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No. 1
Unstable Photometer
Occasionally the panel display on a photometer
may become erratic for no apparent reason when there is no
sampling and the photometer is in the clear position. This
could be an anomaly caused by electro-magnetic interference
(EMI), radio frequency interference (RFI), or a combination
of both.
EMI is caused by electrical and electronic
devices that are operated by line voltage or batteries. RFI
is caused by turning electrical and electronic appliances
on and off. It can also be caused by particular circuits in
certain appliances or electrical devices. For example, EMI
is commonly experienced in an office when a printer is positioned
too close to the computer monitor. The EMI radiating from
the printer affects the video scanning electron beam that
is viewed on the monitor. This causes the images on the monitor
to behave erratically, appearing to be gyrating and wriggling.
The problem can be solved by increasing the distance between
the printer and monitor.
Likewise, EMI and RFI can cause the panel
display of a photometer to behave erratically. Most photometers
contain a photomultiplier tube and the photomultiplier tube
is very sensitive to EMI and RFI. The best method of reducing
EMI and RFI interference in a photomultiplier tube is to shield
it by covering it with a metal tube containing a small hole.
This technique is employed in ATI photometers.
Sometimes an anomaly caused by EMI or RFI
will go away on its own. If it does not, one of the easiest
methods to reduce EMI or RFI in a photometer is to move the
photometer a few feet in one direction or another. Often,
this will reduce the pulsations to a point where they are
insignificant. Another method is to plug the unit into a different
electrical outlet because some of the problem may be interference
through the power line.
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