Ionics Instruments 2004 Page 3-7 DLM 30007-08 Rev. A
DI Water Loop
Deionized water for both CO
2
sensors is continuously generated using a
circulating pump and a mixed-bed ion exchange resin. When used for the
monitoring of low salt samples containing < 10 ppm total carbon, the lifetime of
the resin bed is approximately 5 years. The level of the DI water in the
reservoir (~150 mL) can be viewed through a window and refilled as needed.
The conductivity of the DI water is determined by the Conductivity and
Temperature Measurement Cell, which monitors the ion exchange resin.
When the resin is depleted, the entire DI water loop and pump assembly is
removed and the resin is replaced.
Membrane Module
The membrane module consists of a gas-permeable membrane that separates
the spiral flow channel containing the sample stream from a matching channel
containing deionized water. The membrane is permeable to CO
2
(and other
gases) but impermeable to ions and organic compounds. The sample stream
flows continuously through the membrane module, while a solenoid valve
controls the flow of DI water through the membrane module.
Conductivity and Temperature Measurement Cell
The conductivity and temperature measurement cell is located at the outlet of
the deionized water channel from the membrane module. A thermistor
measures the temperature of the DI water stream as it leaves the conductivity
cell. Conductivity is measured using a bipolar-pulsed technique and the
conductivity electronics are calibrated by measuring a precision resistor prior
to each analysis cycle. The conductivity sensors and thermistors have NIST-
traceable calibrations.
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