II. GENERAL DESCRIPTION
The SEC Signature DIR Process monitor is a microprocessor based intelligent device that continuously and
simultaneously monitors levels of EtO and H2O in sterilization chambers.
The SEC Signature DIR Process monitor is a stand-alone device providing a dedicated 4-20 mA signal
corresponding to actual concentrations of ethylene oxide and a dedicated 4-20mA signal corresponding to the
actual concentration of H2O.
Features
Programmable Heater operates in Relative or Closed Loop Mode.
Continuous self-test automatically indicates a fault, with fail to safe operation.
A multi-layered filtering system protects optics from dirt and water ingress.
Straight optical path eliminates the need for mirrors, reflective surfaces or beam splitters thereby increasing
the stability and reducing the maintenance of the device.
Discourages condensation interference by internally heating optical chamber.
Standard 4 to 20 mA outputs (current sources).
Explosion proof housing designed for duty in harsh environments.
Smart Calibration AutoACcircuit.
Theory of Operation
The SEC Signature DIR Process monitor uses infrared absorption technology for detecting target gas
concentrations. Gases absorb infrared light only at certain wavelengths. The concentration of a gas can be
measured by the difference of two channels (wavelengths), a reference and a measurement channel. The SEC
Signature DIR Process monitor uses an infrared light source at one end and a dual channel receiver at the
opposite end. The dual channel receiver measures the intensity of two specific wavelengths, one at an
absorption wavelength and another outside of the absorption wavelength. The gas concentration is determined
by a comparison of these two values.
The multi channel receivers incorporate single wafer double filter technology with internal optical barriers. The
elements are perfectly matched resulting in overall stability and superior performance throughout the entire
temperature range. By using this multi channel receiver there is no need to use any special lenses or beam
splitters to achieve the dual measurement bands.
The SEC Signature DIR Process monitor utilizes a unique Automatic Analog Control circuit, the AutoACcircuit
(Patent Pending). The AutoACcontinuously makes adjustments to null out effects from temperature,
component drift, dirty optics, interferents and aging. These adjustments are made according to algorithms
derived from infrared gas absorption theory. The AutoACcircuit is continuously checking all unit operating
parameters. If any parameter goes out of tolerance, the AutoACcircuit sets a fault code appropriate to the
problem. The AutoACcircuit ensures that once the unit is spanned, it will remain accurate as long as the zero
is accurate. Simply calibrate (span one time) the unit with a specific amount of gas and the device will
repeatable track the gas concentrations in the chamber throughout the entire load cycle. The only ongoing
calibration that is necessary is an occasional zero.
The optical absorption efficiency of water vapor is a function of the chamber pressure. Higher chamber
pressures cause more absorption resulting in higher signal levels.
The H2O channel is designed to be accurate with pressure rise due to the vapor pressure of the H2O in the
chamber.
Pressure rise due to N2 EtO injections will cause higher H2O readings. Therefore, the H2O signal will be
accurate as long as the chamber contains only water vapor.
