VICI 235 Dynacalibrator - Manual (Page 26)

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Advanced Theory of Operation
Dynacalibrator operation is based on the principle of mixing a known mass flow
of gas permeation device with a metered stream of clean carrier/dilution gas to
generate a precise concentration of span gas.
User-selectable calibration gases originate from Dynacal permeation devices
which are maintained at a user-selectable constant temperature within the Dyna-
calibrator’s permeation chamber. Permeation rates can be precisely determined
gravimetrically as a function of temperature.
An internal flow regulating system ensures a stable gas flow over extended
periods of time for both the carrier air and dilution air streams. The carrier stream
continuously sweeps the permeation gas from the permeation chamber. This
stream then mixes with the dilution stream to provide the final calibration gas
supply.
Different span gas concentrations can be generated by changing one or a com-
bination of the following parameters: (1) the dilution stream flow rate, (2) the
permeation chamber temperature, or (3) the number and/or type of permeation
devices. The fastest, simplest, and most common method is to change only the
dilution flow rate.
The carrier flow is set at the factory, and is controlled by a thermal mass flow
meter. The fixed flow is typically 100 sccm.
The dilution flow is user-adjustable, controlled by a thermal mass flow meter
providing a range of adjustment from approximately 200 cc/min to over
20 liters/min, depending on the capacity option selected. Model 235 Dynacali-
brators contain a single dilution mass flow controller with user specified range
up to 20 SLPM. Since the Model 235 can provide a wide dynamic range of span
gas calibrations simply by changing the flow rate of the dilution gas, it is consid-
ered to be a multi-point calibrator of exceptional range, accuracy, and precision.
Each Dynacalibrator model contains basic pneumatic plumbing, with several
configuration options. The plumbing configuration selected (see the simple
diagram below) processes the input gas supply into a carrier stream and a dilution
stream. The carrier stream picks up the calibrated trace gas and is subsequently
mixed with the dilution stream for the desired output concentration.
PLUMBING
CONFIGURATION
PERMEATION
CHAMBER AND
OUTPUT PLUMBING
SUPPLY
INLET
SPAN/STREAM
OUTLET
CARRIER STREAM
DILUTION STREAM
Figure 4: Basic block diagram