We are trying to simulate an environment of 8000 feet in altitude inside a cylinder. We are located in an area that is 200 feet above sea level. What sort of pressure gauge should we purchase to do this?
Altitude is simulated by controlling absolute pressure between 0 mbar absolute (Space) and 1 atm (1013.25 mbar absolute). Using absolute referenced measurements you can control a certain altitude independently of the local outside environment pressure and altitude. You will need a absolute range pressure gauge which is calibrated to at least your required highest altitude / lowest pressure.
The majority of pressure gauge are scaled in linear pressure units and rarely include an option to display pressure as an altitude due to the complexity of calculation. If 8000 feet is the only altitude you need to set, it would be easier to source a pressure gauge calibrated in psi or millibar and convert the pressure to altitude manually.
If the highest altitude is going to be 8000 feet, the corresponding pressure according to the US standard atmosphere model is as follows:
8000ft or 2438.4m is 752.624mb/hPa, 10.916psi, 564.51mmHg or 22.225inHg absolute.
A pressure gauge with a range of 10 to 17.5 psi absolute would cover altitudes from 8000 feet and below.
The other consideration is how accurately you need to be able to set the altitude. If you know the accuracy in feet, you can refer to the altitude to pressure conversion chart to determine what it would represent in pressure units, if the altitude changed by 100 foot at 8000 feet for example. It is very important that you consider the error at the required altitude because the conversion is non-linear with altitude. A 100 foot accuracy would become harder to achieve the higher the altitude.