Drawing Number: 21067
Revision: A
PIEZOELECTRIC CHARGE MODE PRESSURE SENSOR GENERAL OPERATION MANUAL
4
3.2
Recess Mount
A recess mount 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 effects. See Figure 3.1.
When using a recess mount, note that the length of the
passage may limit the frequency range of the measure-
ment. The effect the passage has is similar to that of an
under-damped second order system, with the resonant
frequency determined by passage length. The passage
length thus limits 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).
Selected Values for 77°F (25°C)
Measured resonant frequencies may differ slightly from
the chart values due to variations in the velocity of
sound in the air from changes in temperature and
pressure of the air in the passage.
To ensure diaphragm integrity, maintain .006 inches
(.153 mm) of clearance ahead of the diaphragm using
seal sleeve Model 065A05, shown in Figure 3.1.
3.3
Flush Mount
In a flush mount installation, there is no reduced area
passage from the sensor diaphragm to the test chamber.
Instead, the sensor diaphragm is mounted flush with, or
slightly recessed from, the inside surface of the test
chamber. See Figure 3.2.
Passage
Passage
Approx. fastest
length
resonance
pulse rise time
(inches)
(kHz)
(microseconds)
.050
66
5
.100
33
10
.200
16.5
20
.50
6.6
50
1.0
3.3
100