We’re going to take a look at pressure sensors. The most obvious use is in measuring air pressure for weather monitoring and prediction. But pressure sensors are also used in cars to measure manifold pressure, in washing machines to measure water levels, and in biomedical applications, such as measuring blood pressure. Another application of pressure sensors is to measure altitude, since air pressure changes with height above sea level. Ocean depth can similarly be measured.

When using pressure sensors, the substance you are measuring can adversely affect the device. Remember that these are sensitive electronic components, and fluids or corrosive gases can destroy them. So unless you’re measuring clean, dry air, you’ll need to provide some degree of environmental protection for your sensor. Just how you do that really depends on what the application is, what environmental conditions you must protect against, and how far your budget stretches.

Pressure sensors work by measuring the deflection of a diaphragm separating two chambers. One chamber is exposed to the pressure that is being measured, while the other chamber holds a reference pressure. The pressure difference between the two chambers causes the diaphragm to deflect, and this deflection is converted into a voltage that is proportional to the pressure difference. Pressure sensors come in three types: absolute, differential, and gauge.

In an absolute pressure sensor , the reference chamber is sealed. Pressure readings are referenced to an absolute pressure, hence the name. Absolute sensors normally have the reference chamber pressure at vacuum or at one atmosphere.

In a differential sensor, the reference chamber is not sealed, and an external pressure reference may be applied. Differential sensors are used to measure the relative pressures between two gases or two liquids. A differential sensor may be treated as an absolute sensor by providing it with a sealed and stable reference pressure.

A gauge sensor is a variation of the differential pressure sensor, where the reference pressure chamber is open to the atmosphere. Thus, the measured pressure is referenced to atmospheric pressure, and variations of atmospheric pressure (such as those caused by weather conditions or altitude) are taken into account. One interesting use of a gauge pressure sensor is to measure airspeed. If the measuring chamber is exposed to the oncoming airflow (caused by the aircraft’s motion), and the reference chamber is exposed to the air but sheltered from the effects of the airflow, then the difference in pressure can be used to calculate the airspeed of the aircraft.

So, with all that in mind, let’s take a look at some pressure sensors. The first sensor is a Motorola MPXA6115A absolute pressure sensor (Figure 12-18). It operates from a 5V supply and will give an output voltage of between 0.2V and 4.8V, proportional to pressures of 15kPa to 115kPa. (Pa is Pascals and is a unit of pressure.) Unlike most other pressure sensors, which require external signal conditioning, temperature compensation, and signal amplification, the MPXA6115A integrates it all in one neat little package. It comes in an eight-pin chip package, with or without snorkel!

Figure 12-18. Interfacing the Motorola MPXA6115A pressure sensor

The NC pins are no connection and should be left unwired. The only additional components required are a decoupling capacitor on the power supply and a resistor and capacitor in parallel at the output. The output may be directly connected to an ADC’s input.

The second pressure sensor we will look at is also an absolute pressure sensor. But, unusually, rather than producing an analog output, it incorporates a built-in ADC. It is interfaced to a microcontroller using SPI, and being digital, it is much less susceptible to noise and interference. The sensor is the KP100, made by Infineon Technologies (http://www.infineon.com) in Munich, Germany.

The schematic for a circuit based upon the KP100 is shown in Figure 12-19.

Figure 12-19. KP100 pressure sensor circuit

The sensor operates off a 5V supply, and this is decoupled to ground using a 100nF capacitor to reduce noise. The sensor has a standard SPI-style interface and is connected to a microcontroller as with any SPI device. The sensor also provides a READY output, which may be used to interrupt the host processor or may simply be connected to a spare I/O and read as a digital status flag. The KP100 also requires a separate clock (CLK) input. This clock can be either 4MHz or 8MHz. If the processor is running at one of these speeds, then the sensor can share the same clock input as the processor. However, if the processor is operating at a different clock frequency, the KP100’s clock may be easily generated using a clock module. These four-pin devices are available in a variety of standard frequencies and require only power and ground to generate a clock output.