BRUEL & KJAER 4220 User Manual

must be used only for short periods of time. (See also the Note p. 17). The
lower limit depends on the leakage time constant in the coupler cavity and
is about 20 Hz when no special care is taken to ensure the tightness (down
to 2 Hz or even less with a simple film of grease). In position "Check" the
stability of the rotating speed, i.e. of the frequency, is directly related to the
stability of the voltage between the motor terminals, and especially at low
frequencies the DC source should have a low internal impedance (less than
2 ohms). Between the given limits of the frequency, the S.P.L. produced by
the Piston phone is constant and independent of the frequency.*)
Batteries.
The Pistonphone is supplied with long-life mercury cell batteries (Mallory
type RM - 3 (R)) upon delivery. The battery compartment holds 7 cells giving
a
total voltage of approximately 8 volts. Though the cost of mercury cell
batteries is higher than the cost of ordinary ones, the adoption of this type
of battery is the cheapest in the long run. The reason is that in normal use,
such as laboratory standard, the Pistonphone is utilized only a few minutes
per day, and perhaps even less. Under these conditions, the batteries' life is
limited by their storage life. Therefore the use of mercury cells, which
feature a long storage life, is recommended. According to the manufacturer,
the batteries RM- 3(R), supplied with the Pistonphone can be, stored for more
than 6 years. This duration corresponds to a utilization of the pistonphone
of one or two minutes per day.
a.
Fig. 9. Sketch of the battery compartment
(a) with Mallory RM-9(R) cells
(b) with Mallory RM-4(R) cells
Under circumstances where extended use is made of the Pistonphone, larger
mercury cell batteries may be advantageous. These are M~llory type RM -
4-(R) with a diameter of 30 mm (the diameter of the RM- 3(R) units is 25
*) The noise from the motor has a resonance peak at high frequency (7-8 kHz), but is
always more than 40 dB below the signal. However, when running the motor at its
highest speed (i.e. 800 Hz) the noise resonance peak can produce a small rise of about
o.2 dB of the total S.P.L. in the coupler cavity. ·
12

BRUEL & KJAER 4220 Specifications

General

Brand	BRUEL & KJAER
Model	4220
Category	Measuring Instruments
Language	English