Calibration of your analyser is essential. It ensures that your meter’s accuracy is maintained and you’re still working to all applicable standards. Flue gas analyser and boiler testers generally measure oxygen, carbon dioxide, carbon monoxide and temperature but may also be fitted with sensors to measure additional parameters.
Every gas measuring instrument requires a method for calibration or verification and there are several means for calibrating a CO2 analyzer. The easiest way is a gas calibration: pure CO2 gas at a known pressure is injected on the sensor membrane. This is a perfect method if the unknown sample is a gas.
However, if the analyzed sample is a liquid, a slight deviation from the actual value can be noticed. In practice, the CO2 measurements in liquid will indicate 1 or 2 % lower CO2 level than expected after a gas phase calibration. One way to prevent this problem is to calibrate in the liquid phase. Using Orbisphere CO2 calibration kits is one way to do that.
The aim of this kit is to provide the end user with an accurate and easy-to-use liquid calibration method. A known and precise amount of sodium bicarbonate powder is combined in a precise PET bottle, with a ready-made dilute citric acid solution to provide defined, accurate carbon dioxide content in the liquid. During the calibration procedure, this solution can then be circulated through the flow chamber of a thermal conductivity sensor.
The calibration tables will help the user to find which number should be entered in their instrument during calibration or which value they should read on their display during a verification, depending on analyzer settings.
At 25°C (77 °F) in sodium citrate liquid, the CO2 concentration computed from known chemical quantities and volumes will be 3.038 V/V or 5.861 g/kg (equilibrium pressure: pCO2 = 4.208 bar).
For an analyzer configured to measure CO2 in beer, the concentration corresponding to this pressure at same temperature will be 3.042 V/V.
For the instrument set for CO2 in water, the value will be 3.157 V/V and 3.323 V/V for cola units.
How to Use the Calibration Kit:
A “starter kit” containing all the items required for preparing 10 samples (including the calibration bottle), and a “refill kit” containing 10 spare samples (no bottle) are available with two carbonation levels.
- The dilute citric acid is poured in the calibration bottle.
- Its temperature measured roughly (the effect of the temperature is not highly critical and a 2°C error in temperature will not generate more than a 0.5% error on the CO2 concentration). The table has temperature increments of 2°C and a coarse linear interpolation between these values will further reduce the error.
- The adaptor piece is screwed on the calibration bottle, and the bicarbonate test tube is gently inserted through the adapter making sure the tube fl oats on the liquid.
- The bottle is then tightly closed. At this time, the bottle can be tilted bottom up and vigorously shaken. The goal is to make sure no bicarbonate powder is left in the tube.
- The bottle can then be left sitting on a table for 3 minutes or more (intermittent shaking would speed up the process). Once all the powder is dissolved, the bottle can be shaken for two or three strokes just before use.
- The calibration sample has to be used the same day it was prepared, preferably within one hour. A CO2 loss of about 1% per day has been typically observed.
The analyzer settings should be clearly identified in order to use the suitable column of the conversion table. As with any other sample, the piercing process is critical for a reliable measurement.
Before piercing, we have to make sure the forcing gas pressure is higher than the sample pressure (depends on the temperature: about 5 bar forcing pressure at 25°C). Also make sure the flow chamber exit is closed. During the piercing step, make sure there are no leaks around the needle and no degassing in the bottle. In case there is a burst of bubble in the bottle at that time, do not attempt to calibrate the analyzer. Prepare another calibration sample instead.
In general the accepted calibration interval for flue gas analysers is 12 calendar months. JMW maintain a database of all previously calibrated equipment and advise the owner prior to the end of the calibration interval that recalibration is recommended.