did need some extra battery power, as it not only needed 12 volts, but used several amp-hours worth to run for a full day. Fortunately though that could be provided by leftover Gel Cells not being used as launch batteries.

I had taken the chart recorder to a couple of NARAMs but had not used it much since it involved so much to set it up and use it. I'd used it at a few local regionals at first too, but more for practice with the recorder as eventually I stopped taking it to regionals. Well, the main reason for building it was for US Team use at WSMC's anyway. It was used in 1989 for the US-USSR Exchange meet at Wallops Island, and at the WSMC's from 1992 thru 2000. It was quite useful on many occasions.

Unfortunately, it has been finicky at times in simply running or not running. I was not good at soldering when I made the circuits of the main control box in 1988, and didn't use shrink-wrap tubing. Over time, the wiring got a bit flaky. Sometimes, a hit to the box was enough to get it going again. Sometimes it was more than that.

Before the 2002 WSMC, I did what I could to find bad wiring inside the main control box, at least what I could find and fix short of getting rid of everything inside and starting over. Also, the recorder assembly was given a bit of an overhaul by mounting it inside of a cheap briefcase. Unfortunately, at the 2002 WSMC, the recorder would not operate right at all. In retrospect, it may not have mattered if it had run, as it was so cold for most of the duration days that it may have been below the temperature range the thermistor Wheatstone bridge could handle (I had found it bottomed out at around 55 degrees of so when testing it in 1988 but figured it would never be used in conditions that cold. Then who expected a WSMC to be held in mid-October). Fortunately, a laptop based Chart recorder developed by James Duffy & Andy Tomasch was on hand, so the team had some use of a recorder, though it had its own practical problems (screen visibility outdoors and battery life).

I plan to make a new control box for my recorder, which will mean junking everything about the old control box. Making all of the circuits brand new, with better soldering skills, heat-shrink, better wiring, and better planning since the old 1988 control box and circuit boards were made up.


Chart Recorder in Briefcase (2002)

(set up for testing at home, with anemometer & thermistor sensors in background. Normally those sensors are atop a tall fiberglass pole)

Closer view with labels added

Servo-Driven Pens

Anemometer, plus Thermistor in Sun Shield

Chart Plots - Pre-printed paper is used, to provide reference grid lines. The time lines are slightly curved to match the radius of the servo pen arms. The paper is driven at a rate of 1" every 5 minutes. The temperature grid is 2" wide (1" above and below center).

At right is a GIF thumbnail from the 1996 WSMC (Click to see a larger version). You can see the temperature and wind plots (there were a few rapid off-scale jumps in the wind plot that was a recorder or servo issue, not the real wind). In general, when the temperature starts rising, and the wind starts decreasing, there may be a thermal upwind. In the example shown, marks and names were written in to indicate when a person's model was actually launched.

I also once tried to work out a remote station to add upwind temperature and wind sensors. Which is why when the recorder was modified for a stepper motor paper drive system, there was room alotted for more servo pens to be added. I was trying to use the same Op-Amp circuits as the main recorder, with the output into a 75 MHz "ground" R/C transmitter, where a 75 MHz receiver would be used with two servos on the paper drive. Unfortunately the RF output of the transmitter fed back into the relatively weak signals of the temperature and wind sensors, making it useless. No amount of shielding could fix the problem.

On a not very windy day that varies the wind direction a lot, usually thermals are causing the wind shifts, "sucking" ground level air into them. It's a lot easier to try to pick lift when that happens, though days of very low wind that allows the wind to vary in direction so much from thermal activity does not happen frequently enough. For a long time I wanted to add a 3rd sensor, a wind direction sensor, but I still haven't come across anything with the right mix of fine resolution and ability to actually point accurately in low winds (very low friction weathervane pivot), that might make such a wind direction plot as useful as theory indicates it might.



Chart Plot from 1996 WSMC (Slovenia)
S4B Boost Glide flights of Dave O'Bryan,
Phil Barnes, and Ben Roberto

A nearly ready-to-use Temperature recorder

Jim Fackert had the idea of using a portable Oscilloscope, with a simple B&W LCD screen, as a thermal chart recorder. It has several limitations, such as resolution, displaying only 6 or 12 minutes of data (with a corresponding plotting delay of 3 or 6 seconds), and only showing one channel unless you pay a lot more for a 2-channel version. But it's a lot easier to set up and use, a lot more portable, and a lot more reliable. It does require making up a relatively very simple sensor box in order to produce a usable voltage output from a thermistor in a Wheatstone bridge, but it's not hard to make up the sensor box (A one night project). This LCD oscilloscope based recorder is portable and easy enough to set up to be practical to take to NARAM and other meets. For more info on it, use this link.

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