How do you measure zero pressure simply???

[mike]

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?

 

[Joerg]

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?

A vane? Essentially a small plate that sits in a square profile, with a
position sensor that signals when it's in the middle (meaning no flow in
either direction). Can be centered by gravity and then balanced around
the pivot, but not completely. How much you balance it determines its
sensitivity, to some extent.

Of course it'll react to pressure from the wind but so will just about
anything else.

 

[mike]

We use a honeywell differential pressure gizmo. I think it sells for ~
$15.
(Newark carries them.)

George H.

Thanks, but the low pressure versions are $72 and digital, so I need a
microprocessor
and a circuit board and power supply and box to put it in and code to run it
and some kind of display....And I've already got that: Omega PCL-200.

I was hoping for a creative option similar in complexity to what I
described.

I've got a UEI EM150. Gonna see what happens when I try to increase
the gain X10.

 

[John Devereux]

Right, like that. Motorola used to make similar things once, but they
spun that off to somebody.

Freescale AFAIK

 

[Don Lancaster]

Freescale AFAIK

Dwyer has all sorts of stuff for this.

< http://www.dwyer-inst.com/ >

--
Many thanks,

Don Lancaster voice phone: (928)428-4073
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
rss: http://www.tinaja.com/whtnu.xml email: [email protected]

Please visit my GURU's LAIR web site at http://www.tinaja.com

 

[mike]

Hi Mike, Looks like the last time we bought these they came from
digikey. part number 480-2516-ND. (The price has gone up a bit since
the first purchase.) They make different sensitivities. I stuck an
instrument amp across it. I’m not sure how much gain... I could look
up the circuit.
I would guess maybe something like 10V full scale.

George H.

Thanks for the inputs.
I've failed at trying to focus the discussion on simple/elegant solutions.
So far, the thermistor bridge seems to be the optimum match to the
requirements.
Think I'll build a more physically robust one in a ball point pen tube.

I don't think there's any chance I can build a diaphragm system
that works any better.
0.001 inches of water is an incredibly small number compared to the
stiffness of diaphragm materials I could easily come by and mount.

I did manage to get 10x more gain out of the UEI EM150 before the
sensor and op amp offsets used up too much of the dynamic range.
I thought about replacing the LM324 with something lower offset,
but the thing is already rather sensitive to flexing the
input port on the plastic case.

I think I'm a happy camper.
Thanks, mike

 

[MarkK]

I'm building some heat recovery ventilation devices.
Are the low cost automotive sensor lacking in resolution for your purpose?

I'm more interested in what you intend to do with the information. You
know that the differential pressure will vary a lot from the basement to the
top floor, it will be negative in the basement and positive at the upper
floors.

Are you going to run a blower to try to equalize the pressure? At what
location in the home? To what end?

I have a dryer type vent installed allowing outside air into the basement
near the furnace. Cold outside air flows in via the vent. My theory is
that this cold air WILL enter the house one way or the other. By allowing
it to flow in __at the location of my choice__ (into the furnace) I reduce
the inflow in other parts of the house.

Also you should try to seal the upper floors as best as possible, any
outflow up there MUST be replaced by inflow in the lower floors.

I'm curious about your strategy for using this pressure info?

Mark

 

[mike]

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.

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.

My initial thought was to use PTC thermistors aka self heated
incandescent light bulbs
with the glass busted off, but moisture and dirt would make that
a short lived sensor.

I'll take another look at self heating. The thermistors are tiny.
I didn't think I could get heat transferred from one to the other
to infer direction of flow
without a very tiny air channel. The hot resistor between them
solves the NTC runaway problem and gives me enough heat to make the
air channel manageable size. Something to be said for keeping the
heat generation separate from the sensing bridge.

The first crude prototype uses 25mW in a 1206 resistor that fills the
channel.
Using a -20-0-+20uA center scale analog meter, I can get full scale
deflection with 5 Pascals of pressure differential. Since the
thermistors are a significant part of the total resistance,
the gain is rather temperature sensitive. Zero is still Zero,
but I'd like to reduce the temperature dependence if I can do it
simply. Yes, measuring voltage helps a lot, but takes active stuff.
Simple is, "hot glue and duct tape to hold together stuff that's already
in the junk box". I'm sure I'll have lost interest in the project
long before anything I ordered would arrive in the mail. ;-)

There's a picture of it here:
http://ecorenovator.org/forum/attac...ventilation-heat-exchanger-pressuresensor.jpg

 

[mike]

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.

Off topic alert:

I've been building heat pipes. If you take a copper pipe
and fill it half full of freon and half full of vacuum,
it makes a great heat pipe.

If you do the math, it's easy to conclude that you
can do the same with water. But they don't work.
Then you realize that the vapor pressure of water
at room temperature is less than the weight of a few
millimeters of water. So the surface boils, but any
heat applied more than a few mm below that can't
get out past the weight of the water column above it,
so all you're left with is convection in the water.

The concept of the heat pipe is the same, but the
physical realization doesn't scale well.

If you're gonna make a water heat pipe, you need a wick
structure and no water column suppressing the boiling.
If anybody knows how to make a useful wick structure
from stuff you find in the average kitchen, I'm interested.

Just like a diaphragm. The diaphragm concept itself is easy.
The structure to support it is the hard part.

My interpretation of this thread is that most people don't
realize how little pressure is represented by a mm of water.

That's why, for a DIY sensor, I skipped directly to the
indirect measurement of heat flow. NO moving parts except
the air...and even that doesn't move at zero pressure differential.
It becomes an electronics problem, and the purpose of this thread was to
eliminate most of the electronics.

 

[mike]

Are the low cost automotive sensor lacking in resolution for your purpose?

You tell me. Which parts of an auto care about a mm of water pressure?

I'm more interested in what you intend to do with the information. You
know that the differential pressure will vary a lot from the basement to the
top floor, it will be negative in the basement and positive at the upper
floors.

The first two lines of this page say what I'm wanting to do.

Are you going to run a blower to try to equalize the pressure? At what
location in the home? To what end?

The HRV has two opposing blowers. The objective is net zero pressure
differential in the vicinity of the HRV.

I have a dryer type vent installed allowing outside air into the basement
near the furnace. Cold outside air flows in via the vent. My theory is
that this cold air WILL enter the house one way or the other. By allowing
it to flow in __at the location of my choice__ (into the furnace) I reduce
the inflow in other parts of the house.

That's a true statement. But the devil is in the details.
As I refine my ability to measure small pressure differences,
I'm finding that you can have pressure differentials throughout the house
even with all the interior doors open.

Also you should try to seal the upper floors as best as possible, any
outflow up there MUST be replaced by inflow in the lower floors.

I don't have any upper floors.

I'm curious about your strategy for using this pressure info?

Holler if you think my math is wrong...
Using round numbers to make the math easy. You can scale for more
appropriate numbers.

If you have 1400 square feet and 8 foot ceilings, you have 11,200 cubic
feet of volume. If you want 0.3 air changes per hour, you have to
heat 3360 cubic feet/hour of incoming air == 56 CFM.

If you have a 30F inside/outside temperature differential, you need
1.08 X CFM x dT = 56 x 30 x 1.08 = 1814 BTU/Hr.
That's 1,306,368 BTU/Month blowing out the cracks.
If gas costs $1/therm, that's $13/month. That represents about a third
of my gas bill.

It's desirable to seal the place and get control that flow.
A heat exchanger can recover a significant percentage of the lost BTU's.
But if it's not pressure neutral, it also forces air thru the cracks.

I'm toying with the idea of dynamically adjusting the fan speeds
to compensate for infiltration due to wind. I'm not optimistic, but
worth a look.

Once you get it set up, you don't have to mess with it...but I'm still
in the messing with it stage.

If you buy a commercial HRV and have it installed and pay for the
electricity
to run it, you can argue that it won't ever pay back.
If you're building it out of coroplast and duct tape, the payback can
be relatively quick.

And you can have a lot of fun learning how to weld polypropylene.

First prototype of the Hillbilly HRV:

http://ecorenovator.org/forum/attac...ntilation-heat-exchanger-hrv-hillbilly-05.jpg

Preliminary results suggest that you can get significant improvement in
indoor air quality while recovering half the BTU's for almost no investment.

 

[josephkk]

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.

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

Totally inappropriate application of f=mg. Open field air pressure is
derived from the weight of the air column above it. Just like hydrostatic
pressure, with the added fun that air is compressible. Wind pressure is
even more fun.