Plant Physiol. (1985) 77, 243-244
0032-0889/85/77/0243/02/$0l1.00/0
Short Communication
Laboratory-Produced CO2 Calibration Gases'
Received for publication July 25, 1984
HERBERT Zvi ENOCH*
Department ofAgricultural Meteorology, Agricultural Research Organization, The Volcani Center,
Bet Dagan 50250, Israel
ABSTRACIT
A simple, inexpensive apparatus for making mixtures of accurately
known amounts of CO2 and CO2-free atmospheric air is described.
Calibration pses with CO2 contents of 200 to 1500 microliters per liter
produced with the apparatus had concentrations which were within 10
microliters per liter of the target concentration.
0D
IR gas analyzers need frequent calibration with gases of known
concentration in order to correct the instrument for drift and
changes in sensitivity (2). Calibration gases stored in steel cylinders under high pressure (15 MPa) can be obtained from many
commercial firms. However, the delivery time, weight, and cost
FIG. 1. Diagram of apparatus for producing calibration gases. A,
of the prefabricated calibration gases represent limitations to the Apparatus for mixing a known volume of COrfree air with a known
user.
volume of pure CO2. B, Measured volumes of CO2 are sampled through
A set of gas-mixing pumps can be used to produce very a thick-walled latex rubber tube in which CO2 is flowing. C, Graduated
accurately known (±0.25% of their nominal value) calibration burette filled with 2 M KOH for estimation of the non-CO2 fraction of
gases (1), but sometimes their cost and bulkiness make them CO2 gases.
difficult to use. Under some circumstances, a somewhat less
accurate but inexpensive and simple method of supplying CO2 absorber.
The closed loop is opened briefly to ensure ambient atmoscalibration gases is sufficient.
A description is given of an apparatus for production of CO2 pheric pressure and then closed again. The atmospheric air
calibration gases that has been in use in this laboratory for some content of the closed loop is alternately pumped through the
time. Minor modifications which can be employed to make the bypass with the CO2 absorber and through the main branch until
apparatus fully portable, more accurate, and also capable of all CO2 and moisture have been absorbed. The bypass is then
closed and the IRGA's zero reading is recorded. Measured
producing other calibration gases, are discussed.
amounts of pure CO2 are introduced into the C02-free air by the
following procedure (Fig. 1B): CO2 is released from a CO2
MATERIALS AND METHODS
cylinder through a reduction valve and a short latex rubber tube
Calibration gases consisting of mixtures of dry atmospheric air into a beaker with water. A syringe with a thin needle (glued to
with well-defined amounts of CO2 can be produced with the the syringe) and a rubber piston is used to transfer measured
amounts of CO2 from the storage cylinder to the closed system.
apparatus shown in Figure 1.
An IR gas analyzer (IRGA) (model URAS II; Hartmann and The needle is inserted through the latex rubber downstream of
Braun, F.R.G.) is connected in a closed loop to a membrane the reduction valve, and the syringe is filled slowly with CO2.
The first two samples are discarded, the third is measured and
pump (Charles Austen Pumps Ltd., U.K.) and a glass bottle (Fig.
IA). The air volume of the closed loop should be determined quickly transferred and injected through the latex tubing upaccurately from its dimensions or by weighing the apparatus stream of the mixing bottle. Complete homogenization is
(except for the IRGA) with and without water. A few centimeters achieved when the IRGA reading becomes constant after a few
of the closed loop consist of thick-walled latex rubber tubing. minutes of pumping. Care should be taken not to change the
The rest is made of gas-impermeable materials. A bypass con- temperature of the CO2 sample by touching the barrel of the
sisting of a glass cylinder packed with a 15-cm layer of moist syringe during the transfer. If the temperature of the CO2 sample
KOH pellets, followed by a 5-cm layer of Soda Asbestos ('Car- equals that of the CO2-free atmospheric air of the closed system,
bosorb' brand, 6-12 mesh; BDH Chemicals Ltd., Poole, U.K.) the CO2 concentration (in volume, /l1-') is readily calculated
and a dust filter, acts as a C02, water vapor, and solid aerosol
from the volume
The
is V
whe
ratio.
' Contribution from the Agricultural Research Organization, Bet Dagan, Israel. No. 1 141-E, 1984 series.
243
CO2 concentration
+
v is the measured volume oftechnical grade CO2, F is the volume
fraction of CO2 in technical grade CO2, and V is the measured
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Copyright © 1985 American Society of Plant Biologists. All rights reserved.
244
Plant Physiol. Vol. 77, 1985
ENOCH
Table I. Measured and Calculated CO2 Concentrations in Calibration
Gases
Co2 Added
to a Volume
Measured Target Concn
C02
of
4545
of Air
ml
1.00
2.00
3.00
4.00
5.00
6.00
7.00
Difference =
Theoretical
Target minus
Al 1-'
220
440
660
880
1100
1320
1540
2=2±8
-10
-10
10
10
5
10
0
mlMConcn.
Valueeasured
Values
230
450
650
870
1095
1310
1540
%
4.5
2.3
1.5
1.1
0.5
0.7
0
volume of the closed system.
A minimum estimate of the impurities of a 'pure' CO2 cylinder
can be obtained by transferring measured samples to an inverted
graduated burette filled with 2 M KOH (see Fig. IC). The KOH
will absorb all CO2. Some gas impurities such as N2, 02, and Ar
will emerge as bubbles in the burette. The volume of the nonabsorbed impurities will give a minimum estimate ofthe amount
of foreign gases in pure CO2.
Due to the outsalting effect, the physical absorption of N2, 02,
and Ar in a 2 M KOH solution is very small. Twenty samples of
50 ml CO2 were transferred to the KOH-filled burette using a
50-ml syringe. From the volume of non-CO2 gas, it was concluded that the CO2 cylinder contained 0.3% impurities.
RESULTS AND DISCUSSION
The system described was calculated to have a volume of4545
ml. Simple plastic syringes with a volume of 1, 2, and 5 ml were
used. Preliminary tests showed that filling the syringes with water
and weighing the amount of water could be done with an
accuracy of better than ±1% of their nominal volume. The
IRGA, when used to measure the CO2 concentration (used in
the absolute mode), and the attached recorder each had an
accuracy of about 0.5% of full scale (which was 1600 l 1-'),
there was little need to improve the accuracy of the CO2 volume
measurement. The apparatus was used in a laboratory where the
temperature was maintained at 23 ± 0.5C.
An IRGA (URAS II; Hartman and Braun, F.R.G.) was calibrated with CO2 in atmospheric air mixtures made by three gas
calibration pumps (model ISA 27/3a; Wosthoff, Bochum,
F.R.G.). The IRGA was used to determine the CO2 content of
calibration gases produced by the procedure described in this
paper.
The measured CO2 values were read to the nearest 5 gl 1-'.
A typical set of measurements is given in Table I.
The mean difference between theoretical and measured value
was 2 ± 8 ,ul 1-' for C02 concentrations over the range 220 to
1540 l 1-'. The relative error was largest (4.5%) at the lowest
concentration, and decreased with increasing CO2 concentration.
The measured concentration (Y) as a function of ml CO2 (X)
is represented by the equation:
Y= 7.1 + 217.7X;
R2 = 0.9997
The IRGA (used in the absolute mode) calibrated with laboratory-produced CO2 caibration gases was used to measure and
control the CO2 concentration at -1000 l -1 in greenhouses
used for CO2 enrichment experiments. For this purpose, an
accuracy of ± 10 LI 1- of the calibration gases is sufficient, as no
plant physiological significance can be attached to such minor
concentration differences.
The method of producing CO2 calibration gases can be improved by using gas syringes with a thermostated water mantle
and micrometer settings for accurate repeated gas delivery which
can be obtained from several commercial suppliers.
A battery-operated (or a manually operated) pump can be used
to make the apparatus portable and thus suitable for field work.
The method lends itself to production of other calibration
gases in an analogous manner, if there is an available supply of
pure gases and specific absorbers.
LITERATURE CITED
1. ENOCH HZ, Y COHEN 1981 Sensitivity of an infia-red gas analyzer used in the
differential mode, to partial gas pressures of carbon dioxide and water vapor
in the bulk air. Agric Meteorol 24: 131-138
2. JANAC J 1971 Accuracy of IRGA. In Z Sestak, J Catsky, PG Jarvis, eds, Plant
Photosynthetic Production, Manual of Methods. W. Junk, The Hague, pp
148-152
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Copyright © 1985 American Society of Plant Biologists. All rights reserved.