You know what, I am going to talk about what this site is all about, just for your information.
We (the members) offer peer support to help you with questions about designs and engineering you are doing. This was a simple question with an equally simple answer. Your previous post showed some grasp of system requirements, but nothing about the desing you envisioned. As I stated, this site is not about doing the design for you, you have to help too. If we do not see desing and engineering information in the posts, we quickly assume the posted is not qualified to do the work they are asking about, and this scares a lot of people on here, including me. We don't like to help someone who does not know what they are doing in the first place.
So back to my point, your first post was a great post but, it doesnt help us to know about the system requirements without your engineering thoughts included. That is why I posted that we cannot do this for you.
I think you would get some great support if you enlightned us.
BobPE,
In my view, you either help or you don’t. Anything else is just inappropriate. Anyway, I am not going to get into this anymore.
Thanks again for your quick response to my question.
Try modeling as a very tall, small diameter gravity tank. Top elevation to match your elevation + pressure head when full; bottom elevation to match elevation + pressure head when empty; volume to match your tank volume.
A Modification on Snafuman's suggestion if you want to be very precise is to:
Vary the diameter of the tank in the 'curves' section to compensate for the changes in volume vs pressure, but of course the air temp would also change, for short duration fluctations adiabatic (no heat exchange) and long duration isentropic (constant temp).