Breweries located at higher altitudes conduct their operations in a “thinner” atmosphere. It is therefore necessary to add less CO² to their products, to achieve the same bottled volumes that can be produced by a brewery at sea level. In theory a beer bottles at a high altitude would be flatter, or contain less carbonation, if consumed by a customer at sea level, and vice versa. In actuality these differences are probably not noticeable in most cases.
The question arises whether a high altitude brewer should make gauge corrections on instruments, when making CO² volumes tests. If the pressure gauge is an “open” type, as used by Zahm & Nagel, one that has its dial face and needle exposed to the atmosphere, there is no need for adjustment. If the gauge is glycerin filled or “closed” and was set at sea level, (the manufacturing plant) then some adjustment should be made for use at higher elevations (@2.5 lbs. for every 5000 feet). Also, if a dead weight tester is used to calibrate gauges, some adjustment would have to be made.
Therefore a brewery at higher elevations, using Zahm & Nagel equipment, should make no adjustments in their CO² testing procedures and should use the normal pressure/temperature charts.
If a product is bottled under CO² pressure and tested independently at greatly different altitudes, different reading will be obtained. This assumes that “open” gauges are used at both locations. The reason for this is that the headspace in the container is actually a sample set at whatever elevation it is bottles at.If this container is transferred to a greatly higher or lower altitude, it is actually a separate atmosphere being measured at the new level. This separate atmosphere only enters the bourdon tube of the “open” gauge upon piercing the container, and therefore reads as bottles at the different altitude. Obviously the greater the elevation difference, the greater the volumes difference. (At 10,000 feet, as an extreme, could possibly show a volumes difference of up to 0.4 volumes.