What Exactly Is Positive or Negative Pressure??
What Exactly Is Positive or Negative Pressure??
(OP)
We provide maintenance and facility management for healthcare facilities throughout the southeast. I am NOT an engineer, so I apologize if my question doesn't rise to the technical level it should. Please forgive.
My question involves checking ventilation for ORs, Procedure Rooms, Soiled Rooms and Clean Rooms. I know these are required to maintain either positive or negative pressure. My company has us check that in a couple of ways. First, there is always the tissue test. If the room is pressurized enough either way it will flutter in or out. Mostly, though, we use a meter off of an Alnor EBT 721 with a tube that slides under the door and get a quantifiable reading.
So my question is, does a reading of .0001" to say even .0005" wc really mean that the room is "positive?" At that level the tissue doesn't even move. Wouldn't that be considered a technically neutral room? Also, when reading the codes in states like NC and FL, I see that they call for Positive or Negative, but I don't see where they quantify that (ie - what constitutes "positive")
Is it a certain CFM of supply over Exhaust/Return? Is it a minimum .001" wc? Would something as low as .0001" wc qualify? Same questions for negative rooms too. Just need some not-over-my-head-technical type guidance.
My question involves checking ventilation for ORs, Procedure Rooms, Soiled Rooms and Clean Rooms. I know these are required to maintain either positive or negative pressure. My company has us check that in a couple of ways. First, there is always the tissue test. If the room is pressurized enough either way it will flutter in or out. Mostly, though, we use a meter off of an Alnor EBT 721 with a tube that slides under the door and get a quantifiable reading.
So my question is, does a reading of .0001" to say even .0005" wc really mean that the room is "positive?" At that level the tissue doesn't even move. Wouldn't that be considered a technically neutral room? Also, when reading the codes in states like NC and FL, I see that they call for Positive or Negative, but I don't see where they quantify that (ie - what constitutes "positive")
Is it a certain CFM of supply over Exhaust/Return? Is it a minimum .001" wc? Would something as low as .0001" wc qualify? Same questions for negative rooms too. Just need some not-over-my-head-technical type guidance.





RE: What Exactly Is Positive or Negative Pressure??
There could be a slight negative or positive pressure, but not enough to cause the tissue to move, because the tissue is less sensitive than a gauge that could read .0001" w.c. It is like measuring the volume of water with a 1/4 teaspoon or a 55 gallon barrel. In the barrel, it would look like a wet spot, but would fill the 1/4 teaspoon.
Hope that helps.
RE: What Exactly Is Positive or Negative Pressure??
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RE: What Exactly Is Positive or Negative Pressure??
RE: What Exactly Is Positive or Negative Pressure??
But if this exceeds approximately 100 cfm/door, there may be issues with doors standing open or slamming shut.
RE: What Exactly Is Positive or Negative Pressure??
AIA/FGI Guidelines (since 2001?) call for at least .01" wc pressure in ORs when measured against a non-like area such as a corridor. Therefore, I'm assuming a tissue test is out since you would have to meet a quantifiable measurement, not just a qualitative one. Of course, some states adopt all or part of the AIA/FGI Guidelines. I believe ASHRAE notates .01" wc as well...but, I'm not confident of this. In industry literature for the medical field, .01" wc is recognized as a target of best practice for infection control.
The .01" wc target for an O.R. seems easy to measure at the door. We just stick a meter under the door crack. If it doesn't meet the target, we know there could be various reasons (high returns, loose room, corridor pressure, etc).
But, when a code calls for just positive or negative pressure how do you determine that? In respect to, say, a positive room I've heard that if you have 50 cfm more supply than exhaust/return per door that meets the intent of the code. But if I have that in the room, yet my meter (yes, we calibrate annually) shows the room measures as low as .0001" or even .0004" wc, is the room arguably "positive?" Does a reading that low at the door satisfy the intent of the code when it calls for a "positive" room?
It would be hard for me to accept this because I've had such a situation and then put a flutter strip against the door and watched it just sit there...even get sucked/pushed into the room when a distant door is opened/closed or someone walked by. To me, that seems like at best a neutral room.
So maybe the question is more like, "What specific measurements are necessary to satisfy a code's intent of a positive or negative room?" For instance, I've looked in the NC Code for ventilation requirements of the physical plant of Ambulatory Surgery Centers and can see where it calls for either, but doesn't explain what either actually looks like.
So, I'm looking for something like, "A room is positive when supply exceeds exhaust/return by 100 cfm and has a differential pressure of at least .001" wc to the corridor." Or, "A room is positive when supply exceeds exhaust/return by 50 cfm/door and a flutter strip blows outward."
Thanks for any clarification and I apologize if you already explained this and I just didn't understand it.
RE: What Exactly Is Positive or Negative Pressure??
RE: What Exactly Is Positive or Negative Pressure??
what you mean is gage pressure, which refers to the pressure difference to a reference pressure. In most cases the gage is open to atmosphere. so you reference pressure is 1 atm (or 14.7 psi).
so if your gage pressure is - 1 psi, you have 13.7 psi actual pressure. Negative to atmosphere.
RE: What Exactly Is Positive or Negative Pressure??
Again though, if you were designing or testing a room, when would you call if positive? What has to be present? I',m assuming it starts at a mechanical design of supply exceeding exhaust. But by how much. If supply > exhaust by 1 cfm does that qualify? If exhasut > supply, but the tissue blows outward is that ok?
RE: What Exactly Is Positive or Negative Pressure??
You measure. You report value. You state if it meets the guideline value or not.
RE: What Exactly Is Positive or Negative Pressure??
So, if you are designing such a room, you would start with the required outward air flow, and assume various configurations of gaps, openings, external environment, etc., and then design the air handler to accommodate that demand, which then results in a desired air pressure difference.
From a testing perspective, if the requirement is non-existent or nebulous, the fall-back would be to measure the pressure under some room configuration variations and see if measured value is at least 10x the rms noise/error of the instrument. This fallback is basically so that the measurement would be repeatably positive, given noise/error in the instrument.
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RE: What Exactly Is Positive or Negative Pressure??
positive pressure as a physical term is any pressure value over an index point such as atmosphere pressure. (it is like when you say +1 or -1, both use 0 as an index point)
pressurized room is a room that the pressure inside the room is greater than the pressure of room surrounding, we could reach this by making the amount of air entering the room is greater than the air leaving the room, even though the difference could be about 0.00001, it meant to prevent air from room surrounding to come in, and if the tissue does not move that does not always mean it is not pressurized room. the tissue has a mass should be taken in the account too.
to check a room, I think you need to use special tools not a tissue.
RE: What Exactly Is Positive or Negative Pressure??
RE: What Exactly Is Positive or Negative Pressure??
when designing, all begins with something like what wilbur say, you plan for excess supply or exhaust, it can be 10% or 5% depending on absolute figures, tolerances.
than the issue on how to control it is often related with what specs require - in pharmaceutical industry specs are always detailed in practice it least to pressure differential sensor tolerance class, procedure for results recording and archiving, procedures for regular maintenance, calibration and correct functioning validation.
RE: What Exactly Is Positive or Negative Pressure??
1. Clean to unclean area
2. Areas of different cleanroom classification
3. Is it important to save the product (safe material)
4. Is it improtant to save operators (containment)
5. If the two adjacent areas are of same class, then which one has more dust.
RE: What Exactly Is Positive or Negative Pressure??
The formula for the calculation is Q= 2610 * A * sq. rt delta P.
Where Q is the volume in cfm,
A is the total leakage area of the room.
delta P is the difference in pressure.
Hope this will help you.
RE: What Exactly Is Positive or Negative Pressure??
Q = C_d * A * v
v = 4005 * sqrt(delta_P)
C_d = discharge coefficient.
With C_d = 0.65:
0.65 * 4005 = 2603.
Rounding up: ~ 2610, as in the previous post.
RE: What Exactly Is Positive or Negative Pressure??
Well, welcome to the world of limited liability.
See, no code (and no ASHRAE does not say anything) indicate set point for negative or positive pressure, especially in hospitals
We get this question all the time when we design an TB isolation suite - "how much negative do you what?" the contract documents do not states the exact set point for the specified pressure differential.
So, we engineers will send you to the Authority Having Jurisdiction, because even the codes and guidelines (IMC, ASHRAE 170, etc) do not dare indicate an exact set point - Reason? Liability
Even the CDCP does not indicate the exact negative or positive set point of pressure differential sensors.
It is common "practice" to have a set point between -0.02 and +0.02 - This practice is as volunteered by instrument manufacturers such as TSI, so if you use TSI instruments, they will recommend you a set point. "Usually 0.01" - Then again, take this advice at your own risk.