ETS-Lindgren HI-2200 RF Survey Meter - Manual (Page 39)

Application | 39
Second, when averaging electromagnetic field measurement data, many safety
standards allow for a process called time-averaging or temporal-averaging. This
process allows for the effect of field intensity exposure variations on the thermal
mass or thermal lag time of the human body. When the EMF exposure varies in a
sufficiently predictable manner, it may be possible to incorporate time averaging
into the exposure evaluation. When time averaging is applied, the field exposure
values are averaged over a moving window in time. The window width is
generally specified by the standard. At typical RF frequencies, this value is often
six minutes.
Again, for most RF exposure evaluations, RMS averaging is recommended. The
six-minute average value is the RMS average of the field intensity values
measured during the previous six minutes (or whatever averaging window time is
selected). With the HI-2200, averaging times of three, six, twelve, and 30 minutes
can be selected.
It is important to note that when using interval or moving averaging, a true
time-average is not displayed until readings have been averaged over at least
one interval (six minutes in our example). Moving averaging assumes that, prior
to beginning the averaging process, the exposure has been negligible (zero) and
that for the first averaging interval, a portion of the averaged data will be zero.
This feature is helpful in that, if the exposure is excessive during the
measurement, it is possible to exceed the full six-minute average in less than
six minutes. For example, if the fields were four times the exposure limit, the
six-minute time averages would be exceeded in approximately 90 seconds.
The moving six-minute average function of the HI-2200 provides this information
to the user in a timely manner. The six-minute average data in combination with
the alarm function can be used in this situation. To alarm on the time-averaged
value, this value must be the primary display value. Take care that personnel are
not exposed to levels that exceed recommended limits.
For Spatial Averaging and similar applications, a Running Average can be
selected. This is the familiar means in which all readings are averaged beginning
from the start of the averaging process and continuing indefinitely until the
averaging function is reset (press the BACK key three times to reset the average
and MAX values).