Not all TDS Meters are Created Equal

[Hanna Instruments]

Not all TDS meters are created equal. TDS meters are actually measuring electrical conductivity or EC. When using a TDS meter for your RODI system it's important that the range is narrow, as accuracy is often based off a percentage of the maximum detection limit. Our HI98308 Pure Water Tester is specifically designed for ultra pure water, having a maximum range of 99.9uS/cm EC or approximately 50 ppm TDS with and accuracy is +/- 2% f.s.

Most TDS meters used in the reef market have a range up to 999ppm with similar accuracy. If your TDS meter is reading 0ppm it could be as high as 20ppm if your meter has that high of a range. In addition over time RODI reservoirs can become contaminated with dissolved ionic particles, its best practice to measure the reservoir and not just the output water. Measured in this photo is our HI70436 Deionized Water which displayed a result of 00.5uS/cm which is less than 1ppm TDS.

This water is great for the HI758 Calcium Checker and for cleaning your cuvettes. Visit our website, hannainst.com, and get your HI98308 PWT and HI70436 Deionized Water today to start testing with confidence

This meter features automatic temperature compensation and has the ability to be manually calibrated using 84 µS/cm Conductivity Standard

 

[jsker]

Awesome

 

[chipmunkofdoom2]

Hi @Hanna Instruments, you mention above that you offer an 84 µS/cm calibration standard. Could you tell me roughly what this equates to in ppm TDS at 25*C? I would like to calibrate my TDS meter with this solution, however my conductivity meter only lets me calibrate in ppm TDS.

 

[Hanna Instruments]

Hi @Hanna Instruments, you mention above that you offer an 84 µS/cm calibration standard. Could you tell me roughly what this equates to in ppm TDS at 25*C? I would like to calibrate my TDS meter with this solution, however my conductivity meter only lets me calibrate in ppm TDS.

Depends on your meter but most TDS meters in the reef market use a 0.5 conversion factor. Meaning the 84µS/cm solution would equate to approx. 42ppm at 77F.

Temperature is key when calibrating so make sure you have a thermometer and use the chart on the outside of the bottle to compensate for variations in conductivity produced by temperature.

 

[chipmunkofdoom2]

Depends on your meter but most TDS meters in the reef market use a 0.5 conversion factor. Meaning the 84µS/cm solution would equate to approx. 42ppm at 77F.

Temperature is key when calibrating so make sure you have a thermometer and use the chart on the outside of the bottle to compensate for variations in conductivity produced by temperature.

Thanks for the info. I have an HM COM-100, which has an integrated thermometer. So temperature correction should be simple.

So let's say I assume a conversion factor of 0.5, which would mean the 84 µS/cm standard would measure 42ppm at 25*C. After I calibrate the meter at 42 ppm TDS, how can I verify that the 0.5 conversion factor is correct? If I measure a standard of a known conductivity, say 342 ppm TDS, should the meter accurately read 342 ppm TDS?

 

[Hanna Instruments]

http://hmdigital.com/product/com-100/

according to their website the meter has a user selectable conversion factor of 0.5 or 0.7.

also you can switch to EC Mode

http://hmdigital.com/wp-content/uploads/2016/11/com100manual.pdf

 

[Hanna Instruments]

Thanks for the info. I have an HM COM-100, which has an integrated thermometer. So temperature correction should be simple.

So let's say I assume a conversion factor of 0.5, which would mean the 84 µS/cm standard would measure 42ppm at 25*C. After I calibrate the meter at 42 ppm TDS, how can I verify that the 0.5 conversion factor is correct? If I measure a standard of a known conductivity, say 342 ppm TDS, should the meter accurately read 342 ppm TDS?

the specifications say 2% which usually means full scale unless they say of the reading. SO assume your deviation would be 2% of the maximum range set, you would want to select the lowest range the meter has available.

 

[chipmunkofdoom2]

Well after a good bit of reading and research (and learning more about conductivity standards and measurement than I think I ever wanted to know), I think I have this all figured out.

Part of my trouble was that I was under the mistaken assumption that the COM-100 couldn't calibrate when reading in EC mode. I was testing my deionized water and thought I should see if it would calibrate in EC mode. I got a calibration error and just assumed that it would not calibrate in EC mode. Turns out the error was due to the conductivity being so low (0.7 µS/cm). When I attempt to calibrate the unit in a 342ppm (700µS/cm) standard, I am able to calibrate using EC. So in fact, it can calibrate in EC mode. There's just clearly a threshold below which this meter won't calibrate, be it in EC or ppm TDS. I'm not sure what that is. I'm going to dilute a 342/700µS/cm standard to around 80 µS/cm to ensure my device can be calibrated with your standard before ordering.

Thanks again for all your help. I really appreciate you taking the time, especially since it's not a device that Hanna manufactured.

 

[chipmunkofdoom2]

Okay, so it looks like the COM-100 can be calibrated in EC mode using a conductivity standard of around 84 µS.

I had a 700 µS conductivity standard on hand, and my DI water is around 0.7 µS, according to the COM-100. I calculated I would need about 80 mL of 0.7 µS DI water and 10ml of the conductivity standard to get an ending conductivity of around 84 µS. After the dilution, the COM-100 read right around 83 µS. Better still, the meter is able to calibrate at this low level of conductivity.

TL;DR: if you have a COM-100 and want an affordable, low conductivity standard in which you can calibrate your meter, it appears Hanna's 84 µS standard will work just fine.

Thanks again for the help @Hanna Instruments

 

[Muffin87]

Temperature is key when calibrating so make sure you have a thermometer and use the chart on the outside of the bottle to compensate for variations in conductivity produced by temperature.

Do you need to compensate for temperature when calibrating the hanna PWT as well?

Both the videos posted on youtube by hanna don't mention compensating for temperature when calibrating the hanna PWT.
They just say to put the meter in the standard solution and use the screwdriver to adjust the value displayed by the meter to 84.

Could you explain in detail the calibrating procedure for the hanna pwt please?

Thank you