|
Mechanical Pressure Elements - Diaphragm Sensor |
Diaphragm Definition
1 a dome-shaped muscular partition separating the thorax from the abdomen in mammals.
2 a thin sheet of material forming a partition. a taut flexible membrane in mechanical or acoustic systems.
3 a thin contraceptive cap fitting over the cervix.
4 a device for varying the effective aperture of the lens in a camera or other optical system
Metallic diaphragms also are used as the elastic chamber
in low-pressure gauges. A diaphragm plate is formed from thin sheet
metal into a shallow cup having concentric corrugations. To make an
element with a low spring rate that generates substantial deflection
from a small change in pressure, two plates can be soft soldered,
brazed, or welded at their periphery to form a capsule, and additional
capsules can be joined at their centers to form a stack, Figure 5.
Generally, the measured pressure is applied to the interior of the
element and no supplemental coil springs are used. A 2-in. diameter
capsule (two plates) will provide about 0.060 in. of motion without
exceeding the elastic limit of the material. This is usually enough to
operate a high-ratio multiplying movement because diaphragm deflection
can transmit high force.
Diaphragm elements often are used in gauges to indicate absolute
pressure. In this form, the diaphragm element is evacuated. sealed, and
mounted within a closed chamber. The pressure to be measured is admitted
to the closed chamber and surrounds the diaphragm element. Changes in
the measured pressure cause the element to deflect, but because
atmospheric pressure is excluded and has no effect on the indication,
the gauge may be calibrated in terms of absolute pressure. If the
applied pressure is atmospheric pressure, the gauge is known as a
barometer.
Diaphragm elements also may be used in an opposing arrangement. By
evacuating one side of the assembly, the gauge can indicate absolute
pressure. If a pressure is applied to one side of the assembly, and a
second pressure is applied to the other side, then the differential
pressure will be indicated. The differential pressure is limited with
respect to the static pressure that can be applied. That is, the gauge
may be suitable to indicate between 10 psi and 12 psi, but not be
suitable to indicate between 100 psi and 102 psi. Also, the consequence
of inadvertently applying full pressure to one side of the element and
no pressure to the other side of the element must be considered.
Diaphragm elements are circular-shaped, convoluted
membranes. They are either clamped around the rim between two flanges or
welded and subject to the pressure of the media acting on one side. The
deflection caused in this way is used as a measurement for the pressure
and is indicated by a pointer. Compared with bourdon tubes, these
diaphragm elements have a relatively high activating force and because
of the circular clamping of the element they are insensitive to
vibration. The diaphragm element can be subjected to higher overload
through load take-up points (by bringing the diaphragm element against
the upper flange).
Wide connection ports, open connection flanges and purging plugs can be
integrated for measuring highly viscous, impure or crystallizing media.
Pressure ranges are between 0...16 mbar and 0...40 bar in accuracy class
0.6 to 2.5.
Used to detect slight changes in pressure
Diaphragm Sensor is Axially flexible, thin, flat and corrugated disk held in place
Increase pressure on one side causes the other side to deflect
Force opposing the pressure is the sum of:
-
Spring constant of the diaphragm
-
Pressure on the opposite side of the diaphragm
-
Spring constant of opposing spring
Axial movement can measure liquid level, rotate a pointer or activate a controller by attaching the free end to linkage.
There two types of elements:
-
Elastic element
-
Limp element
Elastic
Element - Usually metallic
Limp
Element - Metallic or non-metallic
Able to
measure VERY LOW pressure, Common to measure absolute/gauge pressure
down to 0 – 0.2 inH2O, Does not provide much movement which limits their
use in gauges and controllers
| 
