Manual

3
Ashcroft Pressure Transmitters, Series 4000 (refer to Fig. 1),
a
re self-nulling, motion balance instruments. They utilize a
non-bleed force balance, pneumatic relay to convert input
pressure into proportional low air pressure, for transmission
t
o remote indicators or controllers.
T
he Series 4000 operates independently of electrical power
and is not subject to interference from nearby power lines.
As shown in Figures 2 and 3, the transmitter consists of the
following major components:
a. A pressure-sensitive input element to
translate applied pressure into linear
motion.
b. A vane assembly, mounted on the free end
of the input element, to provide a positional
reference which moves vertically to follow
the change in input pressure.
c. A nozzle assembly, which together with the
vane assembly establishes the null condi-
tion, accurately balancing output pressure
against input pressure. This assembly
comprises a supply nozzle and a collector
PRESSURE SENSITIVE
B
OURDON TUBE
VANE
ASSEMBLY
Figure 2. Non-Indicating Pressure Transmitter,
Front View with Cover Removed
nozzle; the supply nozzle sends a jet of air
a
cross a gap to the collector nozzle, to
sense the motion of the lower edge of the
vane assembly. The nozzles are mounted
o
n a temperature-compensated flexure
leaf, which follows the lower edge of the
vane assembly until the null condition is
e
stablished.
d
. A pneumatic relay, which senses pressure
at the collector nozzle and controls the
low-pressure air, to establish output pres-
sure directly proportional to the pressure
applied to the input element.
e. A feedback bellows, actuated by output
pressure, to position the nozzle assembly
so that It follows the lower edge of the
vane assembly, maintaining a balanced
pneumatic circuit.
f. In indicating transmitters, a standard
Ashcroft movement, linked to the input ele-
ment, to drive the pointer of a gauge
mounted on the face of the instrument.
g. An air actuated dash pot is provided on all
pressure transmitters of 100 psig range or
below.
GENERAL DESCRIPTION
NOZZLE
ASSEMBLY
SUPPRESSION
SPRING
FLEXURE
LEAF
ADJUSTING
DISC
FEEDBACK
BELLOWS
PNEUMATIC
RELAY
MOUNTING
BRACKET
Figure 3. Short Span Pressure Transmitter,
Rear View with Cover Removed
I&M008-10061-1-01 250-1969-C_I&M008-10061-1/01 250-1969-C 12/20/13 9:09 AM Page 3