Looking for a Constant Gauge pressure regulator

[salimaye]

First some background:
I am designing an inflation system for an inflatable (balloon)that is descending to the earth from an altitude of 30,000 Meters (98430 ft). The inflation system has to maintain a constant gauge pressure of 34.47 KPa (5 psi)inside the inflatable as it descends to the earth.

I will be using a nitrogen gas bottle as my inflation source.

The pressure regulator that I use will probably be a 2 stage regulator. What makes the design tricky (at least from my stand point) is the fact that the atmospheric conditions (both temperature & pressure) keep changing with altitude. Atmoshperic pressure at highest point is .168 psi and near the ground it is 14.7 psi.

Do you folks know of a pressure regulator (or a similar device) that would do the job?

Regards

Sal

 

[ALEXFV]

Salimaye:
You have a seriuos application because of great changes on barometric pressure!!

You can check at Fisher Regulators, in order to select an approach:

http://www.emersonprocess.com

Hope this can help you

 

[aviat]

You may be able to modify an oxygen reegulator for that purpose. Companies such as Scott Aviation have been doing this kind of work for many years.

 

[quark]

Do you want to maintain constant absolure pressure or gauge pressure? How will atmospheric pressure effect gauge pressure? I think I am terribly missing something.

 

[salimaye]

Hello quark,

I want to maintain constant gauge pressure. The reason behind maintaining constant gauge pressure is that, the fabric used for the inflatable is designed to take a certain amount of skin stress which is driven by how much 'gauge pressure' or differential pressure is present inside it.

Here is what will happen to the balloon without pressure regulator (lets ignore temperature effects for the moment)...

At 30,000 meters, atmospheric pressure is very low (.168 psi). Lets for a minute assume it is zero. Now when you inflate the balloon at this altitude gauge pressure is same as absolute pressure say 5 psi.

Now as this balloon descends towards the ground and sees more air molecules, atmospheric pressure increases. Lets say at a certain altitude it is 2 psi. Now in this case, the gauge pressure in the inflatable will fall to 5-2=3psi. In order to maintain 5 psi, we need to keep filling more gas into the inflatable so that it compensates for the increasing atmospheric pressure.

To achieve this we need a pressure regulator. We however cannot use a standard off the shelf regulator as these devices assume that atmospheric pressures are constant.

I hope I have not added to the confusion.

Thanks aviat & calfersoporte, I will check out those links.

Sal

 

[quark]

Sal,

First I would like to clarify a point. Once you fill a gas in a container at certain gauge pressure, say 5 psi, it remains same irrespective of external pressure though the differential pressure acting upon the cylinder wall varies if you neglect temperature variations.

A pressure regulator may not be a correct option in your case. You can use an electric control valve with a differential pressure transmitter for this purpose.

Regards,

 

[israelkk]

salimaye

I agree with quark. If you need to continuously fill more gas to compensate for mainaining constant differential pressure you probably already may have an electro-pneumatic valve in the system. Therefore, all you need is a pressure differential sensor that controls the electro-pneumatic valve.

On the other hand scuba divers have a breathing systems with a pressure reglator that counts for the different water pressure in the different diving depths a simmilar type of regulator may do the job.

 

[salimaye]

Alright, maybe I am missing a point or need to go back and read my engineering books. But this is my understanding of gauge pressure:

Pressure measured by a gauge that excludes atmospheric pressure is called gauge pressure. When you say you filled the containeer to 5 psi gauge, you actually mean to say you filled it to a pressure of 5 psi above the surrounding atmospheric pressure. It is therefore also called overpressure. Total or absolute pressure on the other hand is the sum of gauge and atmospheric pressure. The total pressure in the containeer on the ground will be 5+14.7=19.7psia

If we now refer to quarks example, we need to specify if we are measuring gauge pressure on ground or at some alititude.

In quarks example, when he mentioned "Once you fill a gas in a container at certain gauge pressure, say 5 psi, it remains same"

When you say "it" remains same you are referring to absolute pressure not gauge pressure. Gauge pressure reading by definition will be different at different altitudes.

In other words, I think differential pressure and gauge pressure are one and the same.

Correct me if I am wrong.

Btw thanks for the suggestion regarding differential pressure transimitter & control valve. I am looking into those options.

Sal