It's just a little more complicated than that (but not a lot).

Each thermistor was run in a feedback network to maintain a constant
resistance (i.e. temperature) and the difference in power required
measured. It needs careful matching of resistors at the required
temperature. Adding the centre one was Raouf's enhancement which
made it easier to balance everything, and improve both linearity and
signal difference. The origin of the system was Harry Cook's electric
vario in the 1950s; everything after that was minor refinements until
solid state pressure sensors came along, doing away with the capacity.

The Ball vario was a good try at a different solution, well engineered, but

a lousy instrument.

At 21:13 17 December 2017, Kiwi User wrote:
On Sun, 17 Dec 2017 12:17:54 -0800, Dave Nadler wrote:

On Sunday, December 17, 2017 at 3:15:03 PM UTC-5,

wrote:
(old) Cambridge varios used 2 thermistors in line to sense flow.

IIRC 3 thermistors; Raouf added a non-sensing thermistor in the
middle
to increase temperature differential. Earlier units like Crossfell used
2.

Thanks: effectively a hot wire solution, then, with the centre
thermistor
serving as the hot wire.

I presume the two-thermistor was effectively the same, i.e. pushing
current through both thermistors and relying on the upstream
thermistor
to warm the air before it reached the downstream one.


--
Martin | martin at
Gregorie | gregorie
| dot org