Techs are Under Pressure
It depends where you live how much pressure you're under. I live in Florida so I'm very close to sea level, therefore the weight of the atmosphere is exerting right at 14.7 pounds per square inch (PSIA) of pressure on every surface in every direction.
There is no such thing as negative pressure… Only pressure lower than the atmospheric baseline
When I hook up to a piece of equipment with my gauges, my gauges read zero, this is because I zero out my gauges so they compensate for the atmospheric pressure.
This is known as PSIG or pounds per square inch gauge, which is the pressure within the system minus 14.7 to compensate for the atmospheric pressure (zero out at the atmospheric pressure)
The issue is, not all locations have 14.7 PSIA.
In the chart above, the far left shows altitude (distance) above sea level and the far right column shows PSIA.
You can see if you are 6000 ft above sea level that there is about 3psi less pressure being exerted on you than where I live in Florida. This means that when you zero out you gauge the actual pressure in the system is 3 psi lower than your low lying brethren.
The problem is that most PT charts and gauges refer to PSIG with the 14.7 already added in. This means that calculating saturation temperatures to calculate evaporator temperature, superheat and subcool will be a real pain.
The easiest way to compensate (if you are at 6000 ft for example) would be to zero your gauge out at 3psi instead of 0, that way the compensation is already built-in.
I know it doesn't seem like much, but a 3 psi difference when taking an accurate superheat and subcool is quite significant, so this is more than just an intellectual exercise.
Some other consequences of altitude is that water boils at a lower temperature, making it more difficult to boil potatoes, but easier to pull a vacuum.
NOW – if you want to get really nerdy. Weather conditions can also alter PSIA based on barometric pressure… but seriously.