Editing Pressure measurement (section)

==Absolute, gauge and differential pressures — zero reference==
{{Anchor|Absolute|Gauge|Differential|Absolute, gauge and differential|Absolute, gauge and differential pressures}}
[[File:Natural gas pressure gauge.jpg|thumb|Natural gas pressure gauge]]
[[File:Pressure sensors.jpg|right|thumb|200px|Silicon piezoresistive pressure sensors]]
Everyday pressure measurements, such as for vehicle tire pressure, are usually made relative to ambient air pressure. In other cases measurements are made relative to a vacuum or to some other specific reference. When distinguishing between these zero references, the following terms are used:
*'''{{visible anchor|Absolute pressure}}''' is zero-referenced against a perfect vacuum, using an [[absolute scale]], so it is equal to gauge pressure plus atmospheric pressure. Absolute pressure sensors are used in applications where a constant reference is required, like for example, high-performance industrial applications such as monitoring [[vacuum pump]]s, liquid pressure measurement, industrial packaging, industrial process control and [[aviation]] inspection.<ref>{{Cite web|last=Taskos|first=Nikolaos|date=2020-09-16|title=Pressure Sensing 101 – Absolute, Gauge, Differential & Sealed pressure|url=https://esenssys.com/differences-between-pressure-sensors/|access-date=2020-09-16|website=ES Systems}}</ref>
*'''{{visible anchor|Gauge pressure}}''' is zero-referenced against ambient air pressure, so it is equal to absolute pressure minus atmospheric pressure. A tire pressure gauge is an example of gauge pressure measurement; when it indicates zero, then the pressure it is measuring is the same as the ambient pressure. Most sensors for measuring up to 50 bar are manufactured in this way, since otherwise the atmospheric pressure fluctuation (weather) is reflected as an error in the measurement result.
*'''{{visible anchor|Differential pressure}}''' is the difference in pressure between two points. Differential pressure sensors are used to measure many properties, such as pressure drops across [[oil filter]]s or [[air filter]]s, fluid levels (by comparing the pressure above and below the liquid) or flow rates (by measuring the change in pressure across a restriction). Technically speaking, most pressure sensors are really differential pressure sensors; for example a gauge pressure sensor is merely a differential pressure sensor in which one side is open to the ambient atmosphere. A [[DP cell]] is a device that measures the differential pressure between two inputs.<ref>{{cite book |title=Process control: modeling, design, and simulation |last=Bequette |first=B. Wayne |page=735 |year=2003 |publisher=[[Prentice Hall]] |isbn=978-0-13-353640-9 }}</ref>

The zero reference in use is usually implied by context, and these words are added only when clarification is needed. [[Tire-pressure gauge|Tire pressure]] and [[sphygmomanometer|blood pressure]] are gauge pressures by convention, while [[atmospheric pressure]]s, deep vacuum pressures, and [[altimeter|altimeter pressures]] must be absolute.

For most [[working fluid]]s where a fluid exists in a [[closed system]], gauge pressure measurement prevails. Pressure instruments connected to the system will indicate pressures relative to the current atmospheric pressure. The situation changes when extreme vacuum pressures are measured, then absolute pressures are typically used instead and measuring instruments used will be different.

Differential pressures are commonly used in industrial process systems. Differential pressure gauges have two inlet ports, each connected to one of the volumes whose pressure is to be monitored.  In effect, such a gauge performs the mathematical operation of subtraction through mechanical means, obviating the need for an operator or control system to watch two separate gauges and determine the difference in readings.

Moderate ''vacuum pressure'' readings can be ambiguous without the proper context, as they may represent absolute pressure or gauge pressure without a negative sign. Thus a vacuum of 26&nbsp;inHg gauge is equivalent to an absolute pressure of 4&nbsp;inHg, calculated as 30&nbsp;inHg (typical atmospheric pressure) − 26&nbsp;inHg (gauge pressure).

Atmospheric pressure is typically about 100&nbsp;[[Pascal (unit)|kPa]] at sea level, but is variable with altitude and weather. If the absolute pressure of a fluid stays constant, the gauge pressure of the same fluid will vary as atmospheric pressure changes. For example, when a car drives up a mountain, the (gauge) tire pressure goes up because atmospheric pressure goes down. The absolute pressure in the tire is essentially unchanged.

Using atmospheric pressure as reference is usually signified by a "g" for gauge after the pressure unit, e.g. 70&nbsp;psig, which means that the pressure measured is the total pressure minus [[atmospheric pressure]].  There are two types of gauge reference pressure: vented gauge (vg) and sealed gauge (sg).

A vented-gauge [[pressure transmitter]], for example, allows the outside air pressure to be exposed to the negative side of the pressure-sensing diaphragm, through a vented cable or a hole on the side of the device, so that it always measures the pressure referred to ambient [[barometric pressure]].  Thus a vented-gauge reference [[pressure sensor]] should always read zero pressure when the process pressure connection is held open to the air.

A sealed gauge reference is very similar, except that atmospheric pressure is sealed on the negative side of the diaphragm.  This is usually adopted on high pressure ranges, such as [[hydraulics]], where atmospheric pressure changes will have a negligible effect on the accuracy of the reading, so venting is not necessary.  This also allows some manufacturers to provide secondary pressure containment as an extra precaution for pressure equipment safety if the burst pressure of the primary pressure sensing [[diaphragm seal|diaphragm]] is exceeded.

There is another way of creating a sealed gauge reference, and this is to seal a high [[vacuum]] on the reverse side of the sensing diaphragm.  Then the output signal is offset, so the pressure sensor reads close to zero when measuring atmospheric pressure.

A sealed gauge reference [[pressure transducer]] will never read exactly zero because atmospheric pressure is always changing and the reference in this case is fixed at 1 bar.

To produce an [[absolute pressure sensor]], the manufacturer seals a high vacuum behind the sensing diaphragm. If the process-pressure connection of an absolute-pressure transmitter is open to the air, it will read the actual [[barometric pressure]].

A '''sealed pressure sensor''' is similar to a gauge pressure sensor except that it measures pressure relative to some fixed pressure rather than the ambient atmospheric pressure (which varies according to the location and the weather).