When we think of pressure measurement or monitoring applications, the first though is usually of high pressure gas or liquid pipelines in industrial settings with an array of
gauges, sensors and controllers to ensure smooth operation. While applications such as this are certainly common, there are also a wide range of pressure applications that
require a more subtle approach to pressure measurement and monitoring. This is where pressure indicators excel.
Pressure indicator is a vague term. After all, any gauge or sensor/readout combination will indicate the pressure. What separates pressure indicators from other pressure
measurement devices is the way they are designed and used. This makes pressure indicators a bit of a catch-all pressure measurement category that stands in contrast to
gauges and sensors.
Industrial and commercial settings have many pressure applications that call for the versatile, self-contained low pressure measurement provided by pressure indicators. Pressure
indicators are useful for monitoring room pressures in demand ventilation systems or in pressure-positive cleanrooms. They are also ideal for a variety of HVAC applications such
as measuring flue draft and setting balancing valves. Pressure indicators are a simple way to test pump performance, pneumatic controls or regulators. Other applications for which
pressure indicators are ideal include: leak testing, quick calibration of gauges and transducers, monitoring pressure drop across filters, or as a temporary replacement for test
gauges.
More about pressure…
Pressure is defined as the amount of force applied over a unit area. Usually involving liquids and gases, pressure is a critical component of a diverse array of applications, both
those that rely on accurate pressure control as well as those that derive other values (such as depth/level or flow) based upon pressure.
As pressure is defined as force over a unit area, there are a number of ways to articulate pressure readings depending upon the unit of force and the unit of area. Most commonly, we
will see PSI (pounds per square inch) or bar. Other units of measure include kg/cm2, inH2O, mmHg, Pa, and many others.
There are also different types of pressure to consider. The type of pressure refers to the zero reference point of a measurement. For example:
Gauge pressure: The sensor is referenced against atmospheric pressure so it does not include the effects of that pressure. It is equal to absolute pressure minus
ambient air pressure. Sealed gauge sensors may use a fixed pressure different than ambient atmospheric pressure.
Absolute pressure: The sensor is referenced against a perfect vacuum so it, therefore, includes the effects of atmospheric pressure. It is equal to gauge pressure
plus atmospheric pressure.
Differential pressure: Similar to gauge pressure although the reference point is another pressure point rather than ambient pressure. The sensor measures the difference
between two pressures, such as each side of a filter to measure pressure drop.
Being the catch-all pressure measurement category that it is, pressure indicators come in many shapes and sizes. They are often handheld and portable, though room pressure monitors
used in demand ventilation systems or in pressure-positive cleanrooms are wall mounted. They are often designed for low pressure applications though some models are quite capable
of measuring up to 10,000 psi.
Pressure indicators are often called manometers, itself a generic term for pressure measurement devices. Barometers, not only used as weather instruments but also as aeronautics
instruments or industrial components, are also types of pressure indicators.
Pressure Sensing Technology
Pressure indicators generally utilize an internal pressure sensor. Though many type of sensors can be used, force collector type sensors are the most common. These electronic sensors
employ a force collector such as a diaphragm or piston to measure the strain caused by force applied over an area. Simply put, pressure applies force to a diaphragm or piston which
causes the piston or diaphragm to move in relation to the amount of pressure. Sensors detect that movement (the strain) and convert to a unit of pressure.
Among force collector sensors, there are two types that we see most often:
Piezoresistive sensors are based upon the piezoresistive effect which describes changes in the electrical resistivity of a semiconductor or metal —commonly silicon, polysilicon thin
film, bonded metal foil, thick film, or sputtered thin film—when mechanical strain (pressure) is applied. Increasing pressure results in changes in the resistivity of the strain gauges
which is detected and converted into an electrical signal proportional to pressure. Generally, the strain gauges are connected to form a Wheatstone bridge circuit to maximize the
output of the sensor and to reduce sensitivity to errors.
Capacitive sensors generally feature two closely spaced, electrically-isolated metallic surfaces one of which acts as a diaphragm by slightly flexing under applied pressure. The flexing
alters the gap between the plates creating, in effect, a variable capacitor. The resulting changes in capacitance can be measured and converted into an electrical signal proportional to
pressure.
If you have any questions regarding pressure sensors, transmitters, or transducers please don't hesitate to speak with one of our engineers by e-mailing us at sales@instrumart.com or
calling 1-800-884-4967.
