BRUEL & KJAER 4220 - Using External DC Power Supply

Millibars
Millimeters
of mercury
Inches of
mercury
Kgfcm
1
Fig. 12.
1 millimeter of mercury- 1.3332 millibar-0 . 0394 inches of mercury• 1Torr.• 0. 001359 kg/em~
1a
1
tm .
1000 90 80 70 60 50 40 30 20 10 900 90 80 70 60 50 40 30 20 10 800 m.bar
740 730
720 710 700
690 680
670
660 650 640
630 620 610
600 mmH't
29.5 29.0
245
28.0
27.5
27 .0
2~5 26.0
25.5 25.0
u.s
24.0
In Hg
I I I
I
kgjem
2
1.05
0.
95
0.9
0.85
160H f
Chart for conversion
of barometer readings
in different units.
Use of Extemal DC Power Supply.
The Pistonphone is normally used at a fixed frequency. However, in certain
instances, use can be made of the fact that the S.P.L. is independent of the
frequency, to check the response of a sound measuring system.
The frequency of the Pistonphone can be varied within the range 30 Hz-
800 Hz by applying an external DC voltage varying from 1.5 to 24 volts
approximately. The necessary connections should be made to the rings which
are seen when the battery compartm e nt is unscrewed. (The - is connected to
the outer ring, and the + to the center ring , marked " BATT+"). The Piston-
phone switch is set in "Check" position . When approaching 800 Hz (at
800 Hz the motor runs at 12ooo r.p.m.), the Pistonphone must only be used
for short periods of time, and as it could be dangerous for the motor to
exceed the maximum speed, the voltage corresponding to the maximum
frequency should be progressively attained, for example by means of a
potentiometer as shown in Fig. 13 . At the lowest frequencies, a round 20 Hz ,
it is necessary to have a DC source with low internal resistance (less than
2 ohms) in order to ensure stability of the rotating speed of the motor.
+
24V=
160583
Fig. 13. Connection of external DC power supply.
An easy, but rather rough way of obtaining low frequencies, consists of
rapidly moving the switch back and forth between the "Off" and "Measure"
positions. When calibrating a sound level meter equipped with an indicating
instrument featuring a convenient dynamic characteristic, a quick check of
the low frequency response of the sound level meter can be obtained in this
way by observing the deflection on the instrument, and an abnormal loss
of sensitivity at the low frequencies can rapidly be detected. Inversely, an
16