Abstract: This chapter highlights pressure sensors ranging from the classical manometers, based on the displacement of the surface of a liquid or on the deflection of the walls of a metallic chamber, to pressure transducers where the deflection of a disk or of a cantilever beam is converted into an electric signal eventually digitalized and sent to a computer. Only the basic principle of pressure-sensitive paints is reported as this technique is still a subject of research.
Key words: hydrostatic manometers, mechanical manometers, pressure transducers.
1.1 Fundamental features
Pressure measurements are essential in aerodynamics for the calculation of velocity, direction and mass flow rate of a fluid stream, and are complementary to balances in measuring the aerodynamic forces acting on a body.
Pressure sensors range from the classical hydrostatic and mechanical manometers to pressure transducers and to pressure-sensitive paints (PSP). The choice depends on the magnitude of the pressure to be measured, on its steadiness, on the required accuracy and resolution, and on the ease with which data can be handled and stored.
It is important to realize that all pressure sensors measure a differential pressure, either between two pressures associated with the flow or between a flow pressure and the atmospheric pressure. Thus, if pressure data are referenced to the contemporary atmosphere pressure (gage pressure sensors), it is imperative to measure this value enabling later reduction of the data to an absolute or other standard condition. Only if the reference is vacuum absolute values of pressure are obtained.
The main desirable characteristics in a pressure gage, whether they are
feasible or not, are given below. In practice, the best possible compromise
is needed to adapt the instrument to the specific case:
1. Range. The lowest pressure obtained with techniques for the vacuum are of the order of 10-14 atmospheres. At the other extreme, in the laboratory pressures greater than 105 atmospheres were obtained. The field is so vast that a particular type of manometer may cover only a small fraction of it.
2. Frequency response. In many applications the pressure changes so slowly that it can be considered constant. On the other hand, the jump in pressure that occurs through a shock wave propagating in a gas at rest can occur in a time less than 10-7 s. Fluctuating pressures that are generated in gas guns, engine cylinders, and in gas turbines have characteristic times that cover the whole field between the two above-mentioned extremes. Frequencies range therefore from 0 to 10 MHz (but no pressure sensor developed so far is able to approach this limit).
3. Accuracy. Usually the accuracy of pressure transducers is referred to the full scale, i. e. the maximum measurable pressure. It is clear that if the instrument is accurate within 1% of the full scale, the measurement of a pressure that is equal to one-tenth of the full scale will be affected by an error of 10%. Therefore, for good accuracy, an instrument should be chosen whose full scale is only slightly higher than the maximum pressure to be measured.
4. Sensitivity. All things being equal, a high sensitivity is always to be preferred: the output signal requires less amplification (pressure transducers) or allows a more precise reading (manometers).
5. Linearity. Usually it is considered convenient to have the output signal proportional to pressure, but this is not only not essential but in some cases not even desirable if the measurement range is very large.
6. Size. The boundaries of the system to be explored can limit the size of the sensor (for example, the need to measure pressure between turbine blades or the need to insert many sensors in a wing model); the size of the sensor may affect its interaction with the stream, disturbing the measurement; because the reading is only an average on the surface of the sensor, size limits the level of detail that can be detected.
7. Calibration. In some cases, the response of the sensor can be calculated by an analytical equation (e. g. hydrostatic manometers). For simple systems, the dynamic response can be inferred from the
static calibration; but in most cases only a dynamic calibration provides a sufficiently reliable basis from which the results can be interpreted.
8. Data acquisition and handling. The hydrostatic or metal manometers require manual reading and transcription of the data. Some types of transducers produce a frequency modulated signal that has to be read by a system of demodulation to obtain pressure. Obviously, the ease and certainty with which data can be interpreted and recorded are important factors in choosing a pressure sensor.