T
HEORY
28
December 2000 Rosemount Analytical 748221-L
Model 7D Thermal Conductivity Analyzer
F
UNCTIONS
A
SSOCIATED WITH
AR2
AR2 Z
ERO
A
DJUSTMENT
P
OTENTIOMETER
R1
This screwdriver-adjustable, factory-set trimming potentiometer is used to eliminate
voltage offset within AR2. When the input signal is zero, R15 is adjusted so that the
output signal also is zero.
F
RONT
-P
ANEL
SPAN C
ONTROL
This potentiometer, connected across TB3-5 and TB3-6, provides continuously
variable adjustment of closed-loop gain for AR2, to permit establishing an upscale
calibration point on the meter scale or recorder chart. With upscale calibration gas
flowing through the analyzer, the SPAN Control is adjusted for the appropriate
reading.
F
RONT
-P
ANEL
M
ETER
The meter is connected from TB3-7 to TB3-8. Potentiometer R22 permits adjusting
meter sensitivity so that meter fullscale agrees with recorder fullscale.
O
UTPUT
S
ELECTION
S
WITCH
S1
The desired output is obtained by appropriate selection of switch contacts: 5V, 1V,
.1V, or .01V.
5.2.2 V
OLTAGE
O
UTPUT
L
INEARIZER
B
OARD
(O
PTIONAL
)
The output signal from the Master Board is a function of the degree of imbalance in
the bridge circuit, but is not linear with respect to the concentration of the measured
component. Providing that the nonlinearity of the calibration curve does not exceed
20% at midscale (for 50 % H
2
max.), the Voltage Output Linearizer Board (PN
633756) may be used to equip a given operating range for linear readout of
concentration on the meter and on a potentiometric recorder.
Straightening of the concentration-vs.-output curve is accomplished by sequential
adjustment of eight odd-numbered trimming potentiometers designated R19 through
R33. Each pot controls the gain of an associated operational amplifier.
During factory checkout of a linearizer circuit board, potentiometers R19 through R33
are initially set at midrange. With zero input signal applied to the linearizer circuit,
ZERO potentiometer R35 is adjusted for zero output. Then an appropriate low-level
signal is applied to the input, and R19 is adjusted for fullscale output. The procedure
is repeated as many times as required to obtain properly linearized output.
5.2.3 I
SOLATED
4
TO
20
M
A C
URRENT
O
UTPUT
B
OARD
(O
PTIONAL
)
This option provides isolated current output for applications which require 4 to 20 mA
into a maximum load of 1500 ohms. This output is
NOT
linearized. A description of
