An Introduction to Presure Sensors

The resistors are realized with a metal foil applied on a plastic substrate (similar to a flex print). These elements are glued on a metal-diaphragm. This is the oldest pressure sensor technology and there are only few companies which produces sensors in this way. It gives very low output signals and is not suitable for pressure below 1 bar.

The resistors are applied on a ceramic diaphragm using the same technology as for the hybrid circuits. They have very low output signals; for pressures lower than 1 bar the diameter of the sensor increases to 30 to 40 mm. This technology is suitable for high volume / low cost production.

A insulating film is applied to a metal diaphragm; after that operation, the resistors are applied on the insulating film using the same technology. These sensors have a low output signal and the limit of the low pressure ranges is 1 .. 2 bars. However they are very suitable (and popular) in the high pressure ranges, especially in hydraulic applications.

The resistors are placed on a silicon chip using standard semiconductor technology. The diaphragm, also on the silicon chip , is realized by etching the back side.

For absolute pressure sensor: the etched silicon chip is bonded to a glass plate under vacuum; the pressure behind the diaphragm is 0 (vacuum)

For gauge pressure sensor: the back side of the silicon diaphragm is exposed to ambient pressure.

The deformation caused by the pressure is measured using a capacitive element. The pressure P will cause a variation of the distance between the two electrodes.

The variation of the distance H between the two electrodes will change the capacity of the system.

Capacitive pressure sensors can be realized in different technologies: ceramic, silicon, etc.

There are some other principles and technologies for the pressure measurement: with Hall element, Inductive sensors, etc.

The pressure range defines the pressure where the sensor fullfills its specifications. The limits of the pressure range are fixed by the initial and the final values (for example 0 .. 3 bar gauge, -1 .. 1 bar gauge, 800 ... 1200 mbar absolute).

The overpressure is the maximum pressure where the sensor will still operate within specification without damage.

The burst pressure is the maximum pressure where the sensor doesn‘t fail (where no process-media can leak through the sensor.

The compensated temperature range defines the temperature where the sensor fullfills his specifications.

The sensitivity is the relationship between the output signal and the pressure changes.

The accuracy of a pressure sensor is the sum of the linearity, the repeatability and the hysteresis errors. It will be indicated in % FS (Full Scale). This indication is valid for a constant temperature; the temperature errors are specified separately.

Linearity error:

the maximum variation of the output signal from a reference line.

Hysteresis:

The maximum signal-difference between 2 measurements at the same pressure performed at increasing and at decreasing pressure.

The offset thermal shift is the variation of the sensor- signal in dependence of the temperature without pressure. The value specifies the maximum signal variation related to the temperature. The units are %FS/°C

The sensitivity thermal-shift is the variation of the sensitivity in dependence of the temperature. The value specifies the maximum sensitivity variation related to the temperature. The units are %/°C.

The long term stability is the capability of the instrument to maintain the same behaviour over time. The parameters which can change over time are offset and sensitivity. The units are %FS/year.

All sensors are measured and qualified individually. Each sensor has to be subjectetd to a temperature cycle to characterize the following parameters:

The full test-cycle will be, at least,about 20 hours long.