Conductivity or TDS

[tjudy]

I went over and read the posts Matt. I am pretty sure that the point about TDS meters using conductivity to estimate TDS is probably accurate. I am going to call the folks over at Hanna and get the scoop on that tomorrow. I now how to phrase the questions now... 8)

Conductivity measures exactly that... the ability for the aquarium 'solution' to conduct electricity. What does that is the ions in the water. No ions... no conductivity. That is pretty clear, and it is also clear how to measure it.

TDS, from the definition that will most likely be presented to me by Hanna, will not include carbonates because those substances do not dissolve (break into ions) in water. I think that the big confusion all along has been the difference between water HARDNESS and TDS... they are not the same thing (though... I guess common sense would say they are :? ).

HARDNESS is defined as the level of carbonates in the water, and results in water have a high alkaline buffering capacity. Since carbonates will not show up on an EC meter, then they are likely not included in the TDS reading of the meter.

I know that this is different from what I posted before, because I have always considered carbonate hardness to be a part of total hardness. I think that the answer I am going to get tomorrow is that carbonates do not count in TDS....

So what do we need in a meter? I want a meter that allows me to reconstitute r/o water accurately. I personally an not overly concerned with test tap water... I know that mine is pretty nasty 8O. I will want to reconstitute in such a way that the pH will be low, but not crash. I can use a small amount of Alkalinity Buffer, then adjust the pH using a good meter and Acid Buffer. Since I am starting with r/o, I know what is going in as far as carbonates are concerned. The meter will tell me when adjustments are necessary. I also want the EC to keep track of ions that inevitably build up in water. I can also use it to make sure my r/o-DI equipment is working well. I can also use it to reconsitute up to specific TDS levels using salts.

One thing that I have discovered during this search for meters is that there are A LOT of meter options. However, not all meters are created equal. I have chosen the Hanna meters (combo pH/EC/TDS/temp) because it is comparably prices to single parameter meters of the same quality.

The Hanna pHep4 pH meter uses the replacable electrode technology, is waterproof, has a simple automatic calibration and will automatically adjust for temperature... it retails for about $70.

The Hanna EC meter with the same bells and whistles is also about $70; as is the TDS meter (though we have pretty much figured out that TDS and EC are really measuring the same thing and converting).

The combo meter has the same features as all three single parameter meters for a retail cost of about $130. Even if yo only wanted EC or TDS, not both, the combo meter at $130 is a better buy than $140 for two separate meters.

I will let everyone know what Hanna says about the meters and carbonate hardness tomorrow sometime.... but my money is on the 'carbonates do not count in TDS' answer. Even money...any takers :wink: .

 

[tjudy]

I got caught up in that last post, I forgot to describe the test that my students and I are going to do on this Hanna combo meter when it gets here (HOPEFULLY tomorrow... :x ).

I had some of my better chem students use R/O-DI water (which we test regularly with our bench-top pH meter and TDS kits here in the lab) make solution samples using 'aquarium salt' and pH buffers. We are going to run the meters through a series of tests, including heating up and cooling down the samples to make sure that the automatic temperature calibration is accurate.

I think that I will set up another battery of tests using calcium carbonate and sodium bicarbonate solutions; first by themselves and then in addition to 'aquarium salt' solution. That might answer the question about whether these meters measure carbonates as TDS once and for all .

 

[farm41]

I am glad you are going to get to the bottom of it. If they are both measuring EC then it wouldn't make any difference which to buy. Although I am interested in the all in one meter.

 

[tjudy]

OK... still no meters today.. :evil: ...

I did have a converstaion with Hanna about TDS vs. EC. Their combo meter uses a conversion factor to conver EC to TDS; however, the combo meter does permit that conversion to be adjusted from .45 to 1.00. That means that you can set it to 1.00 if you are reconstituting r/o without carbonates, and the TDS reading should be right on.

I am going to call back earlier in the day tomorrow and get a techie on the line. I was only able to get a sales rep today.

I asked specifically about testing for carbonate hardness. They are going to send me he information on their kits for that. They have a bench top meter that uses a spectrophotometer to measure color change in a reagent tested water sample. The meter can apparently be used to measure all sorts of stuff, but reagents must be used. It will also convert units automatically between English, French and German unit systems. I have used spectrophotometers for ammonia assays on lake water samples. They can be very accurate, but error can result if reagents are not added correctly.

More tomorrow... I hope...

 

[tjudy]

Here is the technical answer from Hanna instruments regarding EC/TDS and carbonate hardness.

I won the bet.... ... carbonate hardness is NOT considered to be a part of TDS, because calcium carbonate does not dissolve (defined as a disassociation of ions in solution).

These meters (the combo meter by Hanna) uses a conversion factor to change EC, (measured in microsiemens), to TDS (measured in ppm or ppt). Which should you use? The techie's answer was... and I kid you not... 'whichever you were raised with'... :lol:

End of debate... EC and TDS are the same thing. The fact that the conversion factor is adjustable only matters if you are reconsituting or using specific solutions. The techie said that the standard conversion for tap water is 0.5. This means that if the EC measures 1000 microsiemens, then the TDS reading will be 500 ppm.

THERE IS NO METER FOR CARBONATE HARDNESS. ANY METER THAT ADVERTISES SUCH IS USING AN AVERAGE BASED UPON AN EC READING

The only way to measure carbonate hardness is with a reagent test. Even the expensive systems that use a spectophotometer to measure minute color changes require the reagents.

I hope that this clears things up. I think that the big issue is the buffering capacity of alkaline (high carbonate) water. I am going to start a new thread with a question about that.

OH!!!! My new meters came today!!!!

I bought an extra of each.. one Combo and one pHep4 (pH)... if anyone is interested.

 

[Neil]

Ted,

My new meters came today!!!!

Great! Make sure you let us know how you like it and if it seems like a worthwhile piece of equipment. I am very interested and will probably get the extra that you have , if no-one beats me to it.
Are there different solutions for calibrating the pH and TDS?
Neil

 

[tjudy]

The pH is a two point calibration. I have sachets for that. The sachets for calibrating the TDS/EC are on the way. I guess they were back ordered or something.

 

[tjudy]

WE HAD SOME FUN TODAY!!!!

My students and I put these Hanna meters through their paces. I think that they performed very well.

First we calibrated the units.... SIMPLICITY ITSELF!!! pH is a two point calibration with the LCD screeen prompting you when to place the meter into each solution, with a nice clear 'OK' when the calibration is done. The EC is even faster... only one point. We had the meters out of the box, instructions read, and calibrated in less than 15 minutes (we are talking sophomores in high school here).

Here is the test protocol we decided upon:

1) Narrow pH range increments - we set up ten stock solutions for pH ranging from pH 4 to pH 8.5 at .5 pH increments. We used good r/0 water (pH 6.89 on the meter) as a rinse. Then we rinsed in the rinse, then rinsed in a sample of test solution, then measured a sample. We repeated for each sample. Result - no problems. pH reading were within the +/- .05 pH range advertised.

2) Narrow EC/TDS range increments - we set up stock solutions using aquarium salt and r/o water at TDS values of 25 ppm to 475 ppm at 50 ppm increments. At least we tried to. All I can say is that the solution increments should have at least been even based upon the math. The measurment of solutes may have been in error though. We basically ran the same test as before, and the rsults were good. We were off a bit more than the advertised +/- 2%, closer to 3.5%, but I thnk that human error was a problem. The meter was set to a .5 conversion for EC to TDS, and the meter can do the math. If the TDS reading was 150 ppm, the EC reading was 300 microsiemens.

3) Calcium carbonate and sodium bicarbonate - we created a few solutions with lab grade chemicals (not reagent, but better than Arm and Hammer). The r/o water we used measured 40 microsiemans (time to change the DI resin I think), but the tap that the r/o is filtering measured nearly 1800!!!! The EC of the carbonate solutions increased slightly. Less than 5 microseimans per gram of carbonate placed in 100 ml or water. This could easily be due to impurities in the carbonate powders. The pH of these carbonate solutions increased greatly.. up to 8.3 to 8.8.

4) Calibration/Accuracy test - the instructions say to calibrate every 35 tests or so. We decided to give the meters a hard run. We used a 6.0 pH solution and a 7.0 pH rinse. We turned the meter on, calibrated, tested, rinsed, tested, rinsed, tested, rinsed... etc. Until the pH reading fell of the +/- .05 pH range. After 40 tests that class' bell rang and the kids went on vacation. The meter was still testing within parameters.

5) The on/off test - I repeated the above test after school for pH and for EC, but this time I turned the meter off and on again before testing. The pH went to 40 tests without a need for calibration (then I gave up and went on the EC). The EC passed the +/- 2% from the average reading(went to 2.5% for three straight tests) after 32 tests. Not too bad.

Over all, I like the meter. It is very easy to use, accurate and a lot of fun. I brought it home and learned a lot about my tanks. I learned that my Apistogramma panduro pair are in a tank with a tank that is getting 1/2 r/o and 1/2 tap, and the pH is still up at 7.5 and EC is at 750!!! I am going to have to go to almost pure r/o!!! Obviously my tap has some excellent alkaline buffering capabilites.

I also learned that tannins drive up the EC. I have another tank that gets the same mix as the panduro tank, but there is a major tannin producing piece of mesquite wood in there. The EC is at 930!!! I am going to have my students do a more scientific test of tannins and EC.

I have another combo meter if anyone wants it... :wink: I also have two pH meters... one is very, very slightly used.... but at least you know it works !