HAMAR LASER STEALTH Series, General Training Manual

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77
Reducing Atmospheric Effects with Fans
The most practical way of eliminating vertical temperature gradient and substantially reducing the
fluctuations caused by turbulence is to place a fan behind the target with the air blowing toward the laser.
When fans are used in conjunction with the “slow response” position on the readout switch, the effects of
turbulence can be reduced up to a factor of 10 and vertical temperature gradient can be virtually
eliminated.
Note: It is extremely important to use both the fans and electronic damping when maximum system accuracy is
required.
When working to an accuracy of .001 in. in 20 ft. in the summer, the use of fans and damping is
unnecessary. Their use would be necessary, however, if an accuracy level of .0001 in. is required. In the
winter, fans are necessary at all times.
Recommended Fans
Two types of fans are
recommended. The least
expensive and most desirable is
the ordinary box window fan
available at department or
hardware stores. These fans blow
a cylinder of air, rather than a
cone of air that is produced by
more expensive fans and
provide the maximum mixing
of air. A single fan of this type
can be used to a distance of
about 25 ft. If greater distances
are involved, several fans must
be used and positioned as
shown in Figure 31.
The use of fans will always improve the accuracy of readings except when the wrong type of fan is used
or there is an insufficient number of fans. If turbulence proves to be worse with the use of a fan, the more
fans are necessary.
The second type of fan that is particularly useful is the large, propeller-type shop fan. These fans are
usually effective to 50 ft. or more, but they are not as portable as window fans. Oscillating fans with
rounded blades and office fans do not work well.
Electronic Damping
Electronic damping involves adjustment of the selector switch on the front of the readout. Normally, all
adjustments are performed with this switch set at “fast response.” When switched to “slow response,”
additional damping, or electronic averaging, is switched in and substantially reduces the magnitude of
fluctuations from air turbulence. When first switched to the “slow response” setting, the meter readings
will likely change, but the original values will return in 10 to 20 seconds. This change occurs because of
the residual charge existing in the capacitors, which are switched into the circuit. Fluctuations may still
occur but will be greatly reduced. Take readings by observing the limits and splitting the difference. This
procedure works well for turbulence but has no effect on vertical temperature gradient. Do not take
readings until the numbers have returned to the original values.
Figure 31 - Using fans