ANALYTICAL CHEMISTRY, VOL. 50, NO. 2 , FEBRUARY 1978
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Directly Digital Flow Controller with Rapid Response Time and High Precision T. W. Hunter and G. M. Hieftje" Department of Chemistry, Indiana University, Bloomington, Indiana 4 740 1
A new, directly digital device for automated gas flow control at a fixed pressure level is described. Being a pneumatic analog to a weighted-resistor, digital-to-analog converter, the transducer controls gas flow by summing the flow from selected parallel paths. The new transducer exhibits rapid response time, excellent reproducibility and linearity over its entire operating range. Although the present device is intended for gas flow control and operates over an 8-bit (256-fold) flow range, simple modifications would enable its use for liquld flow control or for different flow ranges.
This new method of fluid control requires a minimum of mechanical movement and therefore displays greatly improved response time and reproducibility. When used to control the flow of air, the transducer exhibited a reproducibility of about 2% over two months and showed no significant difference between calculated and observed flows from combined flow paths. However, the present experimental configuration does nothing to eliminate irreproducibility arising from thermally produced expansion and contraction of the needle valve stem. This problem, which is common to all flow-regulating systems which incorporate a needle valve, can be overcome with modifications to be suggested later.
Advances in computer technology have led to increasingly sophisticated control of scientific experimentation and industrial process streams. In many of these applications, on-line computers not only log data but also control many of the experimental or process parameters. With this increased degree of control, improvements in transducer design are imperative for optimal transfer of computer commands to the subservient unit. To achieve optimal transfer of control information, the transducer should ideally be directly digital and respond as rapidly as the computer itself. One parameter which often must be controlled in an automated system is gas flow. When the flow is to be controlled by a computer or other digital network, it is common to employ a transducer consisting of a needle valve connected to the shaft of a stepper motor. With such a device, the magnitude of flow is a function of the position of the valve stem which in turn is governed by the number of increments the stepper motor has moved in relation to an indexed location. Unfortunately, mechanical backlash and changes in valve stem position can lead to irreproducible or unpredictable flows when the stepper motor is returned to a previous location or sent to a new position. Stepper-motor-driven needle valves also suffer from rather slow response times, because of the need to drive the finely pitched screw of the valve over a substantial distance and because of the limited angular rotation rate of most stepper motors. Other disadvantages of this type of flow transducer are the nonlinear relationship of flow as a function of valve stem location and the need to initialize the transducer upon start up (Le., to relate stem location to an index). This paper presents an improved approach to automated control of gas flow which eliminates the shortcomings of the previously described device. The new approach employs a flow network which is analogous to a parallel, weighted resistor ladder used for digital-to-analog conversions; a schematic diagram is shown in Figure 1. As shown in Figure 1, the new controller consists of a number of selectable, parallel flow paths, each of which can be opened or closed, depending on the state of a solenoid valve (S)located in each path. The output gas flow from the transducer is then the sum of the flows through all of the open, individual paths. For convenience and binary compatability, each successive path is adjusted for a flow twice that of the preceding path, so that the total flow is directly proportional to the binary word corresponding to the state of the series of solenoid valves.
DESIGN CONSIDERATIONS The application for which the new flow controller was designed, atomic absorption flame spectrometry, requires the metering of two gases (air and acetylene). A block diagram for the final system used in this dual-flow application is shown in Figure 2. Just as its electronic analog must have a constant voltage across the resistor ladder network, so must the pneumatic transducer have a pressure applied over its alternative, parallel flow paths which is constant regardless of the flow rate. I t was found that a 12-L shunt ballast tank, placed after a dual-stage regulator, made it possible to maintain a sufficiently constant pressure across the transducer ( - 20 psig) regardless of the gas flow rate within the range 0-16 L/min. Both transducers were constructed with input and output tubes 40.6 cm long and 2.54 cm in diameter and fabricated from either brass for transducer 1 (air) or stainless steel for transducer 2 (acetylene). Because of the formation of potentially explosive copper acetylides, it is unwise to employ copper components in any flow system intended for use with acetylene. One should also keep the use of brass to a minimum. The input and output tubes are connected by eight parallel flow paths of l/s-in. tubing (brass and stainless steel, respectively), each of which contains a subminiature solenoid valve (Angar, Roseland, N.J., Model 10, normally closed) and a needle valve (Nupro, Cleveland, Ohio, either models SA or MA) in series. The individual needle valves were adjusted so that the gas flow rates through successive parallel paths increased by a factor of two. The state (open or closed) of each solenoid valve, S,, is determined by the transducer controller. An eight-bit control word and the occurrence of load pulses are employed by the controller circuitry to determine which solenoid valves of each flow transducer are to be open. Auxiliary emergency sensors are also provided, so that all solenoid valves can be closed independently of the eight-bit control word if some defined shutdown condition occurs. In the present application, a photoelectric sensor provides such a shutdown indication if the flame being supported is somehow extinguished. Also contained in the controller is circuitry which amplifies the control word logic information to the voltage and current levels necessary to drive the solenoids. In the present configuration, the load pulses and the eight-bit control word can originate in either of two places: from the digital output of the computer or from a local hardwired selector. The hardwired selector consists simply
0003-2700/78/0350-0209$01,00/0@ 1978 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 2, FEBRUARY 1978 flow in
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The output of the multiplexer is fed in parallel to the inputs of two data latches, one for each flow transducer to be controlled. This design enables the same control word input (hardwired or computer interfaced) to be used for all flow transducers, thereby minimizing the number of required control lines. Which of the flow controllers is to be addressed is governed by the load command system (described below). In Figure 3, only the data latch and subsequent circuitry for one controller are shown. When the data latch receives a pulse through gate G1 from either the local command circuitry or the computer, the control logic level a t input D appears a t output Q. Output Q (whose logic level is the complement of Q) is used to drive a light-emitting-diode display lamp on the front panel. The output a t Q is then fed to a driving transistor T1 through AND gate G2. The passage of the control signal through gate G2 is determined by the "shutdown" input, whose level indicates the existence of safe operating conditions. In the present system, a high or "1" logic level a t the shutdown input indicates that the flame is on. Obviously, this input must be temporarily overridden by switch S2 when the flame is first ignited. If the logic level from gate G2 is high, transistor T1 is biased "on", activating the solenoid valve; if T1 is biased off by the G2 AND gate output being low, the solenoid is turned off. Therefore, if a control bit is loaded high and no shutdown conditions are activated, the solenoid is opened, whereas if the loaded control bit is low or if shutdown is indicated, the solenoid is closed. In Figure 3, signal diode D1 protects the T T L control logic from any switching transients fed back through T1; resistor R1 limits the base current to transistor T1. The solenoid valves used in the present circuitry require a larger current to drive them open (250 mA) than is required to maintain an open condition, once it has been established. This high initial driving current is supplied by capacitor C1, which stores energy until T1 first turns on. When T1 is saturated, capacitor C1 dumps its stored charge through the solenoid, pulling it in. Resistor R2 limits the final collector current to the 50 mA needed to hold the solenoid valve open. Rectifier diode D2 keeps the transistor base-collector junction forward-biased as the magnetic field in the solenoid collapses when the transistor shuts off. T h e 24-V and 5-V power supplies can deliver 24 W and 5 W, respectively. Not shown in Figure 3 are two status flags which are available to the computer. One flag when high indicates that a shutdown condition has occurred. T h e other flag is high for about 10 ms after a load pulse has been received by the control circuitry. This latter flag can be used by the computer as a hardwired delay so that the computer with its cycle time of 1.6 p s does not overrun the flow transducer (the solenoid valves have an opening (pull-in) time of 5 ms).
EXPERIMENTAL Gas flows were measured with three different devices to cover the wide range of flow rates selectable with the transducer. Two different wet test meters (Precision Scientific Co., Chicago, Ill.) with full scale capacities of 3 L (model 63114) and 0.25 ft3 (model 63118) and accuracies of *'/,TO were used to measure flows from 16 to 0.5 L/min. A 22-mL capacity bubble meter was used to measure flows of less than 0.5 L/min. Flow rates were determined by dividing the capacity of the measuring device by the time required for that volume of calibrating gas to flow from the transducer. In this study, the calibration gas used was dry air (Matheson Gas Co., East Rutherford, N.J.). After this initial calibration, other gases could be used with the flow controllers merely by correcting for the differences in gas densities and viscosities. CHARACTERIZATION T o assess the performance of this new approach to automated flow control, flow rates for each parallel flow path and
ANALYTICAL CHEMISTRY, VOL. 50, NO. 2, FEBRUARY 1978
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Flgure 3. Schematic diagram of digital and analog circuitry used to control the state of a single solenoid valve. Multiplexer, M, and data latch are integrated circuits SN74 157 (parenthetical pin designation same as used in manufacturer specifications) and SN7475 (Texas Instrument, Inc., Houston, Texas), respectively; T1 is a NPN, h,, = 60 transistor. Designations C and L refer to computer or local control, respectively
Table I. Statistical Summary of Observed Flow Rates (L/min) from Individual Flow Paths Binary weight of flow 2O 2l 2 2 23 24 25 2.04 0.257 0.540 0.998 0.0641 0.129 Average of 5 data sets 0.009 0.003 0.003 0.03 Std dev among sets 0.0024 0.004 0.001 0.002 0.009 0.02 0.01 Std dev within a set 0.008 % Re1 std dev among sets 3.7 3.1 3.5 0.6 0.3 1.5 % Re1 std dev within a set 1.2 0.8 0.8 1.6 2.0 0.5 for combinations of paths were determined periodically over a two-month period. In order to characterize the reproducibility of flow rates and the uncertainty of a single flow measurement, the flow rates from the individual paths were measured a minimum of three times and then averaged. On five different occasions, the average flow rates for all eight flow paths were determined. The averages of the five average flow rates for each flow path are listed in Table I along with the standard deviations among the five sets of data. Also listed in Table I are the standard deviations within a set of observations (obtained by pooling all observed standard deviations for that flow path). For purposes of comparison, relative standard deviations are also included in Table I. Even though deviations between days of observations are small, analysis of variance reveals that, a t the 95% confidence level, these differences are significant and are probably due to temperature and slight delivery pressure changes from day to day. On the average, day-to-day irreproducibility for all flow paths was about 2% while the uncertainty in a single flow measurement was found to be less than 1% . If the flow transducer were to perform as expected, the observed flow rates for a combination of opened flow paths should equal the sum of the flows through the individual paths. When each of 49 observed flow rates was compared to its calculated flow rate, eight theoretical flow rates were not encompassed by the two-standard-deviation error range of the average observed flow rate. Because the calculated sum of flows is based on measured flows which have an associated error themselves, the calculated flows therefore have an associated uncertainty which has not been taken into account. When propagation of error is used to estimate uncertainties in the calculated flows and the difference between observed and expected flows is statistically tested at the 95% level, only three comparisons showed a statistically significant difference.
26 4.11 0.003 0.02 0.7 0.5
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One would expect that of 49 samples from a normal distribution, 2.45 samples would lie outside the 95% confidence range. Therefore, the three noted differences of the 49 comparisons are probably not significant and the transducer performs as theorized. The same accuracy and precision as stated above were obtained from the new transducer over a wide range of applied pressures. This fact makes it possible to employ the same transducer for a wide range of flows, merely by adjusting the applied gas pressure. Moreover, re-tuning of the parallel flow paths (adjustment of the needle valves) is unnecessary, once their relative flows have been established. CONCLUSIONS The approach of adding flows from parallel flow paths has been shown to be a feasible method of automated flow control. This type of flow transducer has the advantages of minimal mechanical motion, fast response time, good long-term reproducibility, and directly digital operation. The experience gained with this first prototype has shown that performance can probably be improved by further regulation of the gas pressure system and possibly by thermostating the transducer. In addition, the susceptibility of the present device to temperature fluctuations could be greatly minimized if the needle valves were replaced by a set of preset, fixed orifices such as watch jewels. The diameters of fixed orifices would be selected so that a relationship between their transmitted flows exhibited the binary relationship required of the parallel flow paths. In such a device, the desired flow range used could be finally set by adjusting the pressure across the transducer. ACKNOWLEDGMENT The authors acknowledge Maurice Williams and John Dorsett for their help in the construction of the flow transducers used in this study. Thanks are also due to Robert
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 2, FEBRUARY 1978
Ensman, Nick Loy, and Steve Williams for their advice in the design and construction of the electronic circuitry.
American Chemical Society, New York, N.Y., April 1976; taken in part from the Ph.D. thesis of T. W. Hunter, Indiana University, Bloomington, Ind., 1976. Support of this work by Grant PHS GM 17904-04A1 from the National Institutes of Health is gratefully acknowledged.
RECEIVED for review September 16, 1977. Accepted October 31, 1977. Presented in part at the Centennial Meeting of the
Comparisons between Hydrogen Bond Donor-Acceptor Parameters and Solvatochromic Red Shifts Orland W. Kolling Chemistry Department, Southwestern College, Winfield, Kansas 67 156
Transltlon energles were tabulated for three bathochromlc lndlcators In representatlve nonpolar, polar aprotlc, and amphlprotlc solvents. Nonlinear statlstlcal correlation functlons prevlously used for comparing the dye transltlon energy with a second solvent polarity parameter were resolved Into llnear equatlons for polar aprotlc solvents and a devlatlon variable for hydrogen bond donor solvents. I n these comparlsons, the Gutmann acceptor number was useful as a second emplrlcal parameter responslve to solvent polarity. For the lower aicohols, acetlc acld, chloroform, and acetonltrlle, the qualltatlve trend In the devlatlon varlable parallels the order obtained wlth the Kamlet-Taft ac-scale for hydrogen bond donor solvents. Slmllar correletlons between transltlon energles for the bathochromlc dyes and enthalpy-based measures of donor-acceptor lnteractlons generally felled.
Solvatochromic indicators having chromophoric groups absorbing in the UV-visible region have been used t o define empirical scales of solvent polarity. The most commonly used scales deduced from blue-shift chromophores are the Kosower 2 values and the Dimroth-Reichardt numbers. These have been found to be most adequate for representing hydrogen bonding and dielectric characteristics of polar nonaqueous solvents, but are less satisfactory as measures of solvent basicity for nonpolar and aprotic solvents ( I ) . A series of red-shift (bathochromic) dyes have been developed which are more capable of representing distinctions among polar aprotic and nonpolar solvents (2-4) and their transition energies can be used t o quantify medium effects in some instances where the Kosower and Dimroth values fail. Recently, Taft and Kamlet have devised a double scale for hydrogen bond donor and acceptor solvents which is derived from the solvatochromic shifts of nitroanilines and nitrophenols (5, 6). In previous investigations on the solvatochromism of three bathochromic dyes (Phenol Blue, Nile Blue A oxazone, and Brooker's dye VII) a general nonlinear correlation was found between the transition energy ( E T )of the indicator and the "F NMR shift and the 14NESR hyperfine splitting constant for selected model basic probes of solvent polarity (7). A purely statistical treatment of all of the data for both hydrogen bonding and nonhydrogen bonding solvents produced the rational correlation function given in Equation 1
e =cE, E,-
-a
b 0003-2700/78/0350-0212$01.OO/O
where P, is the probe variable, ET the transition energy (kcal/mol) for the solvatochromic indicator, and a , b, and c are constants. The present paper is concerned with the results of a more detailed examination of bathochromic shifts by the three model dyes in pure hydrogen bond donor solvents with the objective of resolving the influence of hydrogen bonding from the correlated parameters of solvent polarity incorporated into Equation 1. For the latter, the Gutmann donor (DMand acceptor (AM numbers were selected for the P, variables. The Figueras concept of a n additive perturbation energy contribution was extended to the response to the probe in hydrogen bond donor solvents ( 4 ) .
EXPERIMENTAL Solvents. Seven solvents were included in this study for which bathochromic shifts of the dyes had not been determined in previous work. These are: benzonitrile, carbon tetrachloride, absolute ethanol, n-hexane, methylene chloride (re-determination), nitrobenzene, and 2-propanol. In all cases, the liquid was initially dried for 1week over anhydrous calcium sulfate. Subsequent steps in purification are those specified below. Anhydrous ethanol was prepared by the usual refluxing over fresh CaO, followed by fractional distillation. Isopropyl alcohol was treated with clean magnesium ribbon prior to fractional distillation. Carbon tetrachloride, dichloromethane, and n-hexane were further dried by passing through a chromatographic column of alumina and then re-distilled. Benzonitrile was passed through two alumina columns, but not re-distilled. Purified nitrobenzene was obtained by reduced pressure (7 Torr) fractional distillation. Solvatochromic Dyes. Purified samples of Phenol blue and Nile Blue A oxazone were prepared by the procedures for recrystallization and column chromatography reported earlier (8, 9). Literature melting points and thin layer chromatography were again used as criteria for purity of the products. Spectra. Measurements of the absorption maxima for specific dye solutions in the visible region were made with a PerkinElmer-Coleman 111 spectrophotometer, following the methods previously described (8). RESULTS AND DISCUSSION Additional experimental values for the transition energies of Phenol blue and Nile Blue A oxazone in nonaqueous solvents are given in Table I, along with published data for Brooker's dye VII. Representative parameters measuring solvent basicity which were used in previous work on red shift vs. basicity correlations were re-examined with special attention being given to the bathochromic shifts in hydrogen bond donor solvents. Possible relationships to enthalpy-based 0 1978 American Chemical Society
