Transcript Slide
Examples of pressure sensor packaging Temperature characteristics of a piezoresistive pressure sensor. Transfer function at three different temperatures (a) and full-scale errors for three values of compensating resistors (b). 1 Slide # Pirani vacuum gauge is a sensor that measures pressure through thermal conductivity of gas. The simplest version of the gauge contains a heated plate. The measurement is done by detecting the amount of heat loss from the plate that depends on the gas pressure. When an object is heated, thermal conductivity to the surrounding objects is governed by G G0 G g Gs Gr ak PPT P PT Gs is thermal conductivity via the solid supporting elements Gr is the radiative heat transfer a is the area of a heated plate k is a coefficient related to gas properties PT is a transitional pressure that is the maximum pressure that can be measured. 2 Slide # • If the solid conductive and radiative loss is accounted for, the gas conductivity Gg goes linearly down to absolute vacuum. The trick is to minimize the interfering factors that contribute to G0. • This can be achieved by the use of both the heated plate that is suspended with a minimal thermal contact with the sensor housing and the differential technique that to a large degree cancels the influence of G0. Thermal conductivities from a heated plate (a). Transfer function of a Pirani vacuum gauge (b) 3 Slide # Pirani vacuum gauge with NTC thermistors operating in self-heating mode • The sensing chamber is divided into two identical sections where one is filled with gas at a reference pressure, say 1 atm, and the other is connected to the vacuum that is to be measured. • Each chamber contains a heated plate that is supported by the tiny links to minimize a conductive heat transfer through solids. • Both chambers are preferably of the same shape, size, and construction so that the conductive and radiative heat loss would be nearly identical. • The bridge automatically sets temperature of Sr on a constant level Tr that is defined by the bridge resistors and is independent of the ambient temperature. 4 Slide # • Condenser Microphone: Produces output voltage proportional to distance if the charge is kept fixed • Fiber-Optic Microphone: Measures diaphragm deflection by comparing reflected beams • Piezoelectric Microphones: Converts deflections directly into voltage • Electret Microphones: No external voltage; high sensitivity, high frequency of operation; high dynamic range; one of the most widely used. • Dynamic microphones: Voltage induced by coil moving in a permanent magnet filed. • Solid State Acoustic Detectors: Measures vibrations to perform detection etc. Examples are SAW devices. Please refer to the handout for detailed information 5 Slide # • Scintillating detectors (example: NaI) Widely used for sensitivity and ease of us Poor energy resolution Scintillating film can have reliability issues Uses a photomultiplier tube Scintillation detector with a photomultiplier 6 Slide # • Ionization Detectors • Oldest and most widely used • Ionization in gas happens at 10 – 20 eV energy • The positive and negative charged ions drift to the electrodes under application of an electric voltage • Could be realized using ionization or proportional chambers Simplified schematic of an ionization chamber (a) and a current vs. voltage characteristic (b) 7 Slide # • Under application of an electric voltage the current increases in a very different manner for different voltage region as shown below: • Region 1: Not all ions are swept away; some recombine • Region 2 (saturation): All ions are swept away; continuous mode operation; no secondary ionization; can be used for energy resolution • Region 3 (proportional region): Avalanche (Townsend) multiplication can occur as the accelerating voltage is large enough for ions to collide and create more ions. Used in pulsed mode; can give energy resolution of the incoming radiation • Region 4 (Geiger-Muller counters): In this region, the energy of incoming radiation don’t matter. No energy resolution. Various operating voltages for gas-filled detectors 8 Slide #