Fluke RUSKA 7252, User Manual

Page: 30 / 128

RUSKA 7252
Users Manual
2-10
Transducer Module
Quartz Bourdon Tube Sensor (Transducer01)
The quartz Bourdon tube sensor is mounted in a machined aluminum housing. The sensor
consists of a helical quartz tube with a mirror affixed to one end, as shown in Figure 2-5.
A rigid beam is attached transverse to the axis of the helical tube. Attached to both ends
of this beam are electromagnetic coils. Mounted beneath the coils are permanent magnets.
A lamp assembly directs light through a quartz or sapphire window onto the mirror
affixed to the helical tube, as shown in Figure 2-6. The mirror reflects the light back
through the window and strikes two matching photodiodes. When there is zero pressure
differential across the helical tube, the photodiode assembly is mechanically adjusted so
that the light spot is centered between each photocell. In this “zero position,” the outputs
of the two photodiodes provide energy used to maintain the quartz assembly in its zero
position; thus, a force balance is created.
As pressure is applied in the helical tube, the entire apparatus attempts to rotate. This
causes the mirror to move the reflected light spot to shine more on one photodiode than
the other. The Sensor Board (see the next section in this Chapter, Sensor Board) then
responds by changing the current to the electromagnetic coils that, through interaction
with the permanent magnets, force the helical tube to return to its zero position. The
amount of current required to do this is proportional to the pressure applied across the
helical tube. Thus, the pressure is determined by the amount of current required to return
the helical tube to its zero position.
A
BS
OLUT
E
M
O DE
L
On Absolute Models, the process is similar, except that the Bourdon tube is
permanently evacuated to less than 0.1 mtorr and sealed at the factory, and the test
pressure is applied to the sensor case. With this configuration, all test pressures are
measured with respect to vacuum
FUSED QUARTZ
BOURDON TUBE
AXIS OF ROTATION
MIRROR
TORSION
HINGE
FLEXURE
PERMANENT
MAGNET
ELECTROMAGNETIC
COIL
glb07.eps
Figure 2-5. Shaft/Magnet Section