What does negative gauge pressure mean?
Answer: When the pressure indicated by the pressure gauge is less than the atmospheric pressure it is called 'negative gage pressure'. This pressure is also called a vacuum pressure. But It does not mean that the absolute value of the pressure is negative. It is still greater than zero absolute pressure but less than atmospheric pressure at that particular location.
You can see from the above figure that the pressure indicator rests at 0 mbar which is used as a reference for atmospheric pressure. When the pressure goes below the atmospheric pressure the pressure indicator shows negative values (in blue) and when pressure is more than atmospheric pressure it shows positive values (in black).
Note: All of the above pressure values are still greater than the absolute zero pressure value.
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