How do you measure zero pressure simply???

On Tue, 10 Jan 2012 08:42:12 -0800 (PST), George Herold

gher...@teachspin.com> wrote:

[snip]

Hi Mike, Why the 'bleep' do you need so much resolution.  If I'm doing
my numbers correctly.
1 atm. = 32 feet of H20 ~ 400 inches.  So 0.001 inches is about
2.5x10**-6 atm.
That's equivalent to a temperature difference of about 1 milli-Kelvin
(ideal gass at 300K)

I should also put that into a height difference... What's the
difference in pressure from the top of your head to the floor?  (I'll
work that out in a minute.)

George H.

[snip]

Sno-o-o-o-ort ;-)

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I love to cook with wine.     Sometimes I even put it in the food.

A diaphragm is simple, doesn't require a flow to maintain it's position
and is easy to dampen mechanically. Position sensing can be as simple
as a pair of point contacts or as complex as you want to make it.
Art


A diaphragm is indeed trivial in concept.
Actually building one is yet another matter.
Using one is a third. Gravity is a bitch.
So is a droplet of water that weighs more than the
force of the pressure difference.

So design it so the plane of the diaphragm is vertical and gravity

"mike" <spamme9_at_gmail.com> wrote in message news:jeg4d6$hem$1_at_dont-email.me...

A diaphragm is simple, doesn't require a flow to maintain it's position
and is easy to dampen mechanically. Position sensing can be as simple
as a pair of point contacts or as complex as you want to make it.
Art


A diaphragm is indeed trivial in concept.
Actually building one is yet another matter.
Using one is a third. Gravity is a bitch.
So is a droplet of water that weighs more than the
force of the pressure difference.

So design it so the plane of the diaphragm is vertical and gravity
isn't a factor. The weight of a drop of water or heat pipes has
as much relevance here as the pressure of sunlight, cosmic ray
impacts, or the phase of the moon.
Art

On Jan 10, 5:01 am,BillSloman<bill.slo...@ieee.org> wrote:

On Jan 10, 2:00 am, mike <spam...@gmail.com> wrote:

dagmargoodb...@yahoo.com wrote:
On Jan 7, 5:43 pm, mike <spam...@gmail.com> wrote:
I'm building some heat recovery ventilation devices.
Need a way to equalize the internal and external pressure of the house.
Don't need a lot of accuracy, but would like to be able to get close to
zero.
Best pressure gauge I have has a resolution of 0.01" of water.
I'd like something simpler I could leave hooked up and running.

I started with a heated resistor flanked on both sides by a thermistor
in a bridge configuration. Stuffed it all into the side of a plastic
tube. Air flow differentially heats one thermocouple depending on
direction.
It sorta works, but uses a lot of power (for a battery operated device)
and has a rather long thermal time constant. The thermistors
are tiny, but they're encapsulated in kapton tape. Don't think I can
get 'em out without breaking 'em.

Is there another simple technique I could exploit to infer zero
net pressure differential?

Using the thermistors themselves as the heat source would save
considerable power. (i.e., wire in series, feed constant current (or
power), feedback from the tap) That avoids relying on air-coupling to
the heat source, which is lossy.

Constant current/power/anything implies active circuitry which
fails to meet my simplicity requirement.

Active circuitry can be pretty simple.

It would help if we knew what we were driving. If it's a uC, then
what's the big deal with a simple interface circuit? If not, your
idea of a SPDT diaphragm-operated switch is as simple as it's going to
get.

I can't seem to get it across that I have lots of complex ways to
do the job. I'm looking for something simple and elegant.
Hot glue and duct tape are my tools of choice.
If I have to make a circuit board and a box to put it in,
I'll just scrap the whole thing and use a commercial manometer.

I'm more interested in the clever design than in the result.

The tiny thermistors I have are negative temperature coefficient
which leads to thermal runaway if you don't have something
limiting the current.

That something can be a simple as resistor in series with the
thermistor. You do need to limit the power dissipated in NTC
thermistors to less than about a milliwatt - too much self-heating and
you start to form "hot channels" inside the device, which wouldn't be
too bad if they were stable, but I've seen an NTC thermistor
resistance measurement fluctuate because the multi-meter doing the job
was dissipating too much power in the device - the fluctuations
stopped when we changed range to one that dissipated less power in the
part, and they had been big enough to have been detectable on the new
range.

You can also prevent thermal runaway by driving the thermistor with a
constant current source, but the added extra circuit complexity
doesn't usually buy you enough extra sensitivity to be worth the
trouble.

CC was so the thermistors could still be used in bridge. I s'pose a
single current-limiting resistor split in two pieces could do as well.

Talking about simplicity, it might even be advantageous to capture a
tiny bit of that thermal-runaway positive feedback, increasing
sensitivity.

On Tue, 10 Jan 2012 08:00:19 -0800 (PST), dagmargoodb...@yahoo.com
wrote:

[snip]

It would help if we knew what we were driving.  If it's a uC, then
what's the big deal with a simple interface circuit?  If not, your
idea of a SPDT diaphragm-operated switch is as simple as it's going to
get.

[snip]

I'm with dagmargoodboat.  The diaphragm approach is simplicity in
itself.

 Using it to drive a switch... zero power.  But you could get
fancy and move the core of a differential transformer,

or even do
differential capacitance fairly easy.

I should also put that into a height difference... What's the
difference in pressure from the top of your head to the floor? (I'll
work that out in a minute.)

On Tue, 10 Jan 2012 08:42:12 -0800 (PST), George Herold

gher...@teachspin.com> wrote:

I should also put that into a height difference... What's the
difference in pressure from the top of your head to the floor?  (I'll
work that out in a minute.)

Should be pretty simple.  Aviation rule of thumb is 1" Hg per 1000'
altitude change.

For a 6-footer that's 6 milli-inches Hg from nose to toes.

In water (13.6 x Hg) that's 81.6 milli-inches

In psi that's 3 milli-psi

Your turn.

Jim

On Jan 12, 3:03 pm, RST Engineering <jwei...@gmail.com> wrote:
On Tue, 10 Jan 2012 08:42:12 -0800 (PST), George Herold

gher...@teachspin.com> wrote:

I should also put that into a height difference... What's the
difference in pressure from the top of your head to the floor?  (I'll
work that out in a minute.)

Should be pretty simple.  Aviation rule of thumb is 1" Hg per 1000'
altitude change.

For a 6-footer that's 6 milli-inches Hg from nose to toes.

In water (13.6 x Hg) that's 81.6 milli-inches

In psi that's 3 milli-psi

Your turn.

Jim

I think I got something like one part in 10**4. A cubic meter of air
is about
1 kg (a bit more).

So F=mg is 10 newtons over a square meter is 10
Pa.

(1 atm = 10**5 Pa)

(I'm slowly converting to MKS units)

George H.

On Thu, 12 Jan 2012 19:10:48 -0800 (PST), George Herold









gher...@teachspin.com> wrote:
On Jan 12, 3:03 pm, RST Engineering <jwei...@gmail.com> wrote:
On Tue, 10 Jan 2012 08:42:12 -0800 (PST), George Herold

gher...@teachspin.com> wrote:

I should also put that into a height difference... What's the
difference in pressure from the top of your head to the floor? (I'll
work that out in a minute.)

Should be pretty simple. Aviation rule of thumb is 1" Hg per 1000'
altitude change.

For a 6-footer that's 6 milli-inches Hg from nose to toes.

In water (13.6 x Hg) that's 81.6 milli-inches

In psi that's 3 milli-psi

Your turn.

Jim

I think I got something like one part in 10**4. A cubic meter of air
is about
1 kg (a bit more).

About 1.3 kg/m^3 assuming 75% N2 and 25 % O2.

I started with Rydberg's constant, 22.4 L/mole.