Listed below are some of the commonly asked questions:
Basic explanation of our technology
Every gas is composed of atoms or molecules. These molecules have various frequencies or wavelengths at which they resonate or vibrate. Our technology uses infrared light to essentially count the number of absorbed molecules in its measurement path to accurately and reliably quantify the active trace gas concentration.
The name of the principle in which our technology utilizes is Wavelength Modulated Spectroscopy which uses Tunable Diode Lasers to absorb gas molecules with the same frequency/wavelength to which it is tuned.
More detailed explanation
Boreal’s analyzers use an open-path gas detector that uses an integrated transmitter/receiver unit (transceiver) and a remote, passive retro-reflector. The analyzers transceiver houses the laser diode, drive electronics, detector module and micro-computer subsystems.
Laser light is emitted from the transceiver through ambient air to the reflector and back. The return light is focused onto a photo-diode. A portion of the laser beam is passed through an onboard reference cell to ensure that the laser beam is locked onto the gas of interest. Measure and calibration signals are then compared to determine the actual gas concentration in the path.
Boreal’s analyzer have a local data display with simple, menu driven set-up functions. Serial and analog signals are also available. The serial signal contains extensive self diagnostic data. A built-in data logger stores up to 10,000 readings. Boreal’s GasView software enables easy transfer of data and diagnostics to a PC.
Our analyzers work by absorption of near infra-red laser light by the specific gas of interest. It therefore provides a direct and immediate measurement of the gas. The laser method uses single line spectroscopy using an absorption line selected to be well isolated from absorption lines of other atmospheric gases. The laser method therefore suffers less from absorption interferences, especially from ever present gases like H2O and CO2, that cause problems with broadband IR and FTIR detectors.
As a consequence, our systems can measure over long ambient paths, up to 750m, using low powered lasers that are completely eye-safe.
The retroreflector is a vital part of the system. The laser beam is transmitted to a retroreflector made with a special type of corner cube arrangement. This causes the signal to be reflected directly back to the detector in the transmitter enclosure. With the appropriate number of reflectors, path lengths up to 750m can be traversed.
A retroreflector is different in that the reflected light returns in the same direction as the incident light. The beam is reflected 180 degrees. A retroreflector is like a section through a corner and has three faces that form the inside corner of a cube. Some highway reflectors are everyday examples. See below.
For short paths, reflective tape or IMOS can be used, these can be attached to the back of an enclosure which can be fitted to a tripod or placed on a wall in any convenient location.
For long paths, gold plated corner cubes are the optimum solution. These are recommend to be placed inside an enclosure to protect the high precession cubes from the elements. These may be mounted on a tripod, or the enclosure can be placed in any convenient location.
If the location is outside and the climate is humid, a rain hood and some heating in the enclosure may be necessary to prevent condensation on the window. This usually requires an outlet nearby to supply power to a heater. The retros can tolerate small vibrations and movement, so the retro mounting does not need to be as robust as the mounting for the analyzer.
The analyzer will shipped already calibrated and does not require any calibration in the field.
The calibration of the analyzer is done by passing a known concentration of gas through a test cell, which is placed in the path of the laser beam. The calibration data are stored in the instrument’s software as a standard reference waveform.
During operation, the analyzer’s internal reference cell is compared with this stored waveform at frequent intervals. Any significant deviation generates a status code to alert the user to a potential calibration problem. The analyzers software can be used to check the reference cell, as well as to download sample, reference, and calibration waveforms (System Menus/Internal Arrays Transfer) to verify that the internal calibration system is functioning correctly. If GasViewOP is being used, these waveforms can be displayed on the computer.
A technical note detailing the calibration procedure and quality assurance is available from Boreal.
Boreal’s analyzers contain an invisible (infrared) laser source. This conforms to Class 3a as per ANSI Z136.1-1993 and does not require the use of protective eye wear, protective equipment, or outdoor control measures. There is no optical ignition hazard presented by lasers of this type.
Caution: The laser output from the analyzers have a very low power (conforms to Class 3a as per ANSI Z136.1-1993) and will not damage eye tissue. However, it is the recommendation of Boreal Laser Inc. that, as with ANY LASER SYSTEM, the user/operator should avoid staring directly into the output aperture of the instrument. Note that the laser beam is infra-red and is not visible to the eye.
The analyzers transceiver unit is not intrinsically safe and is normally located away from a hazardous area.
The retros are intrinsically safe and can be used in hazardous areas. In hazardous areas, where heaters are required to prevent condensation, these heaters should be of a suitable type.
The type of retro used will depend on the path length, atmospheric conditions such as dust or fog, and the type of laser.
When choosing a retro, the prime concern should be to keep the returning light level value between the preferred values of 4,000 and 8,000.
The range of the value is between 1000 and 16,368. Below 1000 the display will indicate “Low Light”.
There is no indication that the returning light has exceeded 16,368. When this happens the receiver saturates and the displayed light value could read very low (<200). In this situation the gas concentration readings will be in error and it is also possible to cause damage to the optical components.
The light value can be adjusted with the use of different retros and attenuators.
Gas concentration is measured in ppm, ‘parts per million’ by volume. If a room measuring 100m x 100m x 100m (1 million cubic metres) has 10 cubic metres of air replaced by a pure gas, then the gas concentration is expressed as 10 ppm. A point sensor measures directly in ppm. Open path monitors, like the GasFinder2, measure the total amount of a specific gas, for example HF, in the path of the laser beam between the transmitter unit and a reflector. This is a ‘total path’ measurement. The units are ‘parts per million metres’, or ‘ppmm’. A uniform background concentration of 10ppm over 50m gives a reading of 500ppmm. 10ppm x 50m = 500ppmm
A concentrated cloud of 50 ppm, 10 m in diameter, in a background of 0 ppm also gives a reading of 500 ppmm. 50ppm x 10m = 500ppmm
When the 500ppmm is divided by 50m the result is a value of 10ppm, which is the ‘path averaged’ concentration.
Explanation of R2
When the analyzer receives the returning laser signal after it has passed through the sample gas, the receiver converts it to the shape of a specific waveform or curve. This is the sample waveform. The analyzer also has a similar signal from the stored waveform of the calibration gas. These curves are then digitized and compared as two numeric arrays.
An accepted mathematical procedure to compare curves or numeric arrays is the Linear Least Squares Regression analysis. This analysis results in a measure of the similarity (R2), between the waveform of the sample gas and that of the calibration gas. A perfect similarity would give a value for R2 of 1.0, and a total mismatch would be 0.0.
The blue line represents the Linear Least Squares fi t of the data and is the best fi t of a straight line between the reference (X) and sample (Y) data points. The slope is a component in the ratio-metric calculation of gas concentration.
A typical plot of concentration versus R2 will give the following graph:
With lower levels of sample gas, the R2s decrease, and equal zero when there is no gas present. As the signal from the gas becomes stronger, the effect of noise, both electronic and optical, is reduced and the R2s will increase (i.e., the signal to noise ratio will increase). The general shape of the plot is the same for all gases; however, the x-axis values will depend on the sensitivity of the instrument to the gas species being observed.