VICI 500 Dynacalibrator - Manual (Page 28)


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Figure 5: Basic block diagram
Advanced Theory of Operation
Dynacalibrator operation is based on the principle of mixing a known mass flow
of gas derived from a 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
Dynacalibrator’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
as supply.
Different span gas concentrations can be generated by changing one or a
combination 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 flows at a fixed rate, maintained at a constant value by a sapphire
orifice plate and a differential pressure regulator. Orifice sizes are available for
rates of 80-376 sccm.
The dilution flow is user-adjustable from approximately 100 cc/min to over
20 liters/min, depending on the capacity option selected. Model 500 Dynacali-
brators contain a single dilution flow control valve and rotameter. Since the
Model 500 can provide a wide dynamic range of span gas calibrations simply
by changing the flow rate of the dilution gas, it is considered 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 dia-
gram 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.