CO2 Measurement in Incubators - Questions and Answers - VAISALA - #6

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- If sampling through a hole or other port in the incubator, insert Nafion® tubing into the incubator and seal around it. - If sampling through a hose barb fitting/sample port, use a very short length of Neoprene tubing to connect the fitting of the Nafion tubing to the incubator fitting. Use the Neoprene as a “connector” to hold the Nafion® tubing as close as possible to the hose barb. It is undesirable to have the sample gas travel through the Neoprene, as condensation is likely to result within the Neoprene tubing - As a precaution, keep the GM70 pump above the level of the chamber sampling port. If condensation occurs in the sample line, this will prevent liquid water from damaging the CO2 sensor. For more information, visit www.vaisala.com or contact us at sales@vaisala.com Ref. B210826EN-A ©Vaisala 2009 This material is subject to copyright protection, with all copyrights retained by Vaisala and its individual partners. All rights reserved. Any logos and/or product names are trademarks of Vaisala or its individual partners. The reproduction, transfer, distribution or storage of information contained in this brochure in any form without the prior written consent of Vaisala is strictly prohibited. All specifications — technical included — are subject to change without notice. Td (°C) -40 -30 -20 -10 0 10 20 30 40 50 60 -60 0.9999 0.9996 0.999 0.997 0.994 0.988 0.977 0.958 0.927 0.878 0.803 -50 0.9999 0.9997 0.999 0.997 0.994 0.988 0.977 0.958 0.927 0.878 0.803 -40 1.0000 0.9998 0.999 0.998 0.994 0.988 0.977 0.958 0.927 0.878 0.803 -30 1.0000 0.999 0.998 0.994 0.988 0.977 0.958 0.928 0.879 0.804 -20 1.000 0.998 0.995 0.989 0.978 0.959 0.928 0.879 0.804 -10 1.000 0.997 0.990 0.979 0.961 0.930 0.881 0.806 0 1.000 0.994 0.983 0.964 0.933 0.884 0.809 10 1.000 0.989 0.970 0.939 0.890 0.815 20 1.000 0.981 0.950 0.901 0.826 30 1.000 0.969 0.920 0.845 40 1.000 0.951 0.876 50 1.000 0.925 60 1.000 Table 3 Dilution coefficients As an example: A gas sample is drawn from a 40 °C (Td) environment and introduced into a 10 °C (Td) enviroment, where the measured gas concentration is 5.32%. In the 40 °C (Td) environment this corresponds to 5% CO2 (5.32% X 0.939= 5.00%) since the higher water content diluted the sample. 5. Why is the CO2 concentration reading higher than expected when using the pump sampling method with drying tubing? When using the Nafion® tubing to dry the sample, the CO2 concentration of the dry sample will be slightly higher than in the wet sample. This is due to a phenomenon called dilution. The CO2 density is “diluted” in the incubator by the volume that the water vapor occupies. If water vapor is removed from the sample, the fractions occupied by other gases, including CO2, will increase accordingly. Table 3 contains the dilution co-efficients for the gas concentration when drying a gas sample. Dew point (at 1013 hPa) of the gas sample in the incubator is chosen on the horizontal axis, and the dew point of the gas sample at the measurement point is chosen on the vertical axis. Dew point of the gas sample at the measurement point can be determined with a humidity probe (HMP75B, HMP76B or HMP77B).