Most solid state pressure sensors are constructed using a small diaphragm, which moves in response to the pressure differential. However, this diaphragm is also sensitive to acceleration in the axis perpendicular to its surface. Care must be taken to mount the sensor in a way that allows motion only in directions parallel to the diaphragm. This can be checked by applying a static pressure to the sensor and shaking it while noting the output.
The pressure differential between various parts of a canopy can be measured most easily using "differential" pressure sensors. This type of sensor has positive and negative (pressure and vacuum) "ports" and the voltage output from these sensors varies only with the difference in pressure between these two ports. Pressure changes due to the local atmospheric pressure and altitude would affect each port identically so that the sensor would be "self adjusting" for these conditions. Differential pressure sensors are available in "unipolar" types which measure pressure only in one direction and produce a voltage proportional to that pressure, or "bipolar" types which measure pressure in either direction and produce a voltage that varies above and below a given point.
"Absolute" pressure sensors could also be used to measure pressure differences in canopies. This type of sensor has only one port and the voltage output corresponds to an absolute atmospheric pressure. Two sensors would be used, comparing the pressure difference between the two. By design this arrangement would be "bipolar", since the atmospheric pressure will always be within the range of both sensors, since that range is so wide.
Although the data obtained from an absolute sensor might be easier to understand due to the absolute nature of the data obtained, this type of sensor would be extremely difficult to implement in testing parachutes, for several reasons:
1. Parachutes exhibit a rather narrow range of pressures between points in and around the canopy, even during inflation (around .25 PSI), but absolute pressure sensors have a very wide range (typically 0-15 PSI). Therefore an absolute sensor would need to be scaled (amplified) and offset to an area where the small signals from these small pressure changes could be effectively used. This adjustment would cause the sensors to produce valid signals only within a narrow range of atmospheric pressures. This means that the parachute being tested would need to be operated within a narrow range of altitudes to obtain valid signals.
2. Even if the sensors were adjusted to produce valid signals in this range and the parachute was operated within this range, there would be variations in the signals due to the descent of the parachute as well as variations due to the relative pressure differences, making this data difficult to interpret.
Other difficulties when using pressure sensors in parachutes:
Most solid-state pressure sensors are non-linear at low pressures. Since pressure sensors must be selected to have a maximum rating that is above what will be expected during inflation so as not to damage the sensor, pressure readings during flight or descent will often be in this non-linear area and will need to be disregarded.
Flexible tubing can be connected to the ports of a sensor to bring these points of pressure and vacuum to specific areas of interest in the parachute. However, adding tubing creates a number of problems itself:
1. The tubing must not be oriented so that it can receive "ram-air" pressure if that is not what is being measured, (dynamic pressure versus static pressure) although covering the end of the tube with foam or other porous material to build what is commonly known as a "static basket", may be useful in preventing ram-air pressure from entering the tube.
2. In pressure measurements that change with time the frequency response of the tubing becomes important since the tubing may behave as a filter. In other words, the pressure output of the tube may be reduced significantly in amplitude at certain frequencies. These variations depend on the length and diameter of the tubing, and the material from which the tubing is made. Tubing can act as a low pass filter (with a cutoff frequency of around 10-30 hertz, and can also have a much higher resonant frequency at which the pressure will be nearly equal to the pressure without the tubing.
3. For sensors with both pressure and vacuum ports, the added tubing should be identical on both ports or calculations involving the tubing will be very difficult.