Sensitivity Calibration - Michell Instruments Condumax II User Manual

OPERATION
As the detection principle of the Condumax II is essentially quantitative it can itself be
utilized to produce a graphical representation akin to the LGR relationship that can then
be used to judge the 'trip point' required for the specifi c gas under test. Refer to Section
3.3.3 on sensitivity calibration.
In applications at monitoring points where a specifi c analysis method is stipulated
in a contractual specifi cation, e.g. a manual visual optical dew-point measurement
instrument, then an alternative method is to select a 'trip point' value that corresponds
to the maximum dew-point found by repeated and careful measurement using the
stipulated method. In practice, the factory default setting for the 'trip point' of 275 mV
for analysis pressure at 27 barg should prove satisfactory if no detailed calibration is
available and will return a measured dew-point 0.5 to 1°C above the value obtained
from manual visual optical techniques applied by an experienced operator observing
best practices (ASTM, D1142). An analysis adjustment to 27 barg is the most common
measurement practice when monitoring transmission quality gas, in order to determine
HC dew-point at the cricondentherm condition (highest temperature at which a two-
phase equilibrium will exist on the phase envelope). Analysis at an alternative pressure
may require adjustment of the trip point to a revised setting, in order to maintain
a consistent sensitivity of analysis. If an alternative analysis is stipulated in the
measurement practices or contractual gas quality specifi cations relevant to a specifi c
application for Condumax II, please contact Michell Instruments for advice.
The processes and signal changes that occur during the formation of condensate on
the sensor surface can be monitored using Modbus RTU digital communications via PC
software. Contact Michell Instruments (see www.michell.com) for more information.
Care should be taken if checking the span of the signal change when using a rich binary
test gas, as a very rapid change in the signal will result in an optical surface wet-out.
3.3.3

Sensitivity Calibration

When a sensitivity calibration is initiated (Section 3.11), the system will check to ensure
the optical surface temperature is within 1°C of the optical surface temperature set-
point and the signal is 0.00%. Once these conditions are met, the system will decrease
the optical surface temperature at a controlled ramp rate until the signal has reached
100% of range (1500 mV), or after 10 minutes if 100% cannot be reached.
In order for the system to compute the required optical surface temperature ramp
rate, it cools the optical surface down to the Sensitivity Calibration Temperature that is
computed from the last hydrocarbon dew-point measurement. This can be changed for
diagnostic purposes by the user interface controls.
Once the sensitivity calibration is complete, a table of signal sensitivity vs. temperature
is displayed by the instrument or can be downloaded by the Modbus communications
to be plotted on a graph.
From the graph below, the signal trip point can be determined by cross-referencing the
known dew point of the gas to a sensitivity value. This sensitivity point should then be
entered into the instrument as the signal trip point, so that, during the Measurement
Phase, the instrument will record the temperature of the optical surface (HCdp) when
the signal reaches the signal trip point.
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Condumax II User's Manual
97081 Issue 28.1, July 2018