Two recent developments in instrumentation technology are making it easier to measure and control gases and vapors released by industry into the atmosphere.
These instruments can:
- Detect pollutants and measure their intensity;
- Measure the efficiency of scrubbers or other devices removing pollution;
- Measure the efficiency of chemical processes and of combustion in furnaces and incinerators.
- Provide feedback signals for control of combustion and chemical processes.
Probes placed in the gas stream would become contaminated and corroded by the harsh chemicals. Detectors that have no physical contact with the gas stream are needed.
The two classes of device that have been developed both achieve non-contact measurement by using optical means. One uses visible light to measure flow. The other uses lasers to analyze chemical content.
Gas Flow Meter
The flow measuring device, developed by Optical Scientific, makes use of turbulence in the gas stream. The key to its operation has been a development of high-speed microprocessors.
The gas stream contains pockets or regions of different temperature or density, resulting in optical scintillation, the same effect seen in the twinkling of stars or shimmering of hot air combining with cold.
The turbulence is caused by combustion or chemical process and by bends and friction with the walls of vessels, pipes, flue etc.
The turbulence has been found to have highly consistent patterns. The only factor affecting turbulence is the rate at which the gases flow.
The optical flow sensor (OFS) uses a light source mounted outside the flue or duct and a photocell detector diametrically opposite. Both are isolated from the gas with quartz windows that withstand heat and corrosion.
The photocell detects the variation in light intensity caused by the scintillation. The variation of the photocell’s electrical output is a precise analog of the pattern of optical turbulence, a pattern repeated continuously because of the consistency of the turbulence.
The analog output is converted to a stream of digital numbers. The microprocessor stores a number and compares it against subsequent numbers until a match is found, indicating that the pattern of turbulence has been repeated. It simultaneously measures the time for the pattern to be repeated.
The device has been calibrated for flow rate in a wind tunnel. Using the time measurement and the diameter of the flue, it calculates flow rate in cubic meters per second.
Chemical Analysis By Laser
The devices for analysis of the chemical content of gases use tunable diode lasers to detect rapid changes as they happen, unlike earlier methods that took samples from the gas stream.
Some lasers produce light over a narrow band of frequencies in the visible, infra-red or ultraviolet spectra. Others produce light at a few highly-precise frequencies. Every atom or molecule absorbs one frequency of light unique to each atom or molecule.
The instrument directs laser light across a flue or duct to a detector at specific frequencies that will be absorbed by specific gases or vapors. The detector measures the amount of light it receives at each frequency, giving a measure of the amount of each gas or vapor.
In the SpectraSensors SS3000 gas analyzer, the laser is tuned to the frequencies of H20, CO2, and H2S (hydrogen sulfide), which it detects 4 times per second.
The Yokogawa TDLS200 measures O2, H2O, CO, NH3 (ammonia) and H2S (hydrogen sulfide).
There are three types of mounting:
- The detector is opposite the laser.
- Laser and detector are housed together and the light is reflected off a mirror on the opposite side; with the longer path length, smaller concentrations can be measured.
- The gas can be continuously drawn into a chamber where mirrors reflect the laser light up to100 times.
Improved Measurement Of Industrial Pollution
These instruments improve measurement and analysis of pollutants released into the air. They can help to protect the environment, and make industrial plants more efficient.