Major snowstorms in my area (Northern Virginia) are often followed by salty-tasting tap water, some days later. Salt that was spread on the roads gets dissolved by the melting snow (or rain), runs off into the creeks, down to the Potomac, and from there, into our drinking water.
I’m pretty sure that’s true. It is known to happen elsewhere, for sure.
But for Northern Virginia, all I have to back it up my claim is a ) my taste buds, and b) my recollection of salty-tap-water events of the past.
So this time, I’m going to try to capture that salt spike in my tap water, in hard data.
Measure it. As it flushes on through the system.
Cheap water quality testers are all water conductivity testers.
If you look on (say) Amazon, you can buy cheap little meters to measure total dissolved solids (TDS) in water. As above. These are often included with high-end countertop water filters, so you can see that something has been removed from the water, in passing it through the filter. (My understanding is that consumers use the TDS reduction as a marker for when to change the water filter cartridge.)
You can also buy remarkably similar-looking meters to measure water salinity. These are often targeted toward (e.g.) aquarium owners, and pool owners, either of whom may need to keep water salinity within a defined range.
You can even buy meters labeled for measuring the electrical conductivity of water. Need I say that those cheap water-conductivity meters look almost identical to the first two?
Turns out, those are all the same meter. They all measure the electrical conductivity of water. They just label the resulting output on different scales.
Maybe — I haven’t quite figured this out one way or the other — there may be non-linear adjustments linked to the named use (salinity, TDS). Maybe not. I don’t think my $5 is going to buy me a lot of sophistication. But these days, you never know.
Starting off with a DIY flop
So, assuming I have deciphered the technical stuff right (below), to capture the salt spike, I need to measure the electrical resistance of my water. Day after day, in a repeatable fashion. For, I’m guessing, a couple of weeks max.
The salt, passing through the system, should show up as a temporary spike in the conductivity of the water. And I’m not looking for a little squiggle in the data. Back-of-the-envelope, I’m hoping for roughly a doubling of the conductivity for the days in which the salt spike passes through. Which I have already predicted will be this coming Wednesday, based on my hazy recollection of the past.
I’ve got an ohm meter. Somewhere. It can measure resistance (ohms). How hard could it be, to rig up some way to use my VOM (volt-ohm meter) to track the resistance (the mathematical inverse of conductivity) of my tap water.
Long story short, this failed. I was unable to make a reliable measurement. After assembling the hardware (two bolts, stuck to a plastic lid, in a mason jar of water, connected to a VOM), the estimated electrical resistance of the water wandered all over the place. Substituting stainless bolts for the galvanized bolts shown above did nothing to correct the problem. I think that, perhaps, my VOM was just not up to the task.
After giving it a couple of tries with this DIY approach, I gave up and ordered the $6 meter pictured above.
I still don’t really know why my DIY water-resistance meter didn’t work. Might have been as simple as a bad battery in the meter. Not worth pursuing, when I can buy a meter for $5.
It really is this simple? The theory.
Pure (distilled) water is a poor conductor of electricity.
But if you add ions to the water — from dissolved salt (Na+Cl-) or calcium carbonate (Ca++ C03–) or baking soda (Na+ HC03-) or hydrochloric acid (H+ Cl-) or whatnot — the ions act as charge carriers, and so allow electricity to flow more easily in the water.
The more ions you add, the better the water conducts electricity. (Within reason or at modest dilution.) All the ions in the water contribute to the increased conductivity of the water. Those could be “dissolved solids” ions, as from calcium carbonate in hard water. Those could be “salt” ions, as in, the salt in a salt water aquarium.
In fact, all of these super-cheap TDS/salinity/conductivity meters measure the conductivity of the water. Period. They just put a different label, and perhaps a different scale, on that measured conductivity.
The first thing to note is that these meters can’t distinguish salt from other ions. All they do is tell how conductive the water is. That depends on the concentration of current-carrying ions in the water.
And the concentration of ions depends on all the ionic substances that have been dissolved in the water, how well they “dissociate” (split into their ionic components) in that water, and (I vaguely recall) how rapidly the resulting ions move, where lighter elements are slightly preferred for speed. (I may be making up that last part).
But the bottom line is that my $5 salt meter does not measure salt in the water. It measures the total ion concentration in the water, of which salt contributes a part. And probably displays it in units that match salt-concentration units (like ppm NaCl and such).
But equally weirdly, while these meters react to all ions in the water, they are blind to dissolved non-ionic compounds. Like, sugar, say. Sugar molecules remain intact (and carry no charge) when dissolved in water. Dissolved sugar does not materially affect the conductivity of water, and so a cheap “TDS” meter will not respond to dissolved sugar or other dissolved non-ionic organic matter in the water.
The upshot is that the thing that’s sold as a “TDS” meter … isn’t. Not if “total” includes things like sugar — dissolved organic material that is not ionic in nature. It’s blind to that stuff, because that stuff doesn’t affect the conductivity of the water.
But that’s only fair, because the “salinity meter” version of it doesn’t measure salinity, either. All ionic substances increase the conductivity of water. For example, I’m pretty sure that adding vinegar to the water will cause the conductivity to increase. On a meter labeled as a “salinity tester”, that increased conductivity would be labeled as increased saltiness.
As far as I can tell — and certainly at this price-point — the only way to measure the different ions separately is through chemistry. Old school, you add reagents to react with certain ions, precipitate them out of the water. You then filter out, dry, and weigh the precipitate to infer the quantity of the selected ion in the batch of water. (Or you buy a meter with exotic-material electrodes that react chemically with certain ions and not other.) Either way, that level of effort and expense is way beyond what I contemplate here.
Separately, and well known, the fact that these meters react the same to all dissolved ions means that “TDS” isn’t a good measure of drinking water cleanliness. For most drinking water, TDS is simply measuring the total dissolved mineral content. For me, here in the Town of Vienna VA, almost all the dissolved solids are from a water hardness of around 5 to 10 grains (per our mandated water quality report.) This is almost entirely from harmless calcium carbonate, dissolved in the water. The relatively high TDS in this case doesn’t mean that my tap water is bad, just that it has dissolved minerals in it.
Conclusion
I hope this has been clarificatory.
There is only one underlying type of cheap water quality meter.
Cheap (sub $10 on Amazon) TDS meters, salinity meters, and water conductivity meters measure water conductivity. Water conductivity is driven by the concentration of ions present in the water. All ions are lumped together by this measurement. And these meters are blind to dissolved non-ionic material, because (e.g.) stuff like sugar doesn’t materially affect water conductivity.
So, really, at least at this price point, there are no salinity meters or TDS meters. There are only water conductivity meters, and the labels placed on them.
The situation isn’t as dumb as I’ve painted it. If you know what’s going into your water — say you are trying to adjust the salt level in a swimming pool — then yeah, that meter will function for you as a salt meter. Because you know that it’s your salt that’s increasing the ion count and pushing up the conductivity of the water.
Similarly, if dissolved organic non-ionic compounds are not an issue for you — no sugar in your water, that you know of — then the same meter may well serve as a useful TDS meter. (Consumers could use it, for example, to decide when to change their water filter cartridge).
For the moment, all I need to do is take a water sample a day, from my kitchen faucet. Just a mason jar, rinsed and filled. Store that away.
And then, if the story is as I think it is, in a couple of weeks, I should be able to go back through the samples and identify the “salty” days through blind taste-test. And, if all goes well, my $6 TDS meter will highlight the same days as high TDS days.
If it all goes to plan, I’ll have documented the post-snowstorm salt spike in our drinking water by both blind taste test, and by measured dissolved solids.
