Different types of sensors and their uses in instrumentation

Pressure Sensors

Pressure measurements can be taken to determine a range of different values depending on whether the pressure is relative to atmosphere, vacuum conditions, or other measuring factors. Pressure sensors are instruments that can be designed and configured to detect pressure across these variables. Absolute pressure sensors are intended to measure pressure relative to a vacuum and they are designed with a reference vacuum enclosed within the sensor itself. These sensors can also measure atmospheric pressure. Similarly, a gauge pressure sensor detects values relative to atmospheric pressure, and part of the device is usually exposed to ambient conditions. This device may be employed for blood pressure measurements.

Aneroid Barometer Sensors

 

An aneroid barometer device is composed of a hollow metal casing that has flexible surfaces on its top and bottom. Atmospheric pressure changes cause this metal casing to change shape, with mechanical levers augmenting the deformation in order to provide more noticeable results. The level of deformation can also be enhanced by manufacturing the sensor in a bellows design. The levers are usually attached to a pointer dial that translates pressurized deformation into scaled measurements or to a barograph that records pressure change over time. Aneroid barometer sensors are compact and durable, employing no liquid in their operations. However, the mass of the sensing element limits the device’s response rate, making it less effective for dynamic pressure sensing projects.

 

Manometer Sensors

 

A manometer provides a relatively simple design structure and an accuracy level greater than that afforded by most aneroid barometers. It takes measurements by recording the effect of pressure on a column of liquid. The most common form of manometer is the U-shaped model in which pressure is applied to one side of a tube, displacing liquid and causing a drop in fluid level at one end and a correlating rise at the other. The pressure level is indicated by the difference in height between the two ends of the tube, and measurement is taken according to a scale built into the device.

 

The precision of a reading can be increased by tilting one of the manometer’s legs. A fluid reservoir can also be attached to render the height decreases in one of the legs insignificant. Manometers can be effective as gauge sensors if one leg of the U-shaped tube vents into the atmosphere, and they can function as differential sensors when pressure is applied to both legs. However, they are only effective within a specific pressure range and, like aneroid barometers, have a slow response rate that is inadequate for dynamic pressure sensing.

 

Bourdon Tubes

 

Although they function according to the same essential principles as aneroid barometers, bourdon tubes employ a helical or C-shaped sensing element instead of a hollow capsule. One end of the bourdon tube is fixed into connection with the pressure, while the other end is closed. Each tube has an elliptical cross-section that causes the tube to straighten as more pressure is applied. The instrument will continue to straighten until fluid pressure is matched by the elastic resistance of the tube. For this reason, different tube materials are associated with different pressure ranges. A gear assembly is attached to the closed end of the tube and moves a pointer along a graduated dial to provide readings. Bourdon tube devices are commonly used as gauge pressure sensors and as differential sensors when two tubes are connected to a single pointer. Generally, the helical tube is more compact and offers more reliable performance than the C-shaped sensing element.

 

Vacuum Sensors

 

Vacuum pressure is below atmospheric pressure levels, and it can be challenging to detect through mechanical methods. Pirani sensors are commonly used for measurements in the low vacuum range. These sensors rely on a heated wire with electrical resistance correlating to temperature. When vacuum pressure increases, convection is reduced and wire temperature rises. Electrical resistance rises proportionally and is calibrated against pressure in order to provide an effective measurement of the vacuum.

 

Ion or cold cathode sensors are commonly used for higher vacuum range applications. These instruments rely on a filament that generates electron emissions. The electrons pass onto a grid where they may collide with gas molecules, thereby causing them to be ionized. A charged collection device attracts the charged ions, and the number of ions it accumulates directly corresponds to the amount of molecules within the vacuum, thus providing an accurate reading of the vacuum pressure.