Where is the next thermal?

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