Hi all. One of the tools we have available to us is the total dissolved solids meters from Hanna Instruments. A couple of models have been looked at so far, the TDS-1 and the DIST-WP-1.
We talked a while back about trying to make a calibration standard for these meters using salt in water. I'd like to figure out if there is a reasonably accurate measure available to everyone that could be used to make such standards. I thought that kitchen measuring spoons might work. My wife was kind enough to do a little experiment for me. I'd like some others to reproduce it so we can define some error bars. If you have access to a balance sensitive enough to measure tenths or hundredths of a gram or better, you can help. Of course if you have such a balance, you can measure the salt directly and don't need this recipe at all. But I'm hoping we'll be able to help out others who don't. I asked her to take a set of common kitchen measuring spoons and a quantity of table salt and accurately measure the weight of salt delivered by some of the spoons. Here are the results of several measurements for each spoon (in grams): TBSP 16.20 16.19 16.50 16.74 16.77 16.47 16.02 16.48 average: 16.4 1/2 TBSP 10.03 9.72 9.60 9.43 9.63 9.53 9.04 average: 9.6 TSP 4.99 5.17 4.96 5.13 5.18 5.17 average: 5.1 The tablespoon and teaspoon are consistent with each other. Three tsp would yield 15.3g which is only about 7% less than 16.4. The "half tablespoon" is closer to 2/3rds if the others are correct. Even from this one experiment, it looks like there will be a lot of variation between different sets of spoons, and even within sets. Something to keep in mind: Measuring spoons are different from your typical silverware. A soup spoon or dessert spoon that you eat from is almost certainly *not* going to deliver a measured tablespoon or teaspoon. The usual "tablespoon" is often about *half* of an actual tablespoon. Remember that when dispensing medicines! Also, I'd ask our friends in metric standard countries to help out by describing the usual measurements used in "metric" cooking, if they aren't the "tablespoon" or "teaspoon." I would ask you to still *weigh* measures of salt, since there's a difference between volume (liquid) measure, which the spoons are, and the weight of different kinds of solids. (A teaspoon of lead weighs more than a teaspoon of salt which weighs more than a teaspoon of air. <grin>) Finally, be sure to level the measure. Start with a rounded spoonful, and then scrape across the top with the back of a butter knife or other straightedge to level it. There are lots of variables: different spoons, coarseness of grind of the salt, the balance used, and measurement technique, to name a few. But if we can get an idea of the limits of error, we might be able to measure salt well enough to make standards accurate to maybe ten percent, for instance. If we find out a tsp delivers 5 grams of salt, then a teaspoon in a liter of water (about 4 cups) would be 5000 ppm. Dilute that by a factor of 100 and you'd get 50 ppm, which is a useful concentration for a standard. Prepare a few dilutions of *that* and we could get an idea of the linearity and sensitivity of the TDS meter at low ppm. Anyway, if this all proves ridiculously inaccurate, then we just have to bite the bullet and buy some kind of balance, or buy the Potassium Chloride calibration standard packets Hanna sells. I've seen 1/40th gram resolution pocket balances for as little as 30 bucks (US). The calibration packets cost only $15 for a pack of 25, which isn't really that much, though I don't know how often you'd need to use them... Let me know if you can help. I'd like to think this is one small step that will help us towards standardizing things. Thanks. Mike D. [Mike Devour, Citizen, Patriot, Libertarian] [[email protected] ] [Speaking only for myself... ] -- The silver-list is a moderated forum for discussion of colloidal silver. To join or quit silver-list or silver-digest send an e-mail message to: [email protected] -or- [email protected] with the word subscribe or unsubscribe in the subject: line. To post, address your message to: [email protected] List maintainer: Mike Devour <[email protected]>
