ANALYTICAL CHEMISTRY, VOL. 50, NO. 14, DECEMBER 1978
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Submersible Liquid Level Alarm for Evaporative Concentration George R. Mapp I V and M. Kent Cueman" Virginia Institute of Marine Science, Gloucester Point, Virginia 23062
Evaporative concentration of large numbers of volatile solutions is at best a tedious process. For example, trace pollution analyses often require t h e concentration of many dilute organic extracts by evaporation a t constant temperature to a specific volume. Constant attention is required to produce these uniform evaporations and t o avoid sample loss by evaporation t o dryness. T o efficiently process large numbers of samples in this manner, an array of submersible liquid level detectors which provides partially automated supervision of the concentration step was designed. Samples contained in glass centrifuge tubes are inserted in sensor units submerged in a thermostatically controlled water bath and evaporation is accelerated with a gentle stream of nitrogen gas. When the desired volume in each tube is reached, the sensor gives an audible alarm and lights an indicator on the central poner supply unit, directing t h e attendant to the proper tube. Technique. T h e sample monitor consists of a number of plastic encapsulated liquid level sensors connected to a power supply and alarm unit. T h e sensors (Figure 1) have an infrared LED and a photo sensor/op amp mounted on opposite sides of a test tube sized hole in a molded plastic block. T h e infrared beam emitted by the LED transects the full sample tube at the desired level of evaporation. 'Cl'hen the liquid level drops below the beam, the empty tube scatters the light and the reduced infrared flux triggers the photo-detector. An integrated op amp circuit designed for on-off applications (Figure 2 ) provides a sharp voltage change to drive the alarm circuits (Figure 3). T h e electrical case connection to the SG1002 LED is made by cementing the diode to a copper sleeve with conductive epoxy. T h e sensor units are cast into polystyrene blocks using hobbyists' embedding plastic and may be immersed in water
3E15" 6800
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Figure 2. Electrical schematic for submersible liquid level sensor ( 1 ) . The trim potentiometer is adjusted for reliable triggering at the operating temperature Sensor I
+50v
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DUAL ONE SHOT OSCILLATOR
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Figure 3. Logical schematic for the alarm unit. OR circuit is made from type 7432 TTL gates and can be designated to accommodate any numer of inputs. A, 6 ,C, D, and E are open collector power NANDs (type 7438)
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Photosensor / OP AMP
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Figure 1. Cutaway view of level sensor unit 0003-2700/78/0350-2169$01 0010
baths and warmed to modest temperatures (50-60 "C). Casting the submersible sensor units requires some practice, so preparation of a test casting is suggested. Clear, symmetrical holes tailored to a variety of glass tubes can be made using polished metal or Teflon rods sprayed with mold release compound. T h e control unit contains a logic circuit to permit several sensors to share a common alarm. Individual LED's indicate which of the ecaporating tubes has triggered an end-point alarm. Series diodes are used to reduce + 7 . 5 V minimum power supply output required by the photo sensors to +5 V for the logic chips. T h e logic circuits are straightforward applications of 7400 series T T L chips which can be tailored to parts in stock. Using a commercially available power supply module and a printed circuit board. the entire control unit c 1978 Amerlcan Chernlcal Society
2170
ANALYTICAL CHEMISTRY, VOL. 50, NO. 14, DECEMBER 1978
is easily packaged in a small mini-box which fits conveniently in a cramped fume hood. The sensors were tested with hexane and pentane extracts and colored water solutions. They appear to function with any reasonably translucent mixture.
LITERATURE CITED (1) RCA Linear Integrated Circuits, file #421, CA3062 Photo Detector and
Power Amplifier, p 6, Figure 15. Reprinted with permission.
RECEIVED for review June 26,1978. Accepted August 7,1978. This work was supported in part by the Bureau of Land Department of Interior, under Contracts Management, U.S. 085501CT5-42 and AA550-CT6-62. VIMS Contribution 884.
