Static Pressure Hose Length

[mathetes]

Quick question:

How much hose should I consider the upper limit when measuring static pressure? Would up-sizing the hose give a length boost?

Is that your opinion, or have you ever installed or seen installed that way?

Just curious.

Thanks for your input.

[Cagey57]

Check the Dwyer web site.

What we have always done, 1/4" FR (poly) tubing 100' max., for runs between 100' and 250' upsize to 3/8" FR tubing.

If you get to the 100' 1/4" FR tube max point consider re-locating the transmitter (usually a max wire length of 100' on 0-10VDC output models) to keep that 1/4" FR tube length under 100'.

[eyeLON]

Disagree w/Cagey.
IMHO Depending on the distance, 3/8" FR is the minimum in order to avoid any kind of lag in static signal. Plus, most techs will check twice before cutting a 3/8" line as opposed to 1/4".

[scrooloose]

We been using setra sensors lately and they suggest 3/16 to 100', 1/4 to 300' and 3/8 to 900'.

[osiyo]

We been using setra sensors lately and they suggest 3/16 to 100', 1/4 to 300' and 3/8 to 900'.

That'd seem about right.

Many of the manufacturer's of outdoor static pressure pickup assemblies in fact provide a length, usually around 50 foot, of either 1/8 or 3/16 tubing, to be used with their devices.

Old school, with pneumatic controls, it was kind of important to worry about tubing/pipe size for the pressure transmitters. Too much restriction to free flowing air (dampening of signal) was NOT good. Since those transmitters used pretty sizable diaphragms, levers, rods, etc to mechanically move stuff. In short ... used energy provided by the differential pressures. You had to overcome the mechanical friction within the device, and provide enough FORCE to move the mass of the materials. Dampen signals and response was slow and maybe not very accurate.

With the new electronic sensors, the problem is usually the reverse. They're sometimes too sensitive and fast acting. So much so that sometimes its a good thing to use smaller/longer tubing or to resort to a restrictor/dampener device (which several manufacturers make for the specific purpose).

i.e. I've seen many a case where exterior doors opening frequently, or gusting winds, have driven controls nuts (unstable) as they try to compensate for every minor fluctuation they sense. VFD's hunting all over, dampers (slow acting) trying to respond to fast changes ... which typically means they assume some position between the high and low pressure signals and you've no real clue what position short of physically looking ... looking at analog output signal from controller in such conditions will just fool yah. If position signal (output of controller) is fluctuating between ... let's say 4 VDC and 8 VDC in a period of 60 seconds, over and over again ... damper actuator is probably somewhere in the middle. It just doesn't move that fast, nor do you really want it to.

As an example, one job we did involved some labs. Spec called for very fast room pressure control. And we used actuators (electronic) which were very fast. LOL ... sounds good? We got a lot of complaints from occupants about fluctuations, and corrections, occurring so fast their ears popped. I spent hours tweaking stuff to find a final set of control parameters that worked well enough to meet their needs ... without making them uncomfortable/distracted.

Under normal conditions, ordinary building, ordinary equipment, you really don't want controls going unstable, constantly changing positions and speeds rapidly. Bad on the equipment and you're typically playing a losing game.

Mostly we just use 1/4" poly. Except in the case of extraordinary long runs. i.e. in your example of 300 foot.

Hasn't been a problem.

Problems I've seen were related to poorly installed poly ... ie tangled, crimped/kinked, etc. And/or poorly installed outdoor pickup. Placement of those can make a LOT of difference.

Sometimes poorly placed indoor pickup. Keep em away from exterior doors.

I am personally fond of placing electronic sensor itself in best location so as to minimize length of tubing run ... MOSTLY as a matter of minimizing chances that poly gets cut, crimped/kinked, etc. Jokers working on stuff above the ceiling tiles just seem to jerk the stuff around willy nilly not caring or thinking about any damage done. And I prefer sensor that is set to produce milliamp signal. Much more stable and less interference prone.

That's my two cents. Don't take it as being worth more than that.

[Russ57]

Man, remember the good old days when we would make those runs in hand bent hard copper with soft soldered joints?

Used to be you had to know what you were doing:)

[CB_Matt]

That'd seem about right.

Many of the manufacturer's of outdoor static pressure pickup assemblies in fact provide a length, usually around 50 foot, of either 1/8 or 3/16 tubing, to be used with their devices.


That is correct, and if you read the install guide that comes with the outdoor reference, you are not supposed to shorten the provided poly tubing. But who reads instructions anyways right?

[crab master]

What are you sensing? Duct static, building static, room static?

For building pressure I find it interesting I'll go to all the steps of running proper tubing only to put restrictors in it to slow down the sensing capabilities of the sensor...

Room static I've only had a few instances of working with it, but building static in some locations has been a real PITA. Lonesome/close to lonesome buildings in gusty areas really suck.

[HVAC Mechanic]

doesnt boyles law state that the pressure will remain the same in all directions in a vessel?? so would that mean that the diameter is irrelevant because all you are measuring is pressure??? you arent requiring any volume
so a 3/16th tube will read the same as a 1/4in tube

[D'Laine]

I've just recently verified static pressures with a Magnahelic guage at the BAS sensor location and at the duct pickup 100 feet away. 1/4" poly.
Same pressures, but... the pickup in the duct was bouncing all over the place. Adding the extra tubing gave me a stable signal for the BAS to sequence the air volume controllers with a steady state.
The air volume doesn't have to change rapidly. It's trial and error for the site until you get the hang of it.

D.

[jaytech1]

What would you guys recommend for a remote vav controller 100' away?

[RuralSpaceman]

I've just recently verified static pressures with a Magnahelic guage at the BAS sensor location and at the duct pickup 100 feet away. 1/4" poly.
Same pressures, but... the pickup in the duct was bouncing all over the place. Adding the extra tubing gave me a stable signal for the BAS to sequence the air volume controllers with a steady state.
The air volume doesn't have to change rapidly. It's trial and error for the site until you get the hang of it.

D.

Good answer.

[metasystech]

I agree with HVAC Mechanic

Your static pressure sensor does not "consume" any air volume per sea unless you are using a hot wire type. A 5/16 line 1000' long will possible have a delay of a second on a very sharp increase of pressure well over the low pressure we are talking about. Whatever split second delay there would be would have no effect on what we are working on.