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Test the SALT CONTENT Do you have a swimming pool with a salt-water chlorinator? Then you will know that you have to periodically add salt to the water to make sure that the chlorination process works properly. So how do you measure salt content in your swimming pool? By LEO SIMPSON T hese days, many in-ground swimming pools use a salt-water chlorinator to keep the water clean and safe from nasty microbes. The chlorinator electrolyses the salt content of the water to produce sodium hypochlorite which then acts like normal “pool chlorine” to sanitise the water. Not having large amounts of chlorine in the water makes it much more pleasant and you don’t come out of the water smelling of chlorine. Nor will your eyes sting or your swimming togs become bleached. However, for a salt-water chlorinator, there must be a minimum concentration of salt in the water for it to work correctly. Just how much is needed depends on the brand and model of chlorinator but typically it is around 3000 to 4000 ppm (parts per million). If the salt concentration goes below the specified level, you must add some salt to the pool. On the other hand, you should not The Pool Salt Meter at our local pool shop. Obviously, it does more than check dissolved salt levels – it also checks total dissolved solids. But it also costs more than $300 and, according to our friendly pool shop owner, “ . . . costs a fortune to repair, too.” We wonder why! 36  Silicon Chip add too much as that is simply wasteful and it might lead to accelerated corrosion of some of the pool hardware. So how do you measure salt water concentration? Most people don’t even bother. They just take a sample of their pool water to the pool shop and ask them to test it (at the same time getting several other important pool chemistry factors checked). If it is below the specified level, this is the perfect opportunity for the pool shop to sell some bags of salt. A couple of shots of the measuring cup to show not only its size but also its construction. The two black circles (next to the green circles) are the carbon electrodes which make contact with the pool water, to give a reading in parts per million on the meter at left. siliconchip.com.au of your swimming pool Here’s our lo-tech – but almost as good – version, total cost 87c (we had to buy the salt!). The rest of the “bits” were from the junk box or test bench. This shot is with tap water, for reference – the multimeter reads 4.42kΩ. The pencil “level” mark inside the tube ensures consistency of measurement. You may note the little puddle of water at the base of the tube: it’d be a good idea to give both the end cap and screw heads (outside the tube!) a smear of silicone sealant to stop this happening! So why not make your own salt meter? Such a device is not likely to be very complex, is it? That’s what we thought too. So we visited a local pool shop and asked the friendly manager if we could have a look at his salt meter. And we took some photos to show It’s not really that hard, is it? We don’t think so! (This is the warning label on the meter at left). siliconchip.com.au what it looked like. It was an analog meter with scale calibrations in ppm and TDS (total dissolved solids). But notwithstanding those obscure labels, our impression was that it was simply an ohmmeter connected to a measuring cup. Measuring cup The measuring cup was interesting. As the pictures show, it was a small cylindrical container with two holes at a certain level up the sides. In the base of the cup were two carbon electrodes which evidently make connection to the solution. In use, the cup is first flushed with fresh water and then, holding your fingers over the holes in the sides of the cup, you fill it up with your pool water. You then unblock the holes and the water flows out so that it is at a precise level in the cup. You then take the reading by pressing the button on the meter. That’s all there is to it. Hmm. So is this really necessary? Since virtually every reader of SILICON CHIP magazine has a multimeter or two or three, whether a digital (DMM) or good old-fashioned analog type and since they all have “Ohms” scales, no other equipment is necessary. In other words, if you want to measure salt content of your pool, you don’t need a $300 salt solution meter or whatever else it might be called. Standard salt solution. What you do need is a salt solution of known concentration. To be more December 2008  37 precise, how do you make up a solution with 3000 ppm salt? In fact, making up such a solution is dead easy. All you need is a measuring jug which will hold one litre of water and a set of measuring spoons. Then you need to measure out 3 grams of salt. Pool salt or table salt will do – they are both pretty much the same thing, no matter how they are labelled. Half a teaspoon of salt is 3 grams. Add that to the water and stir thoroughly until all the salt is dissolved. Voila! You now have one litre of salt solution which is exactly 3000 ppm. Hardly a high-tech exercise, is it? Want a 4000 ppm salt solution? Add 4 grams to your one litre of water instead of 3. But believe it or not, we have seen internet retail outlets which sell such a standard 3000 ppm salt solution for $10.95 for a 230ml bottle! So now that we have a standard salt solution, how do we measure salt content in a swimming pool? It is just a matter of comparing the resistivity of the standard salt solution with the resistivity of the pool water. The more salt in the water, the lower will be its resistance. So the next step is to make a container with a couple of electrodes connected to both sides. You could use almost any cylindrical plastic container but we chose to use a 200mm length of 90mm plastic stormwater pipe fitted with a standard end cap. We drilled 2.5mm holes in opposite sides of the resulting container about 140mm from the bottom. We then attached a solder lug to each of the holes, using a screw, nut and lockwasher. The solder lugs were connected to a length of figure-8 cable with a pair of 3mm banana plugs at the other ends. The banana plugs were connected to a digital multimeter and it was switched to measure “Ohms”. A dollop of silicone sealant should be applied to the screw heads (on the outside only!) to make them watertight. While you’re about it, you might like to put a smear of silicone on the inside bottom of the 90mm tube as you slide the cap on – again, to make it watertight. We didn’t and the results are obvious in our photograph. So that is the test set-up. Checking it out To check it out, first fill the container with fresh water to a mark at, say, 20mm from the top. Note the ohms measurement. Typically our reading was 5000 ohms or thereabouts. It will vary depending on how much chlorine is in your tap water. Tip out the fresh water and note that the resistance reading now becomes very high, typically 40 megohms or more. Then fill the test container with the 3000 ppm salt solution and note the resistance reading. Typically, we measured around 1800 ohms or 1.8kΩ. This will vary depending on the temperature of the solution but we can assume for the purpose of this exercise that the solution water temperature is fairly close to that of the pool. Then fill the test container with water from your pool. If the salt con- centration is more than 3000 ppm, the resistance reading will be lower than 1.8kΩ (or whatever your previous measurement was. Conversely, if the salt concentration is less than 3000 ppm, the resistance will be higher than 1.8kΩ. You can then decide whether or not you need to add salt to your pool. Incidentally, you should not need to add salt to your pool more than once or twice a year. Evaporation from your pool will not reduce the salt concentration; it will increase it. There are only three ways in which salt can be lost from your pool. The first is when swimmers splash water out of the pool and you subsequently have to top it up with fresh water. Second is when back-washing the pool filter, although our experience is that this doesn’t make a huge amount of difference. The third way – fortunately rare because it usually throws virtually all your pool chemistry out of whack – is when you get a lot of rain and a lot of water is lost out of the overflow. Of course, if your pool leaks more than average (most do leak a little!) and you often have to add water to top up the level, your salt level will also drop. So there it is. While we have not described how to make a salt concentration meter reading in ppm (because we don’t believe it’s necessary to actually know the figure), we have described a method of comparing the resistivity of pool water to a standard salt solution. SC www.rmsparts.com.au SMART PROCUREMENT SOLUTIONS o Resistors o Relays o Capacitors o Buzzers o Potentiometers o Switches o Crystals Tel: 07 3219 4735 Fax: 3219 4716 o Connectors o Semiconductors o Hardware o Optoelectronics o Chemicals & Fluxes sales<at>rmsparts.com.au  38  Silicon Chip WHOLESALERS  DISTRIBUTORS  KITTING SOLUTIONS  siliconchip.com.au