Technical
54
The Illuminating Engineering Society provides information regarding radiation dose as follows i :
“ The UV dose is the exposure time on a given microorganism or surface multiplied by the UV irradiance and reported in millijoules per square centimetre ( mJ / cm ²)”. The UV irradiance is the power of electromagnetic radiation incident on a surface per unit surface area and reported in microwatts per square centimetre ( μW / cm ²).”
The required dose depends on both the pathogen species to be eliminated and the desired degree of reduction . For example , eliminating 90 % of Escherichia coli O157 : H7 , the bacterium that can cause sometimes-fatal food poisoning , requires 1.5 mJ / cm ²; doubling the dose eliminates 99 %, tripling eliminates 99.9 %, and so forth . This is referred to as log10 (‘ logten ’), or more commonly ‘ log ’, reduction ( Table 3 ).
Table 3 : Log 10
Pathogen Reduction Log10 reduction
Percentage pathogen
( Number of doses ) elimination 1 90 % 2 99 % 3 99.9 % 4 99.99 % 5 99.999 %
The International Ultraviolet Association published a compilation of dose requirements for many different pathogens , but viruses on average require a dose of about 20 mJ / cm ² for 90 % reduction when directly exposed to the UV-C radiation ( IUVA undated ). Most of the studies referenced in the compilation consider 254-nm radiation from low-pressure mercury vapour lamps , but the required dose from 207-nm and 222-nm excimer lamps should be comparable .
The basics of determining the radiant energy levels to a surface are as follows ii :
• Length of exposure- When disinfecting surfaces , it must be first determined if the target is moving or stationary . This helps to determine if there are any limiting factors associated with the length of exposure time . In most surface disinfection applications time is relative to intensity ; increased source intensity can decrease necessary exposure time . It is important to remember that micro-organisms vary , requiring a higher or lower intensity for inactivation , depending on their structure iii .
• Intensity of source- UV-C lamp and equipment manufacturers normally provide the intensity of a given source ( lamp or fixture ) at a given distance . A distance correction factor may be needed when calculating a desired dose or intensity for a surface . UV-C energy follows the same inverse square law for intensity as visible
RACA Journal I January 2022 light and other electromagnetic sources : the amount of energy at the surface is measured in proportion to the square of the distance from the energy ’ s source ( the UV-C lamp ), assuming no loss through scattering or absorption . Temperature and airflow corrections may also be necessary , depending on the location of the application . The intensity of a source is given in power per unit area ( i . e ., μW / cm ²).
• Distance from source to surface – in a point irradiation application , the distance is relatively easy to calculate . Calculating time requirements and intensity levels for a three-dimensional object or space is more complex . The varying distances from the source are the first challenge , because the object itself creates a shadowing effect , and any shadows from the local environment must be taken into consideration ( for example the back surface of an object that did not get direct light ). However , portable devices are available that effectively measure the reflected dose from shadow areas and offer quantifiable results .
UVGI LAMP TECHNOLOGIES Lamp technologies iv include continuously emitting low- and medium-pressure mercury lamps , as well as pulsed xenon arc lamps . Studies have shown that these technologies are comparably effective for disinfection . Pulsed sources may be more practical if rapid disinfection is required v . Light emitting diodes ( LEDs ) and krypton-chlorine excimer lamps , which emit in the germicidal range ( UV-C ), are emerging technologies .
The most practical method of generating germicidal radiant energy is by passage of an electric discharge through a rare gas ( usually argon ) at low pressures ( on the order of 130 to 400 pascals , or 1 to 3 torr ) containing mercury vapor enclosed in a special glass tube with no fluorescent coating .
Hot-cathode germicidal lamps are identical in shape , electrical connection , operating power , and life to standard fluorescent lamps . Cold-cathode germicidal lamps are also available in various sizes , usually for shorter , smaller diameter lamps . Their operating characteristics are similar to those of hot-cathode lamps , but their starting mechanisms are different .
Approximately 45 % of the input power from such a device is emitted at a mercury-discharge wavelength of 253.7 nm , in the middle of the UV-C band . The second major emission line is at 184.9 nm , but this emission is normally absorbed by the glass , since — if emitted through the glass , as it is with pure quartz — it would create ozone at levels far above the safety limit .
Other mercury lines in the UV-B and UV-A regions are present at much lower emitted-power levels and not considered important in germicidal action . These spectral distribution spikes can , unfortunately cause safety concerns .
www . hvacronline . co . za