e991030.pdf

TEST & MEASUREMENT

conductance tester

with DIY sensor

Conductivity in the sense

it is used in this article is

the ability of a substance

such as water to conduct

electric current. It is

expressed in terms of

current per unit of applied

voltage. It is the reciprocal

of resistivity. Conduc-

tance is the reciprocal of

resistance and is mea-

sured in siemens. It is

therefore the ratio of the

current through a sub-

stance to the potential dif-

ference at its ends.The

tester described in this

article is intended for

assessing the quality of

water, based on the acid-

ity or alkalinity (pH), by

means of a measurement

of the conductance of the

water.

INTRODUCTION

Water with a very high pH is not good

for fish, plants or making tea or coffee.

This is the reason that many aquarium

owners, orchid growers, horticulturists,

and many others use distilled or fil-

tered water. Water filters are very pop-

ular in domestic use, where the quality

of tap water is suspect. However, water

filters themselves present a risk of ger-

mination, requiring good attention to

cleanliness (bottle needs thorough

washing at least once a week).

A very environment-friendly way

of obtaining low-pH water is the use of

rainwater, but this depends heavily on

the area where the rainwater is col-

lected. Such water may be tested for

low pH, that is, low conductivity, with

the present tester.

The pH of water is a logarithmic

index of the hydrogen-ion concentra-

tion in the water. It is given by

CONDUCTIVITY AND

CONDUCTANCE

Conductivity (or specific conductance),

being the reciprocal of resistivity, is

measured in the same way as resis-

tance and expressed in S m –1 (siemens

per metre); its symbol is

in S m –1 :

G= σ A/ l .

S (siemens)

This equation can be used with a solid

as well as with a liquid substance.

The sensor used in the present

tester consists of two annular elec-

trodes having a cross-sectional area of

1 cm which are spaced 1 cm apart.

These dimensions make the calculation

of the conductance of the water being

tested straightforward.

Pure water, sold as distilled water, as

used, for instance, in electric irons, lead-

acid batteries and for horticultural pur-

poses, has a conductivity of 1

pH=log 10 (1/[H + ])

where [H + ] is the hydrogen-ion con-

centration. A pH below 7 indicates acid-

ity and one above 7, alkalinity, at 25

10 –3

Sm –1 , so that the present meter would

Design by P. Baer

Elektor Electronics

1/99

. At constant

temperature, the value of conductance,

symbol G , of a substance depends on

the cross-sectional area, A , in m 2 , the

length, l , in m, and the conductivity,

Probe

LCD1

DP2

DP1

3.5 DIGIT

DP3

BT1

D 3

C11

100n

BAT

BC557B

10 µ

63V

330n

220n

BC547B

D 2

IN HI

330n

REF HI

100k

IC1

ICL7106

TEST

100n

10k

REF LO

BC547B

245A

COMM

IN LO

1N4148

R10

C10

100p

100n

470n

220n

980104 - 11

S (equivalent to a resis-

tance of 100 k

The test voltage is taken from across

R 2 -R 3 -C 3 . During the negative half-

period of the backplane voltage, tran-

sistor T 1 links the test voltage to earth.

In essence, therefore, this FET operates

as a clocked synchronous rectifier with-

out a threshold voltage and conse-

quent non-linearity.

The direct test voltage so obtained

is directly proportional to the resis-

tance of the water. It must, however, be

inverted to provide a test display in

siemens. In the present circuit this is

achieved by applying the test voltage

to the reference voltage input (REF.

HI)of IC 1 and a constant voltage to the

test input pins (IN HI and IN LO). This

results in the display showing U C / U T ,

instead of, as normal, U / U ref . ( U C is the

constant voltage across pins 30, 31, U T

Figure 1. Circuit diagram of the

conductance meter.

). When the water

becomes less pure, the resistance

between the electrodes drops, and the

value of conductance rises. Normal tap

water has a conductance of about

1 mS, and sea water, 100 mS or more.

is the test voltage, and U ref is the refer-

ence voltage). Preset P 1 provides com-

pensation for component and sensor

tolerances.

THE TESTER

From the above, it is clear that the

tester must be capable of measuring

resistance or conductance. This seems

simple enough: take a constant current

source, insert the sensor in a potential

divider and apply the voltage across

the sensor via an analogue-to-digital

converter (ADC) to a suitable display.

Unfortunately, the reality is not so

simple, since the resistance of a fluid

must be measured with an alternating

instead of a direct current. This is

because a direct current would cause

electrolysis which after a while would

distort the sensor electrodes.

The tester, whose circuit diagram is

shown in Figure 1 , therefore uses a rec-

tangular current. This enables the ADC

to drive the liquid-crystal display

(LCD) via pin 21, the backplane voltage

terminal.

The backplane voltage is used to

switch, via transistor T 2 , constant-cur-

rent sink T 3 -D 2 -R 6 , which is combined

with constant-current source T 4 -D 3 -R 7 .

Capacitor C 4 is charged and discharged

in rhythm with the backplane voltage at

a rate of 100 mA. Because of this

arrangement, the 100 µA current from

the source is absorbed by the sink when

the capacitor is being discharged. Con-

sequently, an alternating current of

±100 µA flows through the tester, which

causes a potential drop of ±100 µV

THE SENSOR

To make the sensor, two rings of brass

or other easily soldered metal, a 15 cm

length of RG58U coaxial cable, and a

heavy-duty soldering iron are needed.

The brass rings should have an inner

Figure 2. Parts required for con-

structing the sensor.

RG 58 U

solder braid

here

cover with

silicon cement

disc of brass

or other easily

soldered metal

d = 10 mm

solder conductor

of coax cable here

6 mm

13 mm

cover with

silicon cement

–1

across the resistance, that is, the water.

980104 - 12

Elektor Electronics

1/99

measure 10

+9V

Parts list

Resistors :

R 1 = 470 k

Probe

FIX2

LCD1

C2 C3

C11

Ω

R 2 , R 3 = 1 M

R 4 = 4.7 M Ω

R 5 , R 9 = 100 k Ω

R 6 = 10 k

IC1

R 7 = 20 k

R 8 = 22 k Ω

R 10 = 47 k Ω

P 1 = 10 k

preset potentiometer

Capacitors :

C 1 , C 6 , C 8 , C 11 = 0.1 µF

C 2 = 10 µF, 63 V, radial

C 3 , C 10 = 0.22 µF

C 4 , C 5 = 0.33 µF

C 7 = 100 pF

C 9 = 0.47 µF

C10

R10

FIX4

Semiconductors :

D 1 = 1N4148

D 2 , D 3 = LED, green, 3 mm

T 1 = BF254A

T 2 , T 3 = BC547B

T 4 = BC557B

Integrated circuits :

IC 1 = ICL7106CPL (Maxim)

diameter of 6 mm and

an outer one of 13 mm

to give them an effec-

tive area of just under

1cm 2 . The inner diameter allows them

to just fit over the inner conductor of

the coaxial cable, whose outer insula-

tion must be removed over a length of

about 15 mm from one end and 10 mm

of the exposed braid cut off. The

remaining 5 mm of braid must be

folded back over the outer insulation

of the cable. This ensures that the two

brass rings are about 1 cm apart (see

Figure 2 ). The inner core of the coaxial

cable is then soldered to the outer brass

ring and the braid to

the inner ring. The

outer surfaces of the

rings (but not those fac-

Figure 3. The printed-

circuit board for the

conductance tester.

ing each other) should

then be covered with

silicon cement.

Miscellaneous :

LCD 1 = 3.5 digit liquid-crystal dis-

play (note that IC 1 and LCD 1 are

available as a set)

BT 1 = 9 V dry battery with clip

1 off switch with on contact

Enclosure as appropriate

PCB Order no. 980104-1 (see Read-

ers Services towards the end of

this issue)

CONSTRUCTION

The remainder of the tester is best con-

structed on the printed-circuit board

shown in Figure 3 . Mind the polarity

of the diodes and electrolytic capaci-

tors. The IC should be soldered directly

to the board to allow the display to be

fitted directly above it.

Connect a standard potentiometer

across the test inputs and check that the

display shows corresponding conduc-

tance values when the potentiometer is

turned from, say, 10 k

Conductivity at 20 °C

Silver

1.6

10 –8 S m –1

Copper

1.7

Aluminium

2.8

Tungsten

5.6

Nickel

6.8

Iron

Figure 4. The com-

pleted prototype

board.

When all is well, fit

the completed board

Steel

Manganin

Carbon

3500

into a suitable enclosure in which a

cut-out for the display has been pro-

vided. Connect the sensor to the probe

terminals as shown in Figure 4 . Do not

forget an on/off switch.

Ω

and a maximum test voltage at IC 1 of

2 V. This value will be displayed when

the tester, or rather, the sensor, is dry.

The upper limit of the test range

(1999 µS) is set by the characteristics of

IC 1 . Note also that the basic error of 5

per cent increases slightly when the

test range is given an upper limit of

more than 1000 µS.

[980104]

Elektor Electronics

1/99

to 1 k

FINALLY …

The tester has a range of 50 µS, which

corresponds to a resistance of 20 k

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