"Mike Borgelt" wrote in message
news
On Thu, 15 Jan 2004 13:13:39 -0700, "Bill Daniels"
wrote:


"Bill Daniels" wrote in message
...
I have been conducting some bench experiments with my Bohli taut-band
bellows with capillary leak vario to determine the system time
constant.
The set-up is a 0.45 liter glass vacuum flask attached to the vario
with
.5
meter of 3mm Tygon tubing with a 1cc syringe teed into the tubing at
the
flask. The static/TE port on the vario is left open to the atmosphere.

By injecting 1cc of air into the flask, I simulate a sharp edged
thermal
encounter. This causes the vario reading to instantly jump to about 5
meters per second up and then decay to zero. I am starting a timer at
various points on the scale as the needle falls and timing the decay to
half
that value. (For example, start timer at 4 MPS and stop at 2 MPS) I
am
consistently getting 7 seconds for this decay interval.

The Sage web site suggest the test should be for the decay to 37% of
the
initial value. Using 37%, the decay interval is 11 seconds. Sage
claims
1
second decay.

Repeating the 50% test on a 5MPS vane-in-torus type vario resulted in
values
of only 2.5 seconds.

The decay values for the Bohli vario seem large to me. I am starting
to
suspect that the 0.45 capacity is far too large for the size of the
capillary in the Bohli.

Is this type of experiment of any real value? Any comments?

Bill Daniels


Since I can't edit a previous message, I'm just replying to myself.

I decided that the experiment needed some improvements so I connected the
syringe to the TE side of the vario along with a 1 liter vacuum flask and
a
helicopter airspeed indicator that reads in one knot increments. This
allowed me to closely control the pressure change on the TE side and more
accurately simulate flight conditions.

Setup:
Capacity side = .45 Liter glass vacuum bottle
TE side = 1.0 liter glass vacuum bottle with syringe and ASI connected by
"T" fittings in the tubing.

I injected an amount of air corresponding to 30 knots on the airspeed and
adjusted the syringe to keep it there. (Essentially a instant step change
in
the pressure.) This resulted in just about full scale down (6Knots) on
the
vario. Timing the decay from 5kts to 1.85 Kts (Tau or 37% of 5 Kts.) gets
7
seconds. This is a bit faster than the 7 seconds to 50% decay previously
but
still too long for my taste.

To test the scalar accuracy, I replaced the ASI with a known-good 0-2000
FPM
rate of climb indicator and withdrew air from the TE side so as to hold a
constant 5 meters per second up on the vario until I had a steady reading
on
the ROC. That reading was 1200 Feet per minute or just over 6 MPS. The
vario seems to be under stating the rate of climb by 20%.

Bill Daniels


Bill,

I tested a Bohli about 16 years ago and got similar results to you. It
did seem quite slow.

Now whatever has happened to r.a.s.? Real data with real apparatus and
no arm waving??????

Mike Borgelt

To be fair to the Bohli, I don't think my "desktop instrument lab" is all
that accurate. There are several possible explanations for the slow
response other than the instrument itself - instrument interactions for
example. I'd need a full instrument test bench to do it right.

Still, I have a B40 on order to replace it. I just don't think bellows with
capillary-leak varios are in the same league as altitude derivative units.

Bill Daniels