Ashcroft a4a Manuale di installazione e manutenzione - Pagina 4
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Ashcroft A4A Dial Gauge
2.0
PRODUCT DESCRIPTION – THEORY OF
OPERATION - CONSTRUCTION
The Ashcroft A4A is a mechanical pressure gauge. This is a
device that measures pressure via the movement of an elas-
tic element which is subjected to the pressure to be mea-
sured. This element is "powered" only by the pressure itself. It
requires no external power source to perform its function.
2.1
THEORY OF OPERATION
The Ashcroft A4A precision pressure gauge consists of a
Bourdon tube elastic element, a movement, case, dial and
pointer. The accuracy of a pressure gauge is contingent on
the performance capabilities of the sensing element, move-
ment and the accuracy of the scale graduations which are
imprinted on the dial.
A mechanical pressure gauge measures pressure by con-
verting the movement of the elastic element into the move-
ment of a pointer across a dial.
The Ashcroft A4A precision pressure gauge incorporates a
highly engineered Bourdon tube as a pressure sensing ele-
ment. A Bourdon tube is a piece of tubing that has been
bent into a circular shape. Depending on the pressure
range the Ashcroft A4A gauge may include a single or
multi-coil Bourdon tube.
During the measurement process pressure is introduced to
the inside of the Bourdon tube via the externally accessible
process connection. When pressure is applied to the Bour-
don tube it straightens out a small amount (vacuum actually
pulls the free end of the coil in tighter). Since the process
connection is locked into position by the mounting of the
Bourdon tube into the gauge case, the movement caused
by the application of pressure or vacuum is limited to the
loose end or the tip of the tube. This tube movement is
commonly called deflection or tip travel.
The tip travel is converted to a pressure measurement via a
system of gears that drive a pointer. The gear system is
commonly referred to as the gauge movement.
The pointer moves with the application of pressure or vacu-
um over a dial imprinted with information including pressure
range, engineering units, graduations and other pertinent
information.
The main differences between an Ashcroft A4A gauge and
other "precision pressure gauges" can be found in the
Bourdon tube and the quality of the other components used
in the gauge. The process of making a Bourdon tube for the
A4A gauge takes 15 days to complete and produces the
most highly developed pressure sensing element of its
type. In the A4A gauge this tube is combined with a move-
ment which incorporates precision brass gears and minia-
ture stainless steel ball bearings to provide the most accu-
rate conversion of tip travel to pointer movement possible.
These components are all mounted in a heavy duty cast
aluminum case which provides a rigid base. As a result of
the rigid mounting platform the A4A is immune to inaccura-
cy caused by pipe strain and case flexing. In many other
gauges these are common problems often caused by
stresses generated during the installation process.
2.2
TYPES OF ASHCROFT A4A PRECISION
PRESSURE GAUGES
The Ashcroft A4A is available in four standard dial sizes, 6"
8-1/2", 12" and 16"and pressure ranges from 15 through
100,000 psi. Dials are provided with the required divisions to
meet specification and may be graduated to read in standard
English or metric units of pressure. Special dials in other
pressure related engineering units or with dual scales to
eliminate need for conversion, are available upon application.
The Ashcroft A4A is available to measure gauge, absolute
and compound pressures as well as vacuum. Since the
absolute A4A gauge does not have a hard vacuum refer-
ence it must be set to the prevailing barometric pressure by
the operator. A brief description of the available pressure
types follows:
Gauge Pressure
Measures and indicates pressure
using ambient pressure as a ref-
erence point. Provides the pres-
sure measurement as a differen-
tial between the pressure input
and the prevailing barometric
pressure.
Vacuum Gauge
Measures and indicates pressure
below ambient, using ambient
pressure as the reference point.
All readings taken with a vacuum
gauge are negative. The largest
vacuum level achievable at any
given time is the negative equiva-
lent of the prevailing barometric
pressure at the start of the test
cycle.
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