UVP J-225 BLAK-RAY - Manual (Page 2)

HOW TO READ ANGSTROMS - NANOMETERS - ERGs
The U.S. NIST nomenclature requires measurement of wavelength in nanometers (nm)
instead of Angstroms. Ten Angstroms are equal to one nanometer, which also equals
one millimicron. To obtain measurements in ergs/second/cm², multiply µW/cm² by 10.
GENERAL READINGS
If intensity at the surface (such as on a laboratory bench) is to be measured, the sensor
is placed directly on the surface and connected to the meter housing by the extension
cord. Other measurements can be made at varying distances from the source. For
repeatability of readings, the distance should be noted and the sensor placed at the
same distance for each reading. For making measurements at the source, the sensor
face has four pegs for indexing directly on the tube.
The shortwave meter, J-225, is designed for measuring energy from 220nm to 290nm
emitted by germicidal lamps (peak sensitivity is approximately 254nm). The longwave
meter, J-221, measures energy from 300nm to 400nm (peak sensitivity is approximately
365nm) and is basically designed for measuring "black light" lamps.
NOTE: When the sensor cell is positioned directly on the tube, it cannot measure the
ultraviolet from the entire lamp, only the small area it is on. Readings made by this
method are widely used for monitoring lamp condition over a period of time. It should be
noted that these readings are only relative to each other, not a measure of total lamp
energy, as a result of high loss of incident energy due to cosine reflection losses. The
meter cell should be exposed to the UV only long enough to obtain measurement since
extended exposure will degrade cell calibration.
TEMPERATURE/HUMIDITY EFFECTS
It is preferable to use the meters at normal room temperatures (60-90°F). A short
exposure to higher or lower temperatures may be tolerated although accuracy may be
diminished. Upon prolonged exposure to temperatures below the dewpoint, the
instrument may show increased sensitivity (maximum +7%) when immediately used to
make measurements at temperatures above the dewpoint. The cause of the increase in
sensitivity is the condensation of water vapor on the detector filter surface. When the
detector has reached a temperature above the dewpoint and surface condensation has
dissipated, the above specifications will apply.
INFRARED INTERFERENCE
Although the longwave and shortwave sensor cells have their peak sensitivities at
365nm and 254nm, respectively, they do have a low level response to infrared with little
interference or sensitivity to visible light. Since most ultraviolet sources are filtered, the
infrared emission is decreased and the amount of infrared seen by the sensor is
generally less than 5%, well within the accuracy of the meter reading. For instance, the
unfiltered Model B-100A 100 watt mercury vapor BLAK-RAY Lamp has an average
infrared intensity of 10% of the total reading. The value decreases to an average infrared
intensity of 2.6% of the total reading when the filtered longwave emission is viewed.
For greatest accuracy, the cells should not be used when visible light at the point of
measurement is 40 foot candles or more, or when the equivalent of a 100 watt
incandescent source is within 15 feet. Many high pressure mercury arcs have a high
percentage of infrared emission, as does the sun. For example, the infrared of a G.E.
RS-275 Suntan Lamp typically represents an average of 21.5% of the total longwave UV
reading, as determined by the recommended contrast filter procedure described below.
When such lamps are to be metered, sun ultraviolet studies made, or when ambient
infrared is suspected, the following procedure will provide more accurate ultraviolet
readings.
Accurate ultraviolet measurements with no infrared interference can be achieved by
using the included Clear Snap-On Contrast Filter. Make one reading without the filter,
which will include ultraviolet plus any infrared present. Make another reading by
snapping on the Contrast Filter over the sensor cell. As the contrast filter absorbs all
ultraviolet and the meter sees no visible light, the resultant reading will be infrared only.
When the second (infrared) reading multiplied by 1.06 (an insertion loss correction) is
subtracted from the first reading, the resultant figure will be ultraviolet energy only.
MEASURING HIGH INTENSITY LAMPS
Use the Reduction Screen to measure any high intensity ultraviolet lamp (i.e., a lamp
which drives the needle off the "B" scale). Readings made with the Reduction Screen
are only relative measurements, not in microwatts/cm², although the Reduction Screen
does multiply the reading by approximately 5 times. To install the Reduction Screen,
simply snap it on to the sensor cell.
TO MEASURE FILTERED SHORTWAVE TUBES AND FILTERS
Use the J-225 shortwave meter. Remove the lamp filter if present and take a reading
directly on the tube. This gives a relative measurement of ultraviolet output without the
condition of the filter affecting the reading. Remember to wear UV protective spectacles
when making this measurement. As shortwave filters deteriorate with use, it is often
desirable to know filter condition. The J-225 meter readily determines transmission. The
best technique is to note the transmission of the new filter held between a shortwave
source and the meter. Subsequent readings on the filter will give the percent degradation
by simple calculation.
The meter sensor cell is subject to the same type of deterioration described above. For
this reason, the meter sensor should be exposed to shortwave ultraviolet light only for
the minimum time required to make the reading. This is particularly important at high
intensities (i.e., those beyond the range of the "A" scale).
MEASURING DETERIORATION OF GERMICIDAL TUBES
The sensor cell should be placed directly on the tube, using the perforated metal
reduction screen clipped on to the sensor. Fit the four pegs on the sensor face over the
tube for proper positioning. This assures the same placement of the sensor each time for
reliable comparative readings. Take a reading approximately four inches from each end
of the tube, and another in the middle of the tube. The average of these three readings
should be used. The reduction screen should be used for all germicidal measurements,
even with low intensity lamps. Remember to wear UV protective eyewear and skin wear
when making this measurement.
CORRECTION FACTORS FOR J-221 METER
A meter measures the density of radiant flux incident on a surface. "Radiant incidence
(irradiance)" is measured in units of power (radiant flux) per unit area, i.e., watts/cm².
Meter measurements yield a number; however, this number will be valid only for a
particular type light source and wavelength range, depending on the design and
calibration of the meter being used.