Specific gravity and conductivity standard solutions.

[AUSpool]

I had a HM Digital TDS conductivity meter that I was happy with but after it went for a swim I replaced it with a cheep eBay TDS meter and wanted to check its accuracy. I got a reading of 3780ppm and from two different LPS's I got 4100ppm and 5750ppm as ppm or mg/L NaCl. I suspect the 5750ppm reading is suspect due to anything from a poor sample to a lack of calibration. My Waterco swing arm hydrometer is giving a reading of just over 4000ppm or ~1.0028s.g. Given my target salt level is 6000ppm, neither of the LPS's recommended adding any salt.

Salinity is a little different to most other pool parameters as we tend add an entire bag of salt instead of adjusting to a specific level. One 20kg bag of salt raises the salinity in my 29kL pool by ~570ppm of NaCl. This makes me think that salinity readings don't need to be super accurate but somewhere close would be nice, either the 5750ppm reading from before or the other two readings are way out and it makes me wonder about the other results. And so the search was on for a standard solution and preferably something I could make at home.

I kept a reef aquarium for many years and recall an article by Randy Holmes-Farley that describes the methods for preparing home made standard solutions for refractometers, hydrometers and TDS or conductivity meters. Given the quite low and tight range for pool salinity I think refractometers and the method using standard measures, cups, teaspoons and 2L Coke bottles is a bit useless. The average kitchen scale is not accurate enough for my purpose but I was able to get a half decent scale from eBay for under $10. Natural sea water and the article from Randy Holmes-Farley target a salinity of S=35, 35ppt or 35,000ppm. A dilution factor or 7 neatly converts the salinity to S=5, 5ppt or 5000ppm which I think is ideal.

The original article is found online hear;
Reef Aquarium Salinity: Homemade Calibration Standards by Randy Holmes-Farley - Reefkeeping.com

Without getting into specifics (refer to the article) conductivity and specific gravity standards are a bit different and not interchangeable.

Demineralised or distilled water would be the best and whilst it does cost a bit more than tap water I can get a enough for both solutions for around $5. My tap water has a TDS reading of ~90ppm so I could make a 5090ppm standard but I'll opt for demineralised water.

Using standard iodised table salt the recipes are;

Conductivity: for 5000ppm, 6.2gms of NaCL* in 1,274gms H2O^

Specific gravity: for 5000ppm, 6.2gms of NaCl* in 1,127gms H2O^

Where the conductivity standard is used for a TDS or conductivity meter and the specific gravity standard is used for a hydrometer.


*Iodised table salt is used for NaCl.
^For water (H2O) mgs = mls at 25degC.

 

[Patrick_B]

Ok, a decent meter will be more than accurate enough for pools especially, and good standards are not that expensive if you prefer going this route. You can pick up a very good handheld for about $125 USD, a bottle of standard for about 15 and it will be more than adequate for pool testing.

 

[JoyfulNoise]

Question - is the Taylor K-1766 test kit (chloride test) not available in Australia or just too expensive?


Sent from my iPhone using Tapatalk,16k gal SWG pool (All Pentair), QuadDE100 Filter, Taylor K-2006

 

[AUSpool]

Ok, a decent meter will be more than accurate enough for pools especially, and good standards are not that expensive if you prefer going this route. You can pick up a very good handheld for about $125 USD, a bottle of standard for about 15 and it will be more than adequate for pool testing.

Geday Patrick,

$125 is not excessive for a half decent meter, my old HM that I was quite happy with was less than ~$20 but my real problem is finding a standard that's even close to my target range. I had a bottle of 3000ppm NaCl that I got from eseason gear a few years back which was ok to verify a hydrometer but not suitable to accurately calibrate a TDS meter and shipping from the US does add to the cost.

I just can't find anything over hear either online or through a local store. Which is why the idea of a home made standard is so appealing to me. I can easily make a standard suitable for either specific gravity or conductivity that is close to my target range and easily accurate enough for pools.

Question - is the Taylor K-1766 test kit (chloride test) not available in Australia or just too expensive?




Sent from my iPhone using Tapatalk,16k gal SWG pool (All Pentair), QuadDE100 Filter, Taylor K-2006

Geday Matt,


I don't know, I never seen a chloride test kit like the K-1766 either by Taylor or any another brand but I've never really looked for it. One of my LPS's uses and can supply the Taylor kits but I haven't enquired yet. The Clear Choice Labs kits which we have over hear supply the Watermaid hydrometer which I have but don't really use. I do like the TDS meters though, one dip and it's done but without calibration or verification all you get is numbers which is why I suspect some of the LPS's with expensive meters come up with some huge variations in values.


Cheers Steve.

 

[JoyfulNoise]

The chloride test is based on an argentometric method (also know as Mohr's Method) of measuring the chloride ion (Cl-) concentration. It uses a silver nitrate titrant to precipitate the chloride out as silver chloride and a potassium chromate solution as the end-point indicator. The Taylor test kit is accurate to within 200ppm. It is superior to other methods in that it directly measures chloride which is what matters for an SWG. Hydrometers and TDS (EC) probes only give you an estimate of chloride as conductivity and specific gravity are influenced by other parameters aside from sodium chloride concentration.

However, some countries have strict import regulations that make getting the K-1766 either too expensive or not at all possible. So thanks for the info on the standards used for EC and Hydrometers.


Sent from my iPhone using Tapatalk,16k gal SWG pool (All Pentair), QuadDE100 Filter, Taylor K-2006

 

[Patrick_B]

Get a myronL handheld TDS pen, calibrate it to 3000, and it will be so close, the difference if any, won't amount to a knat's whisker in terms of pool testing needs. I can calibrate mine to 1000, read 5000 and it's more than satisfactory for testing critical parameters in very important systems costing multiple 10's of millions of dollars. Nothing wrong with being accurate, and I understand the fun of tinkering with and tweaking things, but for a practical solution, there is your answer. If you aren't just toying around with this, you are over-thinking it to the power of 100.

 

[AUSpool]

Nice TDS pen Patrick but I think it may be a little over my budget. I found an AU online supplier but they don't list prices, I guess if I have to ask I can't afford it. I can get another HM 4TM (+/-2% accuracy) delivered for AU$35 - good enough for me. I found a 4000ppm NaCl standard for $9 plus postage from Clear Choice Labs (CCL). Happy days, but I still want to make those standards when my new scales arrive and I can verify them from the standard from CCL.

I like tinkering and with a history of marine aquariums where accuracy can be quite important I do tend to over think things. I once wrote 'near enough is not good enough, we share 95% of our DNA with the common fruit fly - it's the last 5% that makes all the difference'. But I still can't get over the variation in salinity results, 3780ppm (cheap eBay TDS meter-factory calibrated), 4100ppm (LPS), and 5750ppm (LPS). And for what it's worth I think form memory my Watermaid hydrometer gave a reading of just under 4000ppm.

I'm always going on about how important calibration is but I never calibrated my old HM meter thinking it didn't need it since it was factory calibrated - to 342ppm NaCl, nowhere near my range. And until this morning I hadn't read the hydrometers disclaimer. The instructions just say to fill to the level and read but hydrometers are not auto temperature compensating. The disclaimer says '+/-400ppm per +/-1degC' and that it has been calibrated to a temperature which is engraved on the bottom, mine is 20 - who swims in a pool at 20degC? And +/-400ppm, that's almost an entire bag of salt for me. The ones CCL sell are calibrated to 25degC and they do instruct to compensate for temperature.

Using the hydrometer this morning I got a reading of 4200ppm @ 25.6degC. After compensating for temp from 20degC, that's 6400ppm. So the previous reading at 5750ppm is quit plausible. Now I'm really confused...

Going back to Matt's first question I came across a bit on CCL site this morning that says a highly accurate drop tests are available but can't be shipped via our post. A lot of our regular postal services use regular passenger aircraft.

Steve.

 

[JoyfulNoise]

Going back to Matt's first question I came across a bit on CCL site this morning that says a highly accurate drop tests are available but can't be shipped via our post. A lot of our regular postal services use regular passenger aircraft.

That makes a lot of sense then. In the USA, chemicals must be shipped by ground transportation as well and I believe only the USPS has the legal waivers needed to ship "pool" chemicals (including test kits). But, the US also has a very extensive network of ground transportation routes and our history is much influenced by the excellence of our postal service (one of only a few government agencies explicitly authorized by the US Constitution). I am often surprised at how quickly and easily my test kit refills get to me from sources located 3000 miles away on the other side of the continent.

One thing that bugged me about CCL is that, in order to hype their sale of hydrometers, they resorted to some fairly infantile and completely transparent scare-mongering about the chemicals in the salt test kit. Yes, potassium chromate contains hexavalent chromium and it is a carcinogen, but, for heaven's sake, does anyone intend to drink their pool chemicals? There really was no reason to go to that level so, while I'm happy you folks in Oz have a local source of decent test kits, their salesmanship, in my opinion, rides only slightly above pool store grade knowledge.

I hope you have an enjoyable swim season!! My pool water is current 50F (10C for you poor souls still clinging to the metric system ) and all I can do is look at the water and appreciate its gleam...

Cheers,
Matt

 

[AUSpool]

Thanks Matt,

I love the US postal system, last year, hang on, the before last now, I bought a few pairs of skis and watched them as they made their way across the states with an update every time they went in and out of a distribution center. We have an extensive road network too but I don't know how they decide what is shipped by which method.

I've never relied on my hydrometer and I thought it might be interesting to compare a sample at 25C to one chilled to 20C to check the temp compensation figures.


Happy new year America,

Steve.

 

[AUSpool]

I tested my salinity with the hydrometer last night at two temperatures;

@20degC - ~3500ppm
@25degC - ~4500ppm

Thats +\-200ppm per degC which is half the published variance.

In in my opinion the hydrometer is a bit useless, I'm not prepared to rely on it.

The NaCl standard I ordered has a +\- 100ppm degree of error which was a bit of a surprise. I haven't attempted to calculate the error for the home made solutions but I'll give it a go when a make them. I'm still waiting for my new TDS meter and need to source a half decent calibration solution.