Absolute Pressure Detecting Circuit for ion Gauge Controllers Thomas R. Edwards' and John C. McCoy2 National Aeronautics and Space Administration, George C. Marshall Space Flight Center, Ala. 35872
point being a virtual ground. As long as the summing point is negative, the amplifier output sits a t the forward voltage drop of the single diode. Use of a silicon diode puts the off level a t positive 0.4 volt. When the absolute pressure is sufficient to cause the summing point to swing positive, the output across the two silicon diodes in series switches to minus 0.7 volt. This voltage is sufficient to turn off transistor T1 of the typical differential amplifier, which opens process control relay R1. Any external equipment associated with the process control relay subsequently shuts down a t the predetermined absolute pressure. The values of the summing resistors are determined as follows. The summing point switching current is set to a maximum value of 100 pA in order to prevent loading of existing circuitry. The maximum value of VI plus V2 is usually in the neighborhood of 100 mV, which sets R1 and Rz a t lo00 ohms. Using the condition a t crossover that the input currents sum to zero, R3 can be found from Equation l.
While many commercial ion gauge controllers are equipped with relative set point indicators, quite a few do not have an absolute pressure detecting feature which allows for the protection of sensitive equipment, e.g., a mass spectrometer, a t a predetermined absolute pressure rather than 80 to 110% of full-scale meter reading, regardless of the pressure range. This report describes a simple zero-crossover switching circuit using a summing network input to an operational amplifier with switching diode feedback (Figure 1). With this circuit, ion gauge controllers with automatic gain change for pressure range or log scale can be operated in their automatic or log mode and at the same time provide absolute pressure protection. This overcomes the problem Torr, below a relative set point of operating a t 5 x indicator of say 90% full scale but far above a safe operating pressure, i.e., l O - - 5 Torr. This adverse situation is relatively easy to achieve with some existing equipment. A typical ion gauge controller has outputs of VI for meter reading and Vz for range, i e . , Granville Phillips 236 Series, or a pick-off point in the circuit representing the log mode voltage VI and V2, i . e . , National Research Corporation Model 836, etc. Summing these signals in conjunction with a negative dc bias, V3, from the controller is sufficient to switch process control relays at a predetermined absolute pressure. The circuit signals V I , VZ, and V3 are summed across R1, Rz, and R3, respectively (Figure 1). All the signals are available within the ion gauge controller.
A 10-turn potentiometer equal to V3 x lo4 ohms may be used for R3, giving an adjustable absolute pressure set point. The type of ion gauge controllers for which this protection circuit is applicable usually have all the necessary voltages and switching relays inherent to their design. All that is required for this modification is one operational
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Figure 1. Absolute
RELATIVE METER READIUQ
pressure detecting circuit
At a predetermined absolute pressure of 3.5 x Torr, typical voltage values are VI = -3.5 mV; V2 = ( N o - N)x 9 mV, where N = 5 and N o = 11 (the value where the decade step voltage is zero, Torr) or V2 = 36 mV; the V3 = -23 V dc, an available negative bias (Granville Phillips Series 236 Model 02). The zero-crossover a t the summing point is obtained when the sum of the input currents across R1 and Rz just equals (or slightly exceeds) that across R3, the summing 'Space Sciences Laboratory *Computation Laboratory.
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TYPICAL DIFFERENTIAL AMP OF PROCESS CONTROL RELAY
amplifier, Zener diode power supply to provide the *15 V dc for the amplifier, R1, R2, R3, and SI, and an output current limiting resistor. Switch S1 allows a manual override, permitting equipment operation above the absolute set point; this returns the ion gauge to its original mode of relative set point operation. The authors recognize that absolute pressure protection is possible only with a calibrated ion gauge and controller. Though many situations obviate the absolute aspect of the circuit, Le., changing gas mixtures, this in no way detracts from effectiveness of the application; it is a safer mode of operation than relative set point indicators. ANALYTICAL CHEMISTRY, VOL. 45, NO. 4, APRIL 1973
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If a filament control relay is available, the absolute pressure protection relay can be combined with the pressure burst protection of the filament. Pressure burst protection turns the filament off regardless of pressure range when a sudden burst in pressure causes a rapid meter reading deflection, and opens the filament control relay, To combine these two safety features, the equipment to be protected is simply connected across both relays, which are connected in series. Now either a sudden pressure burst or gradual increase in pressure to a specified absolute level will cause shutdown of external equipment.
This circuit in the above indicated mode has worked successfully for the past 14 months in two applications. Granville-Phillips Series 236 Model 02 ion gauges and controllers protect a CEC 21-104 Mass Spectrometer and a CEC 21-613 Residual Gas Analyzer from pressure burst and operation above 1 x Torr. During this period there has never been a circuit failure, and the equipment has been successfully protected. Received for review August 7, 1972. Accepted December 11, 1972.
Vacuum-Line Electrochemical Cell for Electrosynthesis C. D. Sc.hmulbach and T. V. Oommen Department of Chemistry, Southern lllinois University, Carbondale, lil. 6290 7
Experience has shown that a vacuum-line electrochemical cell is superior to a conventional cell for the handling of moisture-and-air sensitive compounds. A recent paper described the use of such a cell (1) (hereafter called the Anderson Cell) for polarography, coulometry, and cyclic voltammetry. The Anderson Cell is not suitable for electrosynthetic work, however. The chief limitations of the Anderson Cell for electrosynthesis are: (i) a small capacity of the electrolysis compartment (-10 ml), and (ii) the absence of a separate anode compartment. A conventional H-cell f2), by contrast, has none of these disadvantages, but is not ordinarily attached to a high vacuum line. We have designed a cell that incorporates the useful features of both the Anderson Cell and a conventional cell. For convenience in description, only electrochemical reduction at the working electrode is discussed. The cell is equally suitable for electrooxidation. The cell assembly is shown in Figure 1. The cathode and anode compartments are separated by sintered glass frits and by a middle arm that can be connected to the vacuum line. The cathode compartment can handle 100150 ml of electrolyte. In electrolysis experiments, the working electrode is a mercury pool. The design of the cell enables easy switching from polarography to coulometry or electrolysis. The base electrolyte is contained in ampoule A, and the electroactive species, in ampoule B. Ampoule C contains the anolyte which is usually the base electrolyte itself. The reference electrode may be one of the standard electrodes such as the SCE or the Ag/Ag+ electrode, but for operation under vacuum conditions, the reference electrode must be placed in a separate compartment as it is in the Anderson Cell. On the other hand, a platinum wire dipping in the base electrolyte may be used as a quasi-reference electrode (PQRE) which can be placed in the catholyte, near the DME. The use of PQRE as a reference electrode is recommended where other electrodes are not suitable, though it is known that this electrode is not as highly reproducible as the others (3). The (1) J. L. Mills, R. Nelson, S. G. Shore, and L. E . Anderson, Anal. Chem.. 43, 157 (1971). (2) J F. Coetzee, Ph.D. Thesis, University of Minnesota, Minneapolis, Minn., 1956. (3) D. J. Fisher, W.L. Belew, and M.T. Kelley in "Polarography 1964." Vol. 1 1 , G. H. Hills, Ed., interscience, New York, N.Y., 1966, p, 1043.
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A N A L Y T I C A L C H E M I S T R Y , V O L . 45, NO. 4, A P R I L 1973
auxiliary electrode may be an inert electrode like platinum, or a sacrificial metal electrode such as silver, which was used in our study. The latter has the advantage of preventing generation of oxidizing gases resulting from anodic reactions, thereby avoiding the mixing problems that arise from gas pressure developing in the anode compartment. The selection of an electrode material is also made to avoid undesirable oxidation processes involving the base electrolyte or solvent. The resistance between the anode and working electrode was 300 ohms for a 0.1M solution of the base electrolyte tetrabutylammonium iodide in acetonitrile. This resistance is sufficiently low so as not to restrict coulometry studies with the Electroscan 30. Operation of the Cell. The cell inc1;ding the filled ampoules, electrodes, and magnetically stirred mercury pool is connected to the vacuum line and evacuated. The amount of mercury that drops from the DME during the evacuation is small. The cell is now isolated from the vacuum line, ampoules A and C are opened to the cell, and their contents are poured into the cell compartments until the solution level in the three compartments is equalized. Pure, dry nitrogen is then introduced into all three arms of the cell through the vacuum line to nearly atmospheric pressure. Nitrogen is added also to the tube containing the reference electrode through its nitrogen inlet. The cell may then be isolated from the nitrogen supply. The height of the mercury reservoir is adjusted for a drop-time of 4 to 6 seconds. G polarographic scan on the supporting electrolyte is made before introducing the electroactive species from ampoule B. The electroactive species is then transferred to the cell. A small known volume of the solution containing the electroactive species is first introduced from the graduated ampoule B to obtain a polarogram and then a larger known volume of the electroactive species is admitted for coulometry and electrolysis. Stop, cock G should remain closed during electrolysis. If pres sure differences develop and the solution level become! unequal in the separate arms of the cell during electrolysii because of gas liberation, provision should be made for thc relief of excess pressure or the nitrogen pressure should bc varied to compensate for pressure differences. A t the en( of electrolysis, the contents of the cell are transferrec through stopcock D to evacuated flasks without contact ing the atmosphere. The progress of electrolysis is moni
