Low Cost Conductometer
Parts list:
Power supply section
- 100 volt primary (or mains voltage primary), 12 volt secondary power
transformer 100 miliaperes or more
- 12 volt indicator lamp (optional)
- two 1N4002 Diodes (any diode with piv of 25 volts or more, 1 AMP current
or more)
- two 3300 mfd 25 Working Volt electrolytic capacitors (any capacitor 25
Working Volt or more 1000 microfarads or more)
- LM7812 12 volt positive voltage regulator (any +12 volt regulator, 100 mA
or more)
- LM7912 12 volt negative voltage regulator (any -12 volt regulator, 100 mA
or more)
- two 0.1 microfarad 25 volt ceramic capacitors (these are for high
frequency bypass, any capacitor - any material: ceramic, plastic, any
temperature coeficient, 25 volts or more, in the range 0.033 to 0.47
microfarads)
Conductometer section
- LM324 quad operational amplifier (any quad op-amp with supply range above
24 volts, the schematic figure shows a TL084)
- 15 turn trim potentiometer 10 K (any multiturn pot resistance in the range
5K to 500K)
- 100K 5% 1/4 watt resistor and 0.022 microfarad capacitor (these two form
an RC circuit with a time constant of 2.2 miliseconds, any combination with RC
of around 2 ms is good. Keep R in the range 10 K to 1 megohm)
- 1N4002 diode ( any rectifier with piv above 25 volts)
- 0.1 microfarad 25 volt capacitor (any capacitor - see power supply
section)
- 12 position rotary switch (see circuit explanation below for substitution)
- twelve 1% 1/4 watt precision resistors, 200, 301, 499, 1.00K, 3.01K,
4.99K, 10.0K, 30.1K, 49.9K, 100K, 301K, 499K (see text below for substitution)
- two pair banana jacks for cell and output connections.
Circuit explanation
The power supply is a dual regulated power supply,
provides plus and minus 12 volts.
The conductmeter consists of 4 units identified with letters A,B C and D in
the schematic diagram.
Unit A is a square wave oscillator with a frequency of about 1
kilohertz and peak voltage of 12 volts, a multiturn potentiometer is used to
reduce the peak voltage to 1 volt at the input of unit B.
Unit B is a voltage follower that applies a buffered 1 volt peak
square wave to unit C.
Unit C is the actual conductance meter. The circuit is an inverting
amplifier with voltage gain given by -Rf/Rin, Rf is a resistor selected by a
multi position switch and Rin is the conductance cell. If the equivalent
conductances (Rcell =1/Gcell and Rf=1/Gf) are replaced in the gain equation, we
get:
Vout = -Gcell/Gf x Vin
Vin is adjusted to 1 volt peak, and is
a square wave so the sign of the gain coefficient can be discarded. Substituting
and rearraging terms we get:
Gcell= Vout x Gf
The twelve resistors
have been selected to provide Gf of 5 miliSiemens, 2 miliS, 1 miliS, 0.5 miliS,
0.2 miliS, 0.1 miliS, 50 microS, 20 microS, 10 microS, 5 microS, 2 microS and 1
microS. The switch and resistors can be substituted with values that will
provide usable ranges.
Unit D is a peak detector used to convert the square wave output of
unit C into a DC voltage which is measured with a DC voltmeter.
Calibration and use
To calibrate place a 1% 10K resistor across the cell
inputs, select the 0.1 miliSIemens range (switch positioned between the 4.99K
and 10.0K resistors). Adjust the multiturn trim potentiometer until Vout is 1.00
volts.
To use, connect a conductance cell to the cell input, place the cell into the
measured solution. Select a range that results in an output between 1 and 10
volts. The solution conductance is obtained by multiplying the output voltage by
the range value.
For example, if the range is 0.1 miliS and the output
voltage is 3.4 volts, the conductance is 3.4 x 0.1 = 0.34 miliS.