High Impedance Comparator for Monitoring Water Resistivity Paul K. Holewinski' The Toledo Hospital, Bio-Medical Engineering, Toledo, OH 43606 This paper describes a high impedance comparator suitable for monitoring the resistivitv of a deionized or distilled water feed line supplying water in the 50 Kohmlcm-2 Mohmlcm range. Flow rates up to 500 mL/min are practical. Circult Descrlptlon The operating circuit (Fig. 1) consists of an LM741 operational amplifier driven by a bipolar 9-V power supply. Signals are input to the 741 via the voltage dividers set up between the +9-V supply and ground. R1 and R2 provide a constant voltage to the inverting and noninverting inputs of the 741. The reference voltage corresponding to the level of resistivity desired is adjusted via R3 a t the noninverting input. The comparison voltage generated through the flowing stream is nrovided hv the sensor nrohe RS. When RS exceeds R3 (VS is greater than VR) the o&put of the 741 will swing to t 9 V and lieht the ereen 1.K1). If RS is lower than R:l (VS IS lesser than v:) the output will change to -9 V and light the red LED. A feedback loop is not used in the circuit to insure a sharp transition between positive and negative states. Switch S1 is
a DPST-type enabling both batteries to be disconnected simultaneously when the device is not in use. Current drawn is less than 30lmamp during operation. The circuit board (Fig. 2) is a generai type used for IC experiments. The particular type used in this device is a Radio Shack #276-024 board. The 741 and all resistors are mounted on the board. All other components are mounted on the front panel of the case and connected to the board by the appronriate lengths of &22 stranded wire. The orobe is connected by a shierded cable (single conductor microphone cable is suitable). i ~are s soldered directlv to the . . and the 9-V hatterv , c l. board. The internal arriuigement of pans is not overly critical. The prototype was packaged as shown in (Fig. 3). Probe Assembly The sensor probe (Fig. 4) is constructed from a 1-cm diameter by 18-cm long Pyrex@tube with a 4-mm hole blown in the side at the midpoint. Two 10-cm lengths of #30 Kynar2 wire with 1cm of insulation removed from the ends are fixed in a plug of silicone sealing compound, available from most hardware stores. The wires should be positioned 2-3 mm apart and held until the sealine comoound cures. The wires are then trimmed to a length of 7-8 m i and the assembly fitted into the hole in the side of the Pvrexm tube. Fix the wires in d a c e with more sealing compounz and allow the assembly tocure.
' Address correspondencetoauthor at 1211Winghaven. Maumee.
OH 43537.
Kyniu"is a registered trade& of Penwalt Corporation and refers to a polyvinylidine fluoride insulated wire.
Figure 1. Circuit diagram for high impedance cwnparata.
Parts Llst 61.62 Rt R2 R3 R4
P V Transislw Banery 2.2 Mohm. % w Resista. 5% lo Mahm. % w Resistor. 5% 2 Mohm. Potentiometer 560-720 ohm. % w Rerista, 5%
RS
Sensor Robe
QA 1
LM741 Operational Amplifier
01 02
T t3/, &wen LED T 131, ~ e LED d
(2) 9-V Battery Clips (1) Circuit Board. RS X276024 (1) Case. 5'1. in. X 2% in. X 13/4in.
#30 Kynar Wire Silicone Base Sealer 4 4 0 Screws and Nuts faPC Board Mounting DPST Switch 2 n Coaxial Cable and CIommet 22 Gauge Stranded Hookup Wire 1108
Journal of Chemical Education
Figure 2. Printed circuil board.
KYNAR WIRE
SEALANT
/
\
i
\
WRAP W I T H TAPE
COAXIAL CABLE
Figure 4. Sensor probe: (a) end view: (b) side view Figure 3. Component assembly: (a) front panel, and (b) interior placement.
The wires should be parallel and not touch the bottom of the tube. Cut the wires to a length of 5 cm and solder a 60-cm length of shielded wire to the Kynar wires (one wire to the shield, one to the center conductor). The wire is wrapped around the Pyrex" tuhe several times and fastened securely with electrical tape. Cover the entire portion of the wirewrapped tuhe with electrical tape as well as the sseal where the K y n a F wires enter the tuhe. A thin glass rod may he inserted into the Pyrex" tuhe to align the hare wires parallel as shown in Figure 4. Callbration A resistivity meter will he required to calibrate this device, as well as a container capable of holding several liters of water, and a funnel. Connect the funnel to the end of the P y r e 9 tuhe with a short length of rubber tuhe. Fill the container with
deionized water and add salt until the water has a resistivity of 50 Kohmlcm as measured on the resistivitv meter. Turn on the comparator and turn R3 completely c~unterclockwise. Slowly pour the water through the funnel and adjust I23 so the red LED barely lights. Mark the panel a t this point. Prepare 100 Kohm, 500 Kohm, 1Mohm, and 2 Mohmlcm water and repeat the procedure. The comparator is now calibrated. Connect the electrode assembly in the feed line to he monitored. Set R3 to the level of resistivity desired and turn on the water. When the supply water has a resistivity greater than the level set on the front panel, the green LED with light. If the red LED is lit, the resistivity of the incoming water is less than that set with R3. If air bubbles pass through the line the green LED will flicker momentarily as the higher resistance of the bubbles pass the wires. A constant flow of water is necessary for an accurate reading.
Volume 61 Number 12 December 1984
1109
