Casella CEL CEL-278 - Manual (Page 20)

When a set of batteries has been loaded, check their power output by sliding
the RANGE switch to "On" to ensure that LOW BAT is not indicated.
The instrument is now ready for service and may be coupled or connected to
the sound level meter.
In accordance with accepted measurement practice, it is recommended that
the acoustic calibration of the complete system be checked before and after
each series of readings. The procedure for each Sound Level Meter is
described in the relevant Sound Level Meter handbook. It requires the use of
a CEL-284/2 Acoustic Calibrator (Class 1), a CEL-282 Acoustic Calibrator
(Class 2), the earlier CEL-177 Precision Acoustic Calibrator (Class 1), or an
RFT 05001 Pistonphone.
Page 20 CEL-278/2 Handbook CEL Instruments Ltd