Every pressure gauge in the plant is lying to you, politely and by design. It has the atmosphere on both sides of its element, so the atmosphere cancels and the needle reads the EXCESS over ambient. Open the tap to the room and it reads zero — even though the room is pressing on everything in it at about 101 kPa.

The bookkeeping is one line: $P_{abs} = P_{gauge} + P_{atm}$, P-absolute equals P-gauge plus P-atmosphere. $P_{abs}$ is the absolute pressure, measured from a perfect vacuum. $P_{gauge}$ is the instrument reading, which may be NEGATIVE when the vessel sits below ambient — a condenser under vacuum is the everyday case. $P_{atm}$ is the local barometric pressure on the day, near 101.3 kPa at sea level and lower as you climb. Solve it forward for $P_{abs}$, or backward: $P_{gauge} = P_{abs} - P_{atm}$.

Why it matters: every gas law in this chapter is written in absolute pressure, because they are ratios, and a ratio needs a scale whose zero means none of the quantity. Feed a gauge reading into a pressure ratio and the answer is not slightly wrong — it is wrong by however much atmosphere you left out, which near ambient is most of the number.

One habit worth building early: units guide, they do not confess. Push them through your rearrangement every time. If the units refuse to land on kilopascals, the rearrangement is wrong, no appeal — but units that do work out never prove you right. The check runs one way only, which is exactly why a datum error survives it: kPa minus kPa is still kPa, whichever way round you wrote it.