decreasing signal. Elevated temperatures cause an additional drop in internal resistance and sensitivity. The main effect on the piezoelectric effect is that with increasing pressure loads and temperature, the sensitivity reduces due to twin formation. While quartz sensors must be cooled during measurements at temperatures above , special types of crystals like GaPO4 gallium phosphate show no twin formation up to the melting point of the material itself. However, it is not true that piezoelectric sensors can only be used for very fast processes or at ambient conditions. In fact, numerous piezoelectric applications produce quasi-static measurements, and other applications work in temperatures higher than . Piezoelectric sensors can also be used to determine aromas in the air by simultaneously measuring resonance and capacitance. Computer controlled electronics vastly increase the range of potential applications for piezoelectric sensors. Piezoelectric sensors are also seen in nature. The collagen in bone is piezoelectric, and is thought by some to act as a biological force sensor. Piezoelectricity has also been shown in the collagen of soft tissue such as the Achilles tendon, aortic walls, and heart valves. Principle of operation The way a piezoelectric material is cut defines one of its three main operational modes: Transverse Longitudinal Shear Transverse effect A force applied along a neutral axis (y) displaces charges along the (x) direction, perpendicular to the line of force. The amount of charge () depends on the geometrical dimensions of the respective piezoelectric element. When dimensions apply, , where is the dimension in line with the neutral axis, is in line with the charge generating axis and is the corresponding piezoelectric coefficient. Longitudinal effect The amount of charge displaced is strictly proportional to the applied force and independent of the piezoelectric element size and shape. Putting several elements mechanically in series and electrically in parallel is the only way to increase the charge output. The resulting charge is , where is the piezoelectric coefficient for a charge in x-direction released by forces applied along x-direction (in pC/N). is the applied Force in x-direction [N] and corresponds to the number of stacked elements. Shear effect The charge produced is exactly proportional to the applied force and is generated at a right angle to the force. The charge is independent of the element size and shape. For elements mechanically in series and electrically in parallel the charge is . In contrast to the longitudinal and shear