SENSOR BASICS

   Pressure sensor is a device for pressure measurement of gases or liquids. Pressure is an expression of the force required to stop a fluid from expanding, and is usually stated in terms of force per unit area.

    Pressure sensors are used for control and monitoring in thousands of applications everyday. Pressure sensor can also be used to indirectly measure other variables such as fluid/gas flow,speed, water level and altitude.

    Pressure sensors can vary drastially in technology, design,performance, application suitability and cost. Sensorall International mainly use MEMS pressure sensing element and various packaging technology to design and manufacture different kind of pressure sensor.Pressure sensors can vary drastically in technology, design, performance, application suitability and cost. A conservative estimate would be that there may be over 50 technologies and at least 300 companies making pressure sensors worldwide.

     Pressure sensors can be classified in terms of pressure ranges they measure, temperature ranges of operation, and most importantly the types of pressure they measure. Pressure sensors are variously named according to their purpose.   Detailed pressure types as below

      Absolute pressure sensor

     This sensor measures the pressure relative to perfect vaccum.

      Gauge Pressure sensor

      This sensor measures teh pressure relative to atmospheric pressure.

      Differential pressure sensor

       This sensor measures the difference between two pressures, one connected to each side of the sensor. Differential pressure sensors are used to measure many properties, such as pressure drops across oil filters or air filters, fluid levels or flow rates. Technical speaking, most pressure sensors are really differential pressure sensors, such as gauge pressure sensor is merely a differential pressure sensor in which one side is open to the ambient atmosphere.

        Applications

        There are many applications for pressure sensors

        Pressure sensing

        This is where the measurement of interest is pressure,expressed as a force per unit area, this is usefule weather instrumentation, aircraft, automobiles and any other machinery that has pressure functionality implemented.

       Altitude sensing

       This is useful in aircraft, rockets, satellites, weather balloons, and many other applications. All these applications make sue of the relationship between changes in pressure relative to the altitude. The relationship is governed by the following equation.

   This equation is calibrated for an altimeter, up to 36090 feet (11000meters). Outside that range, an error will be introduced which can be calculated differently for each differnt pressure sensor. These error calculations will factor in the error introduced by the change of temperature as we go up.

    Barometric pressure sensors can have an altitude resolution of less than 1 meter, which is significantly better than GPS systems which is about 20 meters altitude resolution. In navigation applications altimeters are used to distinguish between stacked road levels for car navigation and floor levels in buildings for pedestrian navigation.

      Flow Sensing

       This is the use of pressure sensors in conjunction with the venturi effect to measure flow. Differential pressure is measured between two segments of a venturi tube that have a different aperture. The pressure difference between the two segments is directly proportional to the flow rate through the venturi tube. A low pressure sensor is almost always required as the pressure difference is relatively small.

     Level/Depth sensing

     A pressure sensor may also be used to calculate the level of a fluid. This technique is commonly employed to measure the depth of teh submerged body such as a diver or submarine, or level of contents in a tank such as in a water tower. For most practical purposes,fluid level is directly proportional to pressure. In the case of fresh water where the contents are under atmospheric pressure, 1psi=27.7inH2O/1pa=9.81cmH2O. The baisc equation for such a mesurement is

${\displaystyle P=\rho gh}$

where P = pressure, ρ = density of the fluid, g = standard gravity, h = height of fluid column above pressure sensor

   Leak testing

    Pressure sensor may be used to sense the decay of pressure due to a system leak. This is commonly done by either comparison to a known leak using differential pressure. or by means of utilizing the pressure sensor to measure pressure change over time.

   Ratiometric correction of pressure sensor output

   Piezoresistive pressure sensors configured as wheatstone briges often exhibit ratiometric behavior with respect not only to the measured pressure, but also the pressure sensor supply voltage.

where:

${\displaystyle V_{\mathrm {out} }}$ is the output voltage of the pressure sensor.

${\displaystyle P}$ is the actual measured pressure.

${\displaystyle K}$ is the nominal pressure sensor scale factor (given an ideal pressure sensor supply voltage) in units of voltage per pressure.

${\displaystyle Vs_{\mathrm {actual} }}$ is the actual pressure sensor supply voltage.

${\displaystyle Vs_{\mathrm {ideal} }}$ is the ideal pressure sensor supply voltage.

Correcting measurements from pressure sensors exhibiting this behavior requires measuring the actual transducer supply voltage as well as the output voltage and applying the inverse transform of this behavior to the output signal:

${\displaystyle P={V_{\mathrm {out} }\times Vs_{\mathrm {ideal} } \over K\times Vs_{\mathrm {actual} }}}$

Note: Common mode signals often present in pressure sensors configured as Wheatstone bridges are not considered in this analysis.