CIRAS-2 User Recalibration of CO2 and H2O


Courtesy of PP Systems

The CIRAS-2 CO2/H2O analyzer is an inherently stable instrument and does NOT require frequent calibration. The AutoZero function corrects for nearly all changes that result in calibration drifts. The most common causes of drifts in CIRAS-2 readings are i) incorrect Zeros due to exhausted chemicals or ii) leaks on the desiccant columns from rolled or damaged o-rings and iii) incorrect Diff Bal, applied while reference CO2 (Cr) is changing or unstable.

If needed, recalibration can be accomplished most easily by using the internal CO2 cartridge as the calibration gas source, or a laboratory standard CO2 gas may be used if desired. For H2O recalibration the PP Systems water vapor calibrator is provided with each new system, or another standard humidity source may be used. We recommend use of the water vapor calibrator because the FeSO4 salts have long-term stability and the calibrator requires no humidity or flow setting adjustments.

Our factory calibration ranges of 0-2000 ppm CO2 and 0-50 mb water vapor are ideally suited for most typical applications. The system can also be calibrated to specific upper limits within these ranges, for example, be sure to use a certified gas standard with a concentration slightly greater than the upper limit of expected measurements, e.g. 1700-2000 ppm for A/Ci curves.

Allow the system to warm up for at least 30 minutes. The CO2 cylinder should be fresh and the chemicals must be new or recent. Pay special attention to molecular sieve; be sure that it is recent. If the molecular sieve in its original container is older than 1 year consider replacing it – reopening and closing the container repeatedly exposes the chemical to air, and it will eventually lose its efficacy.

Run the Calibration Check function from the Utilities menu to determine if the system requires recalibration (see PP Systems Application Note AN2011-02 for instructions).

CO2 Calibration – internal source gas

  1. Attach 3-way tubing with AIR OUT and REF IN gas ports sharing a single “T” or “Y” fitting and AIR IN as the third leg. The open leg of the tubing vents to atmosphere and should be about 30 cm in length. Use low-absorption tubing material, such as butyl rubber (See Fig.1 in Application Note AN2011-02).
  2. Go to Utilities > Calibration in the CIRAS-2 main menu, then select ‘CO2’ and check the ‘Internal Calibration’ box (see below).
  3. Click OK, system will perform Zero and return to measurement mode with the solenoid valves set to Diff-Bal (reference air sample passes through both the reference and analysis cells of the analyzer).
  4. Allow system to stabilize for 2-3 minutes, noting ‘Expected’ ppm and difference of Cr (blue plot), Ca (red plot) status values ppm.
  5. Once the readings remain stable click the Steady button.
  6. Upon completion of calculating the calibration (some 20 seconds), CIRAS-2 sends the new calibration constants back to the firmware.

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