BALLISTICS PRESSURE TRANSDUCER MODELS 165B02 AND 118A07 OPERATION MANUAL
2
Before installing the transducer in previously used
mounting ports, clean out the residue remaining from
previous tests as shown in Figure 3.1 . This is
accomplished by hand-reaming the ¼-inch diameter
hole, using a PCB Model 040B20 (English Port,
Drawing 42831) or 040B21 (Metric Port, Drawing
42832) Flat-Bottom Reamer or its equivalent.
It is important to note that the seal surface may require
remachining to get it sufficiently clean following
prolonged use. If this procedure is necessary, do not
forge too deeply into the 1/4-inch (6.35 mm) diameter
hole. Doing so could remove the shoulder, causing the
transducer to bottom out when it is installed.
If waveform distortion occurs during prolonged testing,
remove the transducer and clean out the residue as
shown on 42831, & 42832.
3.2
Preparing New Mounting Ports
Refer to drawings 42831, & 42832 provided for
instructions on mounting hole preparation. For best
results, do not deviate from the outlined steps.
Use good machining practice in preparing the mounting
port. Pay particular attention to the seal surface (.323
inches / 8.2 mm in diameter), keeping it free from tool
chatter marks.
NOTE: It is important that this surface be perfectly
smooth and free from nicks or other discontinuities that
could cause leaks at high pressures.
3.3
Recess Mount
The recess mount technique protects the sensor
diaphragm from the effect of high flash temperature and
particle impingement. This method is often selected
because it can prolong sensor life and increase data
integrity by reducing thermal error. See Figure 3.2.
The recommended range of passage diameters is .090 to
.125 inches (2.286 to 3.175 mm) in diameter.
When using the recess mount technique, note that the
length of the passage may limit the usable frequency
range of the sensor. The effect the passage has is like
that of an under-damped second order system, with the
resonant frequency determined by passage length. The
passage length thus limits the pressure pulse rise time
and may also cause passage ringing.
The function is described by the following equation:
L
4
V
F
r
=
Where: Fr =
resonant frequency passage (Hz)
V =
velocity of sound in air (feet/second)
L =
length of column (feet)
For air at room temperature, the equation becomes:
L
3300
F
r
=
Where: L = passage length (inches)
The natural frequency and approximately fastest
pressure step rise time for various length passages are
shown in the following chart, using a medium of air at
77°F (25°C).
Drawing Number: 21109
Revision: NR
