On Fri, 25 Sep 2009 16:08:02 -0700, snead1 wrote:

On Sep 24, 5:02Â*pm, Martin Gregorie
wrote:
On Wed, 23 Sep 2009 17:53:24 -0700, snead1 wrote:

Shat is a thermal detector?

Typically an DC amplifier watching a tiny, naked fast response
thermistor on a 5m (15ft) pole. The thermistor should be sensitive to
air temp, so it is fitted with a sunshade to keep direct or reflected
sunlight off it. They are also typically high resistance units (20K is
a sensible minimum) so the sensing voltage doesn't warm them. If this
happens the detector is sensitive to wind speed - something we don't
want.

Output is normally an analogue dial or a chart recorder built from RC
servos though I have seen one with an audio output tone that rose and
fell with temperature.

Thermal detectors can be quite sensitive. Full scale deflection with a
0.8 C temperature change is not uncommon, so the better units are
designed to let the zero setting track average day temperature. All
have a gain control, needed because the temperature swing as a thermal
blows through rises during the day, peaking in mid-afternoon.

Some people use digital thermometers, but there are problems - the
sampling rate is often far too slow (usually every 3 or 10 seconds) and
the sensitivity to small temperature variations is limited by the
display. I've seen none that can show changes of less than 0.1 degree.

That's probably more than you wanted to know, but there you go.

--
martin@ Â* | Martin Gregorie
gregorie. | Essex, UK
org Â* Â* Â* |

Does anyone have experience with using thermal detectors to increase the
"get away rate" when auto or winch towing full size gliders?

I should have added that photos and circuits of the thermal detectors
I've built are he

http://www.gregorie.org/freeflight/t..._detector.html

The article mentions 'mylar ribbon' in passing. This is another low-tech
way of finding thermals. 5-10m of the thin aluminised ribbon out of a
mylar capacitor (it is around 25mm [1 inch] wide and 0.5 microns thick)
is attached to the top of a 5m fishing pole. When the ribbon is lifted
horizontal and you see big waves running along the ribbon and/or it
points upwards a thermal is passing over the pole. If there are several
mylars on the field and the day is calm, they will all point at the
thermal - though this arrangement doesn't really belong on a glider
field!


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |