Recall that, as of my last post, my road-salt-in-drinking-water experiment was floundering. My tap water was showing far more variation in measured total dissolved solids (TDS) than seemed reasonable.
Turns out, that’s because a) my tap water is cold, b) temperature strongly affects the conductivity of water, c) this $6 meter measures and adjusts for temperature,
d) extremely slowly. And e) I’m not exactly a patient person. I’m not going to stand around for five minutes, holding this meter in a glass of water.
The solution is as simple. I have to let the glass of tap water sit for a couple of hours, and come up to room temperature.
Once I do that, these “well-aged” water samples all provide consistent readings for parts-per-million total dissolved solids.
Properly measured, my tap water TDS has been around 215-225 ppm TDS for the past couple of days. A little higher than the 170 ppm I expected based on “10 grains of hardness” of the water. But definitely in the ballpark. And seemingly stable.
Which is important, as I expect road salt runoff to produce a big upswing in that, Wednesday-ish, best guess. Based on last Friday’s half-inch of rain washing (almost) all the remaining salt off the roads.
The full story
- This meter measures water’s electrical conductivity.
- That conductivity is increased by ions in the water.
- Such ions are generated when minerals and salts dissolve in water.
- Thus, the meter can infer the amount of ions in the water, from the water’s conductivity.
- It then translates that into something the user can understand, such as parts-per-million total dissolved solids (TDS) or salinity. Depending on the end-use market that is being targeted.
Source: Mettler Toledo white paper, “Reducing Measurement Error in Conductivity Readings”. Annotations in red are mine.
- But water temperature strongly affects conductivity. A 9F decrease in water temperature creates a more-than-10% reduction in water conductivity.
- Hence, this measurement typically requires temperature correction. The goal is to measure the water’s conductivity, adjusted to some standard water temperature.
- And this $6 TDS meter includes that temperature correction via a built-in thermometer (and presumably a look-up table on a chip, or something).
- But the meter is excruciatingly slow about doing that.
I finally got the bright idea of sticking this meter in a glass of ice water and see how long it took to display a temperature of 0 C.
I gave up, it took so long. I got tired of holding the meter in the ice water. I’m guessing it would eventually get there, but it would take five or ten minutes to do so.
In any case, that adjustment is so slow that what I interpreted as the meter reading “setting down” to a final value, in just a few seconds, was nothing of the sort.
And that’s what tripped me up. With incomplete temperature adjustment, cold water registers as “cleaner” water (lower TDS), owing to the lower conductivity of cold water.
Conclusion: Never rule out operator error
On the on hand, I could blame the meter for being so slow to adjust to different temperatures.
On the other hand, it’s up to the meter operator to use it correctly. Or spend the big bucks on one that works faster.
In any case, for $6, I got a very smart meter. Smart enough to do the temperature correction for me.
But the hardware? That’s still the best that $6 can buy. It’s fine, as far as I can tell, but there’s no expectation that $6 bought me some kind of heirloom-quality super-tool.
And, as it turns out, what I got for $6 is a meter that works, but takes forever to settle to a final reading, owing to the glacial pace of adjustment of its internal temperature sensor.
Which I consider fair, for $6. That it works at all is kind of a miracle.
But now that I know that the temperature correction takes forever to register, all I need to do is let my tap water samples warm up to room temperature.
And poof, what seemed like a ridiculously inconsistent meter turns out to be … pretty consistent.
Well worth the $6.
I probably need to buy some distilled water, for another buck or two. Not to test the meter, but to rinse it after I’m done. Per the directions, I am absolutely not supposed to touch the electrodes with anything. Not even to dry them off. I think that if I just let the little electrodes air-dry, after tap water, I risk “poisoning” the electrode surfaces over time with calcium carbonate deposits, a.k.a., water spots. This, by analogy to premature dulling of un-dried razor blades by the thickness of water spots (Post #1699). Distilled water, by contrast, leaves nothing behind when it evaporates.
Otherwise, the experiment is now on track. I have documented a stable baseline of around 215-225 ppm dissolved solids in my (room-temperature) tap water.
I just need to give it a few days for the road salt to work its way from the Potomac River to my water faucet.
