I propose the following definition for the "pressure" tag:

Pressure is the static pressure associated with the measurement or control of fluids such as air, oxygen, water, steam, gas, oil, etc.

The current definition of the pressure Tag is - "Static air pressure measured in "inH₂O" or "kPa"."

Thanks, took your definition until a better one is proposed :)

Shouldn't we define adjective tags to add precision ?

If reading a duct pressure, it'll be static pressure (differential pressure between duct (+) and plenum (-) ). If reading pressure loss through filters (example) it's not static, but differential pressure (before filter (+) vs after filter (-).

We could also define "velocity" and "total" tags to be able to describe all kinds of air pressure :

("total pressure" = "static pressure" + "velocity pressure")

For fluid pressure, the tag "absolute" should be available in addition to the "differential" tag.

Units can be Pa, kPa, psi, inHg, inWC (or inH2O), atm, bar, etc... and I saw head feet also for pumps...

I would suggest (based on Denis OConnor and Wikipedia) :

ref : http://en.wikipedia.org/wiki/Pressure http://en.wikipedia.org/wiki/Pressure_measurement#Absolute.2C_gauge_and_differential_pressures_-_zero_reference

Pressure (symbol: p or P) is the force applied perpendicular to the surface of an object per unit area over which that force is distributed.

Pressure is a variable associated with the measurement or control of fluids such as air, oxygen, water, steam, gas, oil, etc.

Tags related to pressure are :

• static
  • force applied perpendicular to the side of the duct by air
• differential
  • difference between two absolute pressures)
• velocity or dynamic
  • this pressure is used to calculate volumetric airflow : total pressure - static pressure
• total
  • pitot pressure
• absolute
  • zero-referenced against a perfect vacuum, using an absolute scale, so it is equal to gauge pressure plus atmospheric pressure

Thanks for pushing the pressure tag forward Christian.

I have the following comments on the related pressure tags so that they are applicable to both AHUs and Central Plants:

• static - used with pressure to quantify the force applied perpendicular to the side of the duct, pipe, pressure vessel etc.
• velocity - used with pressure to quantify the force resulting from the movement of fluid through the pipe, duct, etc.
• total - the sum of values with common units

I am not sure that we should restrict total to pitot tube pressure.

differential - The existing delta tag has the required generic description and could be used instead of creating a new tag.

I agree that both definition are similar... but differential is used a lot and is, from my point of view, a well-known name... I don't know if delta would be a good choice...

Regarding total... what would you think of that nuance ?

The sum of values with common units When used with pressure, total is the sum of velocity pressure and static pressure (pitot pressure).

Hey Guys,

In aeronautical engineering, we say Ptotal = Pstatic + Pdynamic. I worry about the term velocity as dynamic pressure is 1/2 x rho x velocity^2. I would prefer we use the tag dynamic.

-Adam

Dynamic pressure From Wikipedia, the free encyclopedia

In incompressible fluid dynamics dynamic pressure (indicated with q, or Q, and sometimes called velocity pressure) is the quantity defined by:...

Seems they are synonyme...

BUT,

This said, after verifying ASHRAE Fundamentals p.36.13 We can read : Pressures can be further classified as static or dynamic.

Few pages after that p.36.17, we find an entire chapter about pitot-static tubes for Airlow measurement. In there, the term velocity pressure is used everywhere in the formulas and tables...

Based on that, in most building applications, velocity + pressure tags would be my choice for airflow related information (velocity transmitter in VAV controllers for examples)

Would other fluids (water, etc...) be better served by a dynamic tag ?

In broad fluids applications, I see static and dynamic used and straightforward. My problem with the velocity tag is that it is a component of dynamic pressure.

I don't see this that way.

if Ptotal = Pdynamic + Pstatic

if Ptotal = Pvelocity + Pstatic

Pvelocity = Pdynamic

ref : http://scienceworld.wolfram.com/physics/DynamicPressure.html

This means we face a semantic issue.

For what I know, in VAV boxes, I always see velocity pressure... That said, if there's consensus for using dynamic, let's go for it.

Yep, it's just semantics. I just want us to be very careful distinguishing between velocity based pressure and velocity, that's all. As you said, I'm good with whatever the group decides.

Since we already have delta we should use that for differential. Even though both terms are used, I don't think we should be creating two tags for the same core concept.

Adding tags for total and static to further annotate pressure seems pretty straight forward to me.

The question is which would make more sense: dynamic or velocity. Given that we are looking for an adjective that is the opposite of static, then dynamic seems a better choice for two reasons:

• in programming dynamic is common opposite of static
• velocity is better served to mean speed as a core point versus an adjective of pressure

That assuming that both fit the bill from an industry terminology perspective (which I am not qualified to say)

Ok for me... but in the doc, I would specify that velocity pressure should be tagged as dynamic pressure... or something like that.

That way, if someone looks for velocity pressure, it'll find the right tag to use.

That sounds good to me. Thanks, guys. It makes sense from a fluids perspective.

Thanks, we are moving forward!

Ok, here is my proposal based on some research Paul did:

Add the following tags:

• dynamic
• static
• total

Those tags will have generic definitions, but reference the pressure tag.

The pressure tag docs will be updated to include:

The pressure tag should be used with the following tags:

• dynamic pressure is kinetic energy per unit volume of a fluid particle; in HVAC this commonly referred to as velocity pressure
• static pressure exerted by a fluid that is not moving or flowing
• total pressure is dynamic pressure plus static pressure
• delta pressure is a differential pressure between two points

If I may... static pressure definition isn't true.

There's a static pressure measured on a duct where air is flowing...

The applicable definition would be something like :

the resistance to flow, measured perpendicular to the flow

I agree with Christian. Water 10m deep can be moving, but still have static pressure exerted by the 1atm of water on top of it. I would prefer Christian's definition.

Agreed that in the two examples there is static pressure. I took a standard definition, but I also think that it can be interpreted incorrectly for the same reason. In lieu, I would recommend we use an alternate standard definition.

:the force per unit area that is exerted by a fluid upon a surface at rest relative to the fluid

That surface at rest part concerns me a little bit. I can't speak for anyone else, but I find it easily confusable.

From an HVAC point of view the P static is affected by the sum of all the resistance the air flow encounters as it travel through the duct work (coils, filters, curves, duct narrowing or widening and the duct it self) reducing the P total in the end.

I agree with Paul's definition, but I like the simplicity of Christian's because it also includes the method to measure.

while I understand the point about measurement to try and clarify the definition, I think measuremnent is outside the scope of what we are to trying to accomplish with the term definition. Furthermore, I don't think we want "measured perpendicular to the flow"..because you can have static pressure with no flow as we often do.

In ASHRAE Fundamental (F36.13) they describe static and dynamic pressure this way (I think it's an interesting angle)

After explaining Absolute and Vaccum pressure... they talk about Differential pressure and Gage pressure (a special case of differential pressure).

"Pressures can be further classified as static or dynamic. Static pressures have a small or undetectable change with time; dynamic pressure include significant pulsed, oscillatory, or other time-edpendent component."

Christian, that is the easiest to understand explanation I've ever seen. We should consider a gage tag too then. :)

Hi all - this thread was brought to my attention because we are currently working on a new proposal for enhanced weather-related tags for project haystack.

One of the common weather points is atmospheric pressure. I was planning to propose the tagging of such a point as atmospheric pressure. In the case of atmospheric pressure, it is usually measured in millibars (or sometimes inHg).

Do you all feel that this tag is correct and sufficient. Does it need further qualification? Your thoughts are most appreciated.

As I understand it atmospheric pressure = static pressure, just specifically related to weather, altitude, etc.

I agree that atmospheric pressure is a static pressure when discussing it versus a dynamic pressure.

I think that the real concept to be discussed here is what should be our definition of pressure. We need to keep in mind that it’s a semantic tag and while it doesn’t have to exactly match the physics definition, it should be kept close. “Pressure” will always be force applied over an area, but pressure should be something close.

As far as the tags used in the the weather proposal (and I know I'm getting ahead of things here), but I would recommend barometric pressure (or maybe weather barometric pressure). The reason for this is that if we use atmospheric, at some point in time it will conflict with outside.

Hey Keith,

Can you clarify? I would think static atmospheric pressure should always be force per area where the area is perpendicular to the vector of gravity. I don't see us as having any wiggle room here.

-Adam

Keith - thanks for your feedback. You make a good case for using barometric instead of atmospheric and I don't have a problem with using either. They are synonyms as far as weather points are concerned.

Every weather point also has the weatherPoint tag, so at a minimum the tag would be weatherPoint barometric pressure.

I guess one less detail to hash out would be do we always need to specify static or dynamic when using the pressure tag? In the case of barometric pressure it self-describing or redundant to also use the static tag. But if it is more consistent to do so I'm fine with that.

static, dynamic, or total

It is probably easier if pressure is always assumed to be static unless another descriptor is used. I base this on the fact that 99+% of the readings in buildings are static pressure readings. Shouldn’t we avoid having the vast majority of points have a static tag when we can just say that it is assumed unless otherwise stated?

Another topic related to nailing down the pressure discussion surrounds the reference from which pressure is measured. This topic is most commonly encountered with psi vs psia vs psid vs psig. Psia is a measurement of pressure referenced to absolute (or vacuum) and this is what we encounter with the pressure associated with weather. Psid is a differential pressure measured across a point in the system which I believe we have covered with delta pressure. Psig (often shortened to psi) measures the pressure at a point referenced to atmospheric pressure and is what we normally find on gauges. I bring this up not because I think we need to take action on it but to ensure that it is part of this conversation in the event someone else would like to weigh in on it. In my opinion, delta pressure is already handled and generally atmospheric pressure is so small as compared to a gauge pressure (psig), the psia factor is generally ignored. I believe we should continue with using pressure with regards to either psia or psig values and delta pressure to represent psid values.