Vacuum can be measured in two ways:
By a gage pressure sensor measuring pressure below atmospheric pressure (i.e., referenced to atmospheric pressure).
By an absolute pressure sensor measuring pressure greater than absolute zero, but less than atmospheric pressure.
For user convenience, the gage pressure sensor, designed for vacuum applications, is usually scaled to report a decrease in pressure below atmospheric pressure as an increase in positive voltage. Thus, at current atmospheric pressure, a vacuum pressure sensor actually reports 0 psi, but at absolute zero pressure. A vacuum pressure sensor reports the value of the current atmospheric pressure as psi of vacuum.
Because vacuum pressure sensors are measuring small pressure changes (max of 15 psi), the output can be scaled inches of water, inches of mercury or in psi. Consult the Pressure Reference Chart to compare how the various pressure sensors reference pressure.
Silicon Versus Bonded Foil Sensor Comparison Chart
| Silicone-based Pressure Sensor | Bonded, Foil Strain Gauge-based Pressure Sensors | |
| psi | Up to 10,000 | Up to 300,000 psi |
| Temperature | Up to 240º F | Up to 400º F |
| Overload Capability | Up to 400% | Up to 50% |
| Low Pressure Applications | Down to 0.3 psi (10 inches of water) | Down to 1 psi |
Note: Honeywell uses the term "vacuum gages" to define gage pressure sensors scaled in psi of vacuum or inches of water of vacuum. Absolute gages that are scaled to 15 psi are just considered absolute gages.