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Anonymous Poster #1

# HVAC - Pressure Measurement

02/26/2018 5:53 AM

In one of my site, I have to maintain positive pressure (in few Critical rooms) with respect to atmospheric pressure.

Right way is to use differential manometer but the problem is I cannot use the tube to outside atmosphere. Is there any other way to do pressure measurement of the room? Can absolute pressure meter work?

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#1

### Re: HVAC - Preassure Measurement

02/26/2018 6:37 AM

Not at the sort of pressures that a room runs at, as daily ambient pressure swings due to local weather conditions will swamp it. There really isn't any other way than to get a reference to the outside pressure and use a differential manometer.

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#2

### Re: HVAC - Preassure Measurement

02/26/2018 7:54 AM

Usually these + pressure are a few inches of water, as measured by a manometer. Since 32 feet = 14.7 PSI, so one inch is .0382 pounds differential. You can use a blind manometer. Which uses a static volume (say a one foot cube) to represent the outside pressure. As long as you standardize to a barometer and adjust the blind manometer measurement for atmospheric variation, this should work. A sliding scale you move to represent the varying air pressure will work, or you can get more complex, as needed.

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#3

### Re: HVAC - Preassure Measurement

02/26/2018 8:25 AM

It doesn't need to be a manometer (liquid filled). Use a differential pressure gauge, with one port open to the outside atmosphere.

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#4

### Re: HVAC - Preassure Measurement

02/26/2018 8:35 AM

In the original post, the Original Poster claimed to be unable to do this, nonsensically.

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#5

### Re: HVAC - Preassure Measurement

02/26/2018 9:24 AM

OK I was assuming he meant he couldn't use a tube to outside atmosphere, perhaps because of concern over loss of fluid (but nonsensical as you say) so my idea was eliminate the tube.

In any case a simple U-tube manometer only gives a reading, not much use if nobody notices pressure loss. With a gauge there's the option of adding a switch for alarm, start a fan, etc.

Perhaps OP will come back.

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#6

### Re: HVAC - Pressure Measurement

02/26/2018 10:11 AM

I would install something like this TV2 which monitors and logs pressure, temp and humidity....

https://www.e2di.com/cleanroom.html

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#7

### Re: HVAC - Pressure Measurement

02/26/2018 11:49 PM

Don't use a tube; use wires! Wires, properly sealed, can't contaminate the inside space, nor allow inside air to escape.

With two identical electronic pressure gauges, one inside and one outside, properly compensated for temperature, any voltage difference will indicate the ∆P, and loss of that voltage will indicate loss of pressure differential. A very simple comparator circuit could easily be made to trigger an alarm if the pressure differential dropped below some threshold.

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#8

### Re: HVAC - Pressure Measurement

02/27/2018 1:12 AM

will this help ?

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#9

### Re: HVAC - Pressure Measurement

02/27/2018 2:16 AM

Surely you need the positive pressure to be relative to 'local outside' - which may be inside the building, esp as you say you cannot access 'external outside'.

So if the room is inside the building, with no outside ventilation access, you need the +ve pressure (with an air lock probably) to be rel. to inside the building, (just to the other side of the door?)

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#10

### Re: HVAC - Pressure Measurement

02/27/2018 3:09 AM

Whilst a true statement the Original Poster claims, nonsensically, to be unable to gain an outside pressure reference. Therefore the problem does not have a viable solution.

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#12

### Re: HVAC - Pressure Measurement

02/27/2018 4:44 AM

That's right, but the OP didn't mention "external outside". The way I read it, by outside atmosphere he meant outside the room.

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#11

### Re: HVAC - Pressure Measurement

02/27/2018 4:34 AM

Use your differential manometer but connect the pipe outside the building to a U tube partially filled with mercury. Place a vented cap over the external exposed end to prevent ingress of water or other foreign matter. Make sure that both ports to the manometer and U tube are at equal heights. The mercury will create a seal without disrupting the pressure measurement.

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#13

### Re: HVAC - Pressure Measurement

02/27/2018 10:37 PM

Of course you can run a tube to the atmospheric reference. I give you permission to do so. That's the way it's done all the time. There is no flow through the tube, the tube's volume deadheads against the sensing port so no DP can develop between the sensor end and other end of the tube.

1/4" or 3/8" OD Polyflo tubing. I see runs of 100m all the time using 3/8" OD tubing.

Terminate either end with a wall mount sintered filter, like this one
http://www.dwyer-inst.com/Product/Pressure/Accessories/ModelA-465

No, you can not use one absolute pressure sensor/transmitter. If you could afford it, you'd need two absolute transmitters, one in the critical room and one at the atmosphere reference point and then you'd have to subtract the two. Depending on the DP target value, two subtracted measurements could well fall outside the accuracy spec of sensors themselves and invalidate the measurement by regulator's auditor.

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