TDS or Conductivity meter confusion

[GBryant890]

I want to get a meter to measure the purity of my RO water. Which would be better suited to the task? The TDS meter and the conductivity meter measure different things, but do they both relate to water purity? The TDS meter is a lot cheaper. So, based on that, I'm leaning toward getting the TDS meter. Are there other uses for the conductivity meter that would justify my getting it instead?

 

[taikonaut]

Post the specs of the conductivity meter and the TDS meter. Both use the same technology (conductivity), but give you different unit (uS or ppm). You can convert from one to another, so which one is better depends on the specs.

I am leaning toward a conductivity meter...

 

[GBryant890]

I guess you're right. On closer inspection, the price difference between the meters I'm looking at seem to be based on quality of construction rather than the cost of technology.
I was looking at the Pinpoint Conductivity meter and the Sharp TDS meter. The Pinpoint has an accuracy of ±1% and a range of 0-1,999 uS and 0-20,000 uS. While the Sharp has an accuracy of ±2% and a range of 0 - 19,99 mS (this looks like a typo on the website.). But what tipped me off to the quality difference was the Sharp Conductivity meter (which I hadn't looked at) had the same price and accuracy as the Sharp TDS meter. Both of the Sharp meters give an accuracy of ±2% full scale. I found them on the Foster & Smith site.
http://www.drsfostersmith.com/produ...atid=4527&Ne=40000&R=12076&Nao=18&N=2004+2022
For comparison, the Poinpoint conductivity meter is here.
http://www.drsfostersmith.com/produ...atid=4485&Ne=40000&R=12250&Nao=18&N=2004+2022
So, having done a little more research and answering my own question about the uses of a conductivity meter, I see why you're leaning towards the conductivity meter. Sincere thanks for the KITB to get the research done before posting Taikonaut.
Now the question is this... Based on the accuracy difference, is the Pinpoint meter really that much better than the Sharp meter? Are there any other meters that are comparable in price to Pinpoint?

 

[taikonaut]

There are several meters out there, but pinpoint seems to be one of the most popular ones. Check out some of the sponsors of RDO and see if any of them offers good deal.

As far as the accuracy/precision, I am going to say that there is little difference between 1% and 2%. There are several things that I want to you be aware of:

1, most accuracy is full scale deviation, not the reading that you see. So if you see 10 uS, and the accuracy is ± 1 %, is is not 10±0.1 uS, but rather, 10±(1999uS*0.01) or 10±19uS. You really need to read the specs carefully.

2, most TDS meter will give you only one digit of readout when you use it on RO water. My RO reading is 13ppm, for example. For RO/DI, almost all TDS meter is useless when it comes to monitoring the DI resin, since all you see is "0" ppm (underrange reading, not that it is perfectly pure).

3, fortunately, most of the hobbyists were able to make good use of TDS/conductivity meter despite the short comings mentioned above, but I just want people to be aware of them.