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Pressure Measurement Techniques
Published in Ethirajan Rathakrishnan, Instrumentation, Measurements, and Experiments in Fluids, 2020
Low-pressure gauges are usually referred to as vacuum gauges. Measurement of low pressures requires considerable care on the part of the experimentalist. For moderate vacuum measurements manometers, dial gauges, and diaphragm gauges may be employed with reasonable accuracy. But for measurement of absolute pressures below 1 torr (1 mm of Hg or 133 Pa), the McLeod gauge, Pirani gauge, Knudsen gauge, ionization gauge, and alphatron gauge are some of the commonly used pressure gauges. The McLeod gauge is used as a standard for measuring low vacuum pressures. It can be used for measuring low pressures in the range from 10−2 to 102μm of mercury, i.e., from 0.00133 to 13.3 Pa, for dry gases. Pirani gauge is a device that measures pressure through the change in thermal conductance of the gas. Pirani gauges are suitable for measurement of pressures in the range from 1 μm to 1 torr. The Knudsen gauge is capable of measuring low pressures in the range between 10−5 m and 10 μm of Hg. The ionization gauges are capable of measuring pressures as low as 10−12 torr.
Measurement of Pressure
Published in Pramod K. Naik, Vacuum, 2018
pressure measurement range of the Pirani gauge is 1 × 10 -1 Pa to 65 Pa. The lower limit of the pressure measurement is governed by the fact that at lower pressure the gas density is too low to cause any significant change in the wire temperature as the heat loss by conduction through gas becomes too small and not measurable. The thermal conductivity and heat capacity of the gas influence the measurement of pressure in this gauge. In the thermocouple gauge, a constant electrical current is supplied to the filament inside the gauge to which a thermocouple is spot welded for measurement of temperature which is indicative of the gas pressure. Chang et al 5 reported gas pressure sensor capability of a ZnO nanowire. They studied the growth and characterization of lateral ZnO nanowires on ZnO:Ga/glass templates. The average length and average diameter of the laterally grown ZnO nanowires were 5 µm and 30 nm, respectively. A single nanowire was bridged across two electrodes. By measuring the current–voltage characteristics of the samples at low pressure an increase in currents from 17 nA to 96.06 nA was observed by lowering the pressure from 1.3 × 10 –1 Pa to 7 × 10 –4 Pa. The authors have claimed that ZnO nanowires prepared in the study are potentially useful for pressure sensing in vacuum.
Pressure Measurement Techniques
Published in Ethirajan Rathakrishnan, Instrumentation, Measurements, and Experiments in Fluids, 2016
Low-pressure gauges are usually referred to as vacuum gauges. Measurement of low pressures requires considerable care on the part of the experimentalist. For moderate vacuum measurements manometers, dial gauges, and diaphragm gauges may be employed with reasonable accuracy. But for measurement of absolute pressures below 1 torr (1 mm of Hg or 133 Pa), the McLeod gauge, Pirani gauge, Knudsen gauge, ionization gauge, and alphatron gauge are some of the commonly used pressure gauges. The McLeod gauge is used as a standard for measuring low vacuum pressures. It can be used for measuring low pressures in the range from 10−2 to 102μm of mercury, i.e., from 0.00133 to 13.3 Pa, for dry gases. Pirani gauge is a device that measures pressure through the change in thermal conductance of the gas. Pirani gauges are suitable for measurement of pressures in the range from 1 μm to 1 torr. The Knudsen gauge is capable of measuring low pressures in the range between 10–5 m and 10 μm of Hg. The ionization gauges are capable of measuring pressures as low as 10–12 torr.
Process analytical technology for monitoring pharmaceuticals freeze-drying – A comprehensive review
Published in Drying Technology, 2018
Davide Fissore, Roberto Pisano, Antonello A. Barresi
A simpler and more reliable method is that called “comparative pressure measurement” suggested by Nail early in 1980[64,122] and still considered one of the cheapest and most reliable methods to detect the end of primary drying, also in industrial apparatus. It takes advantage of the dependence of the signal of a thermal conductivity gauge, like the Pirani gauge, on the gas type and, in case of mixtures, like water and inert gas, on the composition. This would be a strong limitation for simple pressure monitoring, because the chamber gas composition differs in different cycles depending on setup, loading, and product features,[122] and, as a consequence, this type of pressure sensor, which is quite cheap, but has also a lower accuracy than the capacitance manometer, is used only in low price lab equipment. The use of the ratio of pressure signals given by the two gauges, the thermal conductivity and the capacitive ones, that approaches unity (offset point) at the end of primary drying as the Pirani is generally calibrated for air is quite reliable, because it eliminates the possible effect of a variation of the total pressure, and also quite sensitive. In fact, as the partial pressure of water falls when the sublimation rates strongly decreases, but it reduces below detection limits only when the sublimation flux is extremely low, the method allows to detect the last vials that complete primary drying: thus, it offers a safe-side indication even if the estimated drying time may be slightly longer than real as the change in the chamber atmosphere is also influenced by geometry, condenser operation, and pumping rate.