Photohelic Differential Pressure Switch

I presume you know that on earth, at sea level, the column of air on every square inch of (normal) surface weighs about 15 pounds. Thus the air pressure at sea level is about 15 pounds per square inch (psi) (on one scale of pressure, known as absolute pressure).

If the earth had no atmosphere, and instead was surrounded by a vacuum (without the atmosphere), the weight of the column of air on every square inch of (normal) surface would weigh 0 pounds, or 0 pounds per square inch (psi) (on one scale of pressure, known as absolute pressure).

For various reasons, the normal pressure at sea level is often described as 0 psi. This is on a different scale of pressure known as gauge pressure.

When using the gauge pressure scale, the pressure of a vacuum is negative (in fact, the absolute vacuum I described if the earth would have no atmosphere) is -15 psi gauge pressure.

Thus, negative pressures (gauge) represent a vacuum, positive pressures (gauge) indicate a pressure greater than the normal pressure at sea level.

Temperature is something very different than air pressure, it deals with the amount of heat (or energy) in an object.

Humidity is still something else, it is a measure of the water vapor present in a gas or mixture of gases (like our atmosphere).

I hope this helps, but I think, depending on what you are trying to do, you need some help or education.

All pressure measurement is a differential measurement, with a pressure measured with respect to a reference pressure.

- Gauge pressure measurements use atmospheric pressure as the reference.

- absolute pressure measurements use absolute zero pressure as the reference.

- Differential measurement have two measurement ports. The port labeled "low side" is the reference side. A differential measurement is the difference: high side minus low side.

If the high side pressure is higher than the low side pressure then the differential is positive.

If the high side pressure is lower than the low side pressure then the differential is negative.

It's basic arithmetic.