Abstract: A differential refractometer measures the difference of refractive index of electromagnetic centimeter waves of samples of gas with high sensitivity, and can be used to measure the gradient of refractive index of electromagnetic waves in air, and more particularly, to measure radar ducting conditions low over bodies of water. The instrument uses two microwave oscillators, each with its own frequency controlling cavity, with the frequencies of the oscillators differing by a specific value. Air from each of two intakes, vertically separated a specific distance, is directed into each cavity, and the difference frequency measured for a short time, to provide a first measured difference frequency. Then, the air flow from the intakes are cross-fed to the cavities, and the difference frequency is measured for a short time to provide a second measured difference frequency.
Abstract: An isokinetic sampling system and method of operation assures the taking of more valid data over a long period by automated isokinetic sampling of flue gases in the stack for a stationary combustor. Sample air is extracted from the flue gases by a sampling probe having a sampling nozzle of a precise inner diameter, a thermocouple to measure the air temperature in the stack, and a pitot tube that gives readings representative of differential pressure across sampling nozzle. A separate transducer senses absolute pressure of the flue gases. Temperatures, absolute pressures and differential pressures of the flue gases are measured and representations of the temperatures, absolute pressures and differential pressures are fed to a micromanometer which feeds signals representative of these representations to a computer that generates responsive control signals for a mass flow controller.