Interference between instruments

Linking a couple of recent threads: Connecting multiple varios and how to change ASI markings....

I discovered the reason for a flight computer's inconsistent readings was the bezel on the ASI not being tight enough to seal. Snugging it up resulted in a more accurate ASI and flight computer.
Checking each instrument is a good winter project. Techniques are described in the back of Reichmann's "Cross-Country Soaring".
Jim

On Sun, 17 Dec 2017 09:23:17 -0800, JS wrote:

Linking a couple of recent threads: Connecting multiple varios and how
to change ASI markings....

I discovered the reason for a flight computer's inconsistent readings
was the bezel on the ASI not being tight enough to seal. Snugging it up
resulted in a more accurate ASI and flight computer.
Checking each instrument is a good winter project. Techniques are
described in the back of Reichmann's "Cross-Country Soaring".
Jim

There is a good description of how to prevent interference between varios
sharing the same TE probe he http://www.borgeltinstruments.com/
and select "Articles" from the menu bar and then "Installation" from the
menu on the left.

In summary, there is no chance of interference if all devices sharing the
probe are modern pressure sensing instruments, otherwise locate the
splitter as near the TE probe as possible and never just behind the panel.

================

Semi-related supplementary question: some of the older electronic varios,
e.g the Cambridge Mk 4 used capacity bottles, so were obviously, like
mechanical varios, flow-sensing instruments. What I'd like to know is how
did they measure the flow? Did they use a hot wire sensor or something
more exotic?


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

On Sunday, December 17, 2017 at 2:40:15 PM UTC-5, Kiwi User wrote:
On Sun, 17 Dec 2017 09:23:17 -0800, JS wrote:

Linking a couple of recent threads: Connecting multiple varios and how
to change ASI markings....

I discovered the reason for a flight computer's inconsistent readings
was the bezel on the ASI not being tight enough to seal. Snugging it up
resulted in a more accurate ASI and flight computer.
Checking each instrument is a good winter project. Techniques are
described in the back of Reichmann's "Cross-Country Soaring".
Jim

There is a good description of how to prevent interference between varios
sharing the same TE probe he http://www.borgeltinstruments.com/
and select "Articles" from the menu bar and then "Installation" from the
menu on the left.

In summary, there is no chance of interference if all devices sharing the
probe are modern pressure sensing instruments, otherwise locate the
splitter as near the TE probe as possible and never just behind the panel.

================

Semi-related supplementary question: some of the older electronic varios,
e.g the Cambridge Mk 4 used capacity bottles, so were obviously, like
mechanical varios, flow-sensing instruments. What I'd like to know is how
did they measure the flow? Did they use a hot wire sensor or something
more exotic?


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

Cambridge varios used 2 thermistors in line to sense flow.
UH

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.

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

On Sunday, December 17, 2017 at 3:17:57 PM UTC-5, 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.

One early version I took apart had 2. Later ones may have added a center one to help with linearity.
I recall hearing that Raouf made more money selling flow sensors to computer makers than he ever did making soaring instruments.
UH

On Sunday, December 17, 2017 at 5:30:57 PM UTC-5, wrote:
...Later ones may have added a center one to help with linearity.

Gain more than linearity.

I recall hearing that Raouf made more money selling flow sensors to
computer makers than he ever did making soaring instruments.

Right, that's why he sold Cambridge and started a separate company to sell
flow sensors. And sold that company prior the biggest customer tanked ;-)

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

On Sunday, December 17, 2017 at 12:23:21 PM UTC-5, JS wrote:
Linking a couple of recent threads: Connecting multiple varios and how to change ASI markings....

I discovered the reason for a flight computer's inconsistent readings was the bezel on the ASI not being tight enough to seal. Snugging it up resulted in a more accurate ASI and flight computer.
Checking each instrument is a good winter project. Techniques are described in the back of Reichmann's "Cross-Country Soaring".
Jim

Whether the advances on the original dual thermistor vario ( Crossfell)
made by Raouf were new thinking or just tweaking, is the stuff for
patent examiners to argue.
In 1962/3 I made several three thermistor varios based on what I knew about the Crossfell. They were donated to Mid Atlantic club, being an improvement on
the pellet type. Like the Crossfell they all suffered from temperature related
drift, which I solved by adding a shutoff valve in the flow line, plus
zero adjuster.
Raouf analysed the problem as unbalance in the slope of the temp/res
characteristic; he then chose balanced thermistors and trimmed the current setting.
This I think, was game changer, producing a reliable instrument.

I used one for years.

John F