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Forget about messy chemicals for pH checks A pH meter for • • g pools If you hove o swimming pool you will wont to measure the water's pH from time to time to ensure healthy water condition. This new pH meter is much cheaper than previous instruments. By JOHN CLARKE & LEO SIMPSON Years ago, pH probes were virtually a laboratory curiosity but they are now finding use in the home for monitoring the swimming pool and fish tanks, and in gardening. Perhaps the most obvious use is in monitoring the family swimming pool to help combat the dreaded scrurge of all pools - algae. To prevent the growth of algae and kill all those nasties such as E. coli, proteus and giardia, chlorine is added to the water. This can't be done indiscriminately though because the pH of the water must be kept within A small plastic case houses the low-cost electronic circuitry. The pH probe is supplied complete with buffer solutions which are needed for calibration. 60 SILICON CHIP fairly narrow limits, between 7.2 and 7.6, for the chlorine to work effectively. If you keep marine or tropical fish, the pH of the tank water is a very important consideration, if the fish are to survive and thrive. Similarly, if you're a keen gardener, you will know that many plants require acid or alkaline soils to do best. If you have a pH meter, you can keep a close check on soil conditions. Soil pH can be measured by mixing 10 grams of soil in 100ml of water and then measuring the pH of the resulting solution. The pH probe Previously, pH probes have been very expensive and used fragile glass construction. This new pH probe has a body made of clear polypropylene and a 'spear' shaped membrane (the 'business' part of the probe) which allows easy cleaning with a cotton bud. The probe is suitable for measuring solutions with a pH range from zero to 14 and has a reasonably quick measurement response time, to 98 % of ultimate reading, within 20 seconds. The probe is supplied complete with buffer solutions which are needed for calibration. As soon as we became aware of this new probe we were enthusiastic about the possibilities for an economical pH probe. We decided on an analog meter which could be driven by a few op amps. The result is as you see it; a compact instrument using just two integrated circuits, one three terminal regulator and three pots. That makes it all seem very easy but the design was no pushover 3.9k +5V VR4 10k 2.2k 14 TP2 ------<Asv~rlTRY 1k POWER ~ T 9V : ...L. OUT 10 16VWJ -=t +5V + 2.2 16VW! -~M pH METER GND T SC4-2-488 Fig.2: the op amps all operate as voltage followers. ICl provides a high input impedance to match the probe, while IC2a, 2b & 2d buffer the slope, temperature and asymmetry controls. IC2c provides a + 2V reference voltage. 60D 500 400 "' r---... ........ ~~ 3D0 "'o~ ~'oo"c 200 100 -100 "' ~~ ~ ~ ' ~~ ~ ~ ::--.... --~ ~ ..... -200 -300 "' - 400 -500 r--...--.........:: ---....... ~... "- -600 0 10 11 12 13 14 pH Fig.I: this graph shows the pH probe's output for three temperatures (0°C, 25°C and 100°C) over the full pH range from zero to 14. since pH probes have an output voltage which varies all over the place. They need compensation for three parameters: slope, temperature and asymmetry. Of these, temperature has the biggest effect on the probe. As well, a pH probe is a very high impedance device, around 200 to 500 megohms, so it needs a measuring circuit which can provide an extremely high input impedance. Let's have a look at how these parameters vary the pH probe's output and then we can see how the circuit provides compensation. Compensation Fig. 1 shows the variation in the pH probe's output for temperature extremes of 0°C and 100°C, over the full pH range of zero to 14. At a pH value of zero. the. probe output ranges from + 518mV at 100°C to + 379mV at 0°C. Similarly, for pH14. the probe output varies from - 518mV at 100°C to - 379mV at 0°C. Note that temperature variations have no effect on the pH probe's output at pH 7 and so therefore the characteristics show that the probe has an output of zero millivolts at this pH value. In fact though, this is not the case. Real probes can have an output anywhere between ± 50mV at a pH value of 7. Our circuit compensates for this " offset" voltage by providing an adjustable compensating voltage. This is varied by the Asymmetry control. Another variation in the probe's output is brought about by ageing. Typically, this can cause the output voltage to diminish down to about 94% of the output when new. Our circuit compensates for this ageing process with a "Slope" control. This can accommodate for a 15% reduction in output due to ageing. Circuit description The circuitry of the pH meter uses one quad op amp package and one single op amp package, three potentiometers and a few other passive components. ICl is the single op amp package, a CA3130 Mosfet input op amp which is used as a unity gain voltage follower. This provides the extremely high input impedance of one Teraohm, or one million megohms. So high is this impedance Al'll/1 , 1088 61 ~ pHPRO~E INPU~ / 10 9 Fig.3: the PCB has been designed to fit the meter terminals. The pH probe cable may either be soldered direct to the PCB or connected via a BNC socket as shown. Install PC stakes at the two test points, TPl and TP2. that even the smallest leakage and capacitance effects can degrade it. These effects are minimised on the printed circuit board by a "guard ring" which is connected to the op amp output, via a 3.9k0 resistor. This effectively negates leakage effects. The output of ICl feeds VR2 and a series 5.6k0 resistor. VR2 is the Slope control, referred to previous- ly. Following VR2 is another voltage follower stage, IC2a, but this time it's an ordinary op amp, one of an LM324 quad op amp IC. IC2a's output is fed to a voltage divider comprising 3.9k0 and 6.Bkn resistors in parallel and a lk0 resistor in parallel with a lOk0 potentiometer, VR3. VR3 provides the correction for temperature variations in the measured solution. IC2b is yet another voltage follower , for the output of VR3. When you look at the rest of the circuit, you'll realise that all five op amps are connected as voltage followers or, to put it another way, as non-inverting unity gain buffers. IC2b drives the negative side of the meter movement. On the other side of the circuit, VR5 allows adjustment to compensate for the probe offset referred to earlier. VR5 is the Asymmetry control. Its output is buffered by IC2d which drives the positive side of the meter movement. Now comes the clever bit. Note that the "ground" side of the input socket and negative connection for the 5.6kn, 3.9k0 and 6.Bkn resistors is not the negative rail for the whole circuit. Instead these points are connected to the output, pin 8, of IC2c. IC2c is used to provide a + 2V voltage reference for the pH probe and the slope and temperature dividers. The output nominally sits at 2V by virtue of the voltage divider at the non-inverting input, pin 10, of IC2c. So the four op amps of IC2 and the associated offset and the resistor networks associated with VR2 , VR3 and VR5 all provide compensation for the probe so that it reads pH correctly. VR4 sets the meter current for full scale deflection and VRl provides offset voltage compensation for ICl. So there it is, a simple but clever circuit. It allows a rather intrac- t;~ ~----------- tt«n SC 4-2-488 OO At left is a view inside the prototype while above is an actual size artwork for the PCB. 62 SILICON CHIP the lid of the case, the printed board can be mounted on the meter terminals. Two solder lugs, which are supplied with the meter, can be soldered in place over the meter terminal holes on the board. The board is then secured using the nuts on the meter terminals. The photos show how we did it. We used short lengths of rainbow cable to connect the pots and the switch to the respective points on the printed circuit board. The probe cable The two solder lugs supplied with the meter should be soldered in place over the meter holes on the PCB. The board is then secured on the back of the meter using the two meter screws. table extremely high impedance device, the pH probe, to drive a 50µA meter and yet provides quite a lot of tricky compensation. The circuit does all that without providing any voltage gain and does it with two common IC packages, the CA3140 and the LM324 . As a bonus, the LM324 is not only·cheap, but is ideal for battery-operated circuits since its current drain is typically only 800 microamps, or only one milliwatt per op amp, at 5V. The pH meter is ideal for checking swimming pools but can also be used to check fish tanks and soil acidity. Wash the probe and replace the plastic cap after use. The 5V supply rail for the circuit is derived from a 9V battery using a 7805 3-terminal regulator. The regulator is critical since any change in the supply voltage will alter the offset voltage of the op amps and thus degrade the compensation provided. Construction Our pH meter was built into a standard plastic utility box, with plastic lid, measuring 130 x 68 x 44mm. We designed a printed board to suit the case and to fit onto the terminals of a standard MU-45 50µA meter movement. We do not recommend the use of Veroboard for this project, as the layout is critical, especially the guard ring for the CA3140. Our printed board measures 76 x 58mm and is coded SC4-2-488. Befo,re mounting any components on the board, make sure that it will fit onto the terminals of the meter. Make any adjustments to the mounting holes at this stage. Mounting the components is straightforward. Just follow the wiring diagram. When the board is complete, the case must be worked on. If you have purchased a kit, it is likely that it will come with a ready-drilled and silk-screened case. If not, you will need to make and attach a Scotchcal panel to the lid and then make the cut-out for the meter and drill holes for the three potentiometers and the on/off switch. When the meter is mounted on That done, a BNC socket needs to mounted on one end of the case and a connection made from it to the printed board. Don't use ordinary shielded cable for this job; it is not suitable. You can use a short length of RG58 coax cable or, as we did, a short length of the ea ble from the probe itself. PARTS LIST 1 plastic case, 130 x 68 x 44mm 1 PCB, SC 4-2-488, 76 x 58mm 1 lonode pH probe (G101 NFE) with pH 4.00 and pH 6 .88 buffers 1 front panel artwork 1 side panel artwork 1 meter scale artwork 1 MU-45 50µA meter 1 SPDT miniature toggle switch 1 panel mount BNC socket 1 line BNC plug 1 91 6 9V battery 1 9V battery clip 3 knobs Semiconductors 1 7805 5V 3-terminal regulator 1 CA3140 FET input op amp 1 LM324 low power quad op amp Capacitors 1 1OµF 16VW PC electrolytic 2 2. 2µF 1 6VW PC electrolytic Resistors (0.25W, 5%) 1 x 6.8k0, 1 x 5.6k0, 2 X 3 .9k0, 2 x 2.2k0, 1 X 1k0, 1 X 100k0 miniature vertical trimpot, 1 x 1 Okn miniature vertical trimpot, 1 x 1OkO linear potentiometer, 2 x 1 kn linear potentiometer Miscellaneous Rainbow cable, grommet, solder. A l'!l/L 1988 63 I- OFF O ON 'f!l::ilff& 1L'JLti/l/;-l;! pH METER SLOPE ASYMMETRY A A 100% 40 50 60 30~70 J__: 20 10 0 oc 80 90 100 :-l Above are actual size artworks for the front and side panels. The probe cable must have a BNC plug fitted to it. Ideally, it should be a crimped BNC plug but few readers will have the facilities to do that. The alternative is to use an ordinary BNC plug but the thin cable from the probe needs to be have an insulation sleeve to build its thickness out to the 7.5mm diameter needed to mate properly with the plug. Another alternative is to solder the probe cable directly to the board and forget about BNC connectors altogether. The ea ble should be passed through a small grommetted hole in the end of the case and clamped before being terminated to the board. This is electrically satisfactory but makes it less easy to store the probe safely in its plastic case. Remember that, even though this pH probe is made of plastic, it is still a relatively fragile instrument which should be treated with care. 64 SILICON CIIII' Calibration Initial setting up of the pH Meter should be done firstly by shorting the pH probe input and adjusting the input offset of ICl. Now connect a multimeter set to read DC millivolts between the output of ICl and the output of ICZc. Adjust trimpot VRl until a reading of 0mV is obtained on the meter. 0 0 This meter artwork is designed to fit an MU-45 50µA meter movement. Set the VRZ Slope control to the 100% position and the Temperature control to the 100°C setting. Connect the multimeter between test points TPl and TPZ (positive lead to TPZ) and adjust the Asymmetry pot VR5 for a reading of + 320mV. Then adjust trimpot VR4 so that the pH meter itself reads pH 7 or centre scale. Calibration requires two buffer solutions, one very close to pH 7 and another several units away from pH 7. The Ionode pH probe is supplied with two bottles of buffer solution, one a phosphate buffer of pH 6.88 and the other a phthalate buffer of pH 4.00. (Buffers are solutions of known pH which can absorb large quantities of contaminants with little change to the pH value). Note that the pH probe comes with a protective cap. This cap must be carefully removed before the probe is used and replaced when you are finished. The cap physically protects the probe from damage and slows down any drying out of the probe memebrane. To start calibration, set the Temperature control to match that of the buffer solution. Set the Slope control to 100%. Place the probe into the 6.88 buffer and adjust the Symmetry control until the meter reads 6.88. Leave the probe in the solution for several minutes to check for drift and reset the Asymmetry control if necessary. Remove the probe and wash the membrane in distilled water. Then place the probe into the pH 4.00 buffer and use the Slope control to obtain the correct reading. Now wash the probe again and place it in the pH 6.88 buffer. Adjust the Asymmetry control for the correct reading, then wash the probe again and repeat the process for the ph 4.00 buffer. This procedure may need to be repeated several times to correctly calibrate the unit. This is because the asymmetry and slope are not known. If pH readings are being taken regularly, say every few days or so, then it won't be necesary to calibrate before each use. If used less frequently though, you should go through the calibration process to ensure accuracy. ~