and P. J. Hannan U. S. Naval Research Loborotory Washington, D.C. 20390
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A C O ~ ~ ~ ~ UGas-Mixing OUS Apparatus
In t,he studies of photosynthetic processes of algal cells, we found it is often desirable to vary the concentration of C02 being supplied to these cells during growth. Previously in this laboratory when a change in the C02 concentration was required, a COz.air mixture of the proper concentration had to be prepared in advance by charging a cylinder with these gases to the appropriate partial pressures. This procedure was laborious, timeconsuming, and required the periodic filling of large cylinders. Also, care had to be taken to assure that the resulting mixture was homogeneou,s. To overcome these difficulties, an apparatus was designed which would allow the preparation of gas mixtures of any desired COz concentration directly from t.he laboratory compressed air supply and a single tank of Cot. A schematic representation of the apparatus is presented in Figure 1 and a photograph in Figure 2. The mixing tank is a steel cylinder (Standard Type A military C 0 2cylinder) 17 in. long X 6.5 in. in diameter with an internal volun~eof approximately 1 ft3. Two holes have been drilled and tapped near the forward end to accommodate a pressure switch (Penn Controls, Model 1016) and an outlet pipe, respectively. The outlet pipe leads to a tee fixture, one arm of which serves as a manual valve-controlled access to the
atmosphere, while the other leads to a pressure-reducing valve through which the effluent gas passes. The tank is charged through a gas manifold containing two inlet ports, one for COZand one for air, which are controlled by normally-closed solenoid valves (Valeor, Series SV-61). The Cop port is also equipped with a needle valve. Within the tank is a copper mixingtube which serves as an extension to the intake manifold. The walls of the tube, which extend nearly to the end of the tank, have been collapsed diametrically, forming a series of constrictions about '/2in. apart with the axis of each constriction being rotated 90" to that of the preceding one. The two gases unite at the manifold end of the mixing tube, and as a result of the turbulence created by the tube, are satisfactorily mixed by the time they rnterthe tank.
Figure 2.
Figure 1.
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Schematic diagram of continuous gas-mixing apparatus.
Journol of Chemical Educalion
PhMogrrrph of ont ti nu om go.-mixing opporotua.
For our study, the COz and air are both supplied a t a pressure of 50 psi to the solenoid valves, whose operation is controlled by the pressure switch. The airflow is at a fixed rate dictated by the plumbing; the flow of the Cop is under needle valve control. The concentration of C02 in the mixture is varied by altering its rate of flow into the manifold by the needle valve, and the composition of the effluent is measured with a COz analyzer. When the gas pressure in the tank has increased to 40 psi the solenoids are de-energized, and the flow is stopped. The solenoids remain de-energized until the gas pressure in the tank is reduced to 20 psi. At this pressure they are re-energized and the cycle repeated. The pressurereducing valve in the effluent line is set a t 10 psi to ensure a constant rate of gas flow to the algal chamber. By proper calibration of the Cop needle valve control, the desired concentration of CO2 can be approximated initially, thereby reducing the amount of fine adjustment necessary. The change-
over time from one C02 concentration to another can be reduced considerably by opening the atmospheric vent on the tank and allowing the apparatus to cycle and flush the tank. Besides the advantage of pro-
ducing large volumes of binary gas mixtures of varied concentration quickly and with a minimum of manipulation of heavy gas cylinders, this apparatus also hm the advantages of extreme mobility, ruggedness, and low construction cost.
Volume 44, Number 7, July 1967
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