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UL 920004-2017 pdf free download

UL 920004-2017 pdf free download.Performance Requirements for Open Path Toxic Gas Detectors.
5.4.1 Use of gas cells
The test fixtures shall be designed such that the test gas in individual cells can be changed and that when using the equipment, the cells can be exchanged sufficiently quickly In order that transient obscuration during the exchange by the walls or window retaining structure shall not create a abeam blocked condition. The transverse dimension of the cells shall be large enough not to cause partial blockage of the beam.
The test described in 5.6 and 5.14 may require cells of large dimensions or the use of an alternate gas simulation filter.
Cells shall be located as close as possible to either the transmitting or receiving aperture of the equipment having regard to minimizing unwanted effects on the equipment of reflections from the cell on the receiver and the need not to cause partial blockage of the beam.
The characteristics (e.g., material, thickness and flatness) and inclination of the windows of the cells shall be chosen to minimize the effects of reflection, distortion and attenuation of the beam over the effective bandwidth of the measuring radiation. Non-uniform gas cells shall be characterized by average path length.
The axial length of the cells may be chosen in relation to the concentration of the gas filling the cells to provide standard values of integral gas concentration for use in calibration.
Cells may be filled with test gases including, for example, clean air (for setting the zero point) and the gas to be measured. Cells used for the zero point setting shall have minimal effect on equipment calibration. Heating may be applied to cells to ensure that vapor, condensable at room temperature, can be maintained in the gaseous state.
To avoid using large volumes of potentially toxic gas and air mixtures, cells of appropriate length filled with test gas of substantially less than the full-scale gas concentration may be used for small path integral concentrations (e.g., 0.5 ppm x 1 meter).
For the water vapor interference test of 5.20, the cell shall be 2m in length and be capable of containing water vapor at atmospheric pressure. To prevent condensation, the cell walls and windows may be heated. Gas cells used for tests with toxic gases shall be constructed such that errors in measurement arising from variations of attenuation with wavelength in the windows of the cells shall be less than 2% of measuring range or 5% of the measured value, whichever is greater.
5.4.2 Mask for beam attenuation
The attenuation produced by precipitation and dust in the optical path, and material deposited on optical surfaces shall be simulated by a mask (for example, an opaque grid in the form of a mesh with matte black surfaces) inserted into the beam path. The mask, used for the test described in 5.22, shall have a signal intensity at the receiver of no more than 6% of signal intensity measured or calculated at half the maximum distance to the receiver. Alternatively, to simplify calculation of the open area of the mask, a mask with at most 6% open area can be used.UL 920004-2017 pdf free download.

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