by WHAT IS A GAGE PRESSURE SENSOR ?
A gage pressure sensor is a bonded foil strain gage-based pressure sensor
designed to measure applied pressure referenced to sealed atmospheric
pressure. The atmospheric pressure in a gage pressure sensor, which is sealed
to prevent moisture and other air particles from entering the sensor, always
reflects the atmospheric pressure on the day of manufacture, as opposed to the
current or other date.
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A gage pressure sensor operates in the same way as a bonded foil strain gagebased
pressure sensor. As pressure is applied to a diaphragm, pressure is
transmitted to a gauged element via a mechanical connection, such as a bolt. As
the gauged element is deflected, strain gages, which are bonded to the element,
reflect this as a change in resistance. The bridge circuit identifies the resulting
change in electrical resistance as a change in applied pressure to the sensor.
(Consult the Pressure Reference Chart to compare how the various pressure
sensors reference pressure.)
WHAT IS AN ABSOLUTE PRESSURE SENSOR ?
An absolute pressure sensor is a bonded foil strain age-based pressure sensor
designed to measure applied pressure referenced to sealed vacuum, or absolute
zero pressure.
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An absolute pressure sensor operates in the same way as a bonded foil strain
gage-based pressure sensor. As pressure is applied to a diaphragm, pressure is
transmitted to a gauged element via a mechanical connection, such as a bolt. As
the gauged element is deflected, strain gages, which are bonded to the element,
reflect this as a change in resistance. The bridge circuit identifies the resulting
change in electrical resistance as a change in applied pressure to the sensor.
(Consult the Pressure Reference Chart to compare how the various pressure
sensors reference pressure.)
WHEN SHOULD I USE AN ABSOLUTE PRESSURE SENSOR RATHER THAN A GAGE PRESSURE
SENSOR ?
The primary consideration concerns whether the atmosphere plays a role in the
application for which the pressure sensor is being used. If any part of the
application is exposed to the atmosphere, such as an automotive emission
system or in level measurement in an open tank, a gage pressure sensor should
be used. But if the atmosphere has no effect on the application, either type of
pressure sensor can be used. However, when measuring extremely high
pressures, such as those found in hydraulic applications, an absolute pressure
sensor is typically used. (Consult the Pressure Reference Chart to compare how
the various pressure sensors reference pressure.)
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WHAT IS A DIFFERENTIAL PRESSURE SENSOR ?
A differential pressure sensor designed to detect a change in pressure as the
difference between two applied pressures. A differential pressure sensor
employs a dual diaphragm with each diaphragm linked to a common central
gauged element (foil based sensors) or silicon substrate (silicon based
sensors). If both applied pressures are equal, both diaphragms are balanced,
and the reading shows zero; but if either diaphragm see a different pressure
from the other this differential pressure is transmitted to the gauged element.
As the gauged element or silicon substrate is deflected so the gages register a
change in resistance. The bridge circuit identifies the resulting change in
electrical resistance as a change in applied pressure to the sensor.
Differential Pressure Sensor
Differential pressure sensors are commonly used to measure the rate of flow in a
pipe. By monitoring the pressure drop on either side of an orifice plate, the mass
flow rate of the fluid in the pipe can be determined.
(Consult the Pressure Reference Chart to compare how the various pressure
sensors reference pressure.)
WHAT IS A VACUUM PRESSURE SENSOR ?
Vacuum can be measured in two ways. By a) a Gage Pressure Sensor measuring
pressure below atmospheric pressure (i.e. referenced to atmospheric pressure) or
b) 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.
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.
Pressure Reference for Vacuum Pressure Sensor
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.)
Sensotec Note:
Sensotec 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.
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WHAT IS A BAROMETRIC PRESSURE SENSOR ?
A barometric pressure sensor is an absolute pressure sensor that is used to
measure barometric pressure. The full scale output can be calibrated in a number
of different ways but is typically 16 to 32 or 0-30 inches of mercury.
(Consult the Pressure Reference Chart to compare how the various pressure
sensors reference pressure.)
Pressure Reference for Barometric Pressure Sensor
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