Fluke RUSKA 7252, User Manual

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Theory of Operation
Transducer Module
2
2-11
PHOTOCELLS
LEFTHAND RIGHTHAND
MIRROR
BEAM OF LIGHT
REFLECTED FROM
ROTATING MIRROR
SHIFTING SPOT OF
LIGHT, SHOWN AT
NULL POSITIO
BETWEEN PHOTOCELLS
PHOTOCELLS GENERATE VOLTAGE
ACCORDING TO ILLUMINATION
POTENTIOMETER FOR ZERO-ADJUST
CALIBRATION OF THE QBT GAGE
ERROR SIGNAL TO CONTROL AMPLIFIER IS
ZERO WHEN PHOTOCELLS ARE EQUALLY
ILLUMINATED
ROTATING AXIS
glb08.eps
Figure 2-6. Photocell/Light Spot
Sensor Board
A temperature sensor and the quartz Bourdon tube pressure sensor are monitored by the
Sensor Board. The Sensor board is also used to control and maintain the sensor housing
at 50 °C.
Linearization Term
As described in the previous section, the relationship between the pressure being
measured and the current required to keep the quartz Bourdon tube in its zero position is
the main principle behind the operation of the Calibrator’s sensing element.
Ideally this pressure-current relationship would be a linear equation of the form
, kP l
=
where I is current, k is a constant of proportionality, and P is pressure. However, due to
certain mechanical characteristics of the helical tube and its supporting structure, this
pressure-current relationship is slightly nonlinear. The nonlinear portion of this
pressure-current relationship closely follows the form of a second order polynomial, or
,
2
c bP aP l
+ + =
where again
P
is pressure and
a
,
b
, and
c
are coefficients generated during the
calibration procedure as discussed below.
When the user performs a three-point calibration, the Calibrator’s software creates the
three coefficients based on the user’s zero, mid-point, and full-scale adjustments. From
then on, the nonlinear term given above is subtracted from the total pressure-current
curve to achieve the desired linear pressure-current relationship.
On the RUSKA 7252 and RUSKA 7252i, multiple quartz sensor ranges are used in order
to provide the percent of reading specification. In these instruments, a total of three points
per range with one point shared between two ranges is required. Therefore, for the
RUSKA 7252, a 5 point calibration is performed and for the RUSKA 7252i, a 9 point
calibration is performed to fully characterize the RUSKA quartz sensors.
Auxiliary Sensors
Auxiliary sensors are sensors such as the oven temperature sensor. These are reference
sensors aligned at the factory and are utilized by the firmware.