TDS meter

Total dissolved solids TDS meter is an electronic device measuring electrical conductivity of water. Pure water has low conductivity and water with dissolved inorganic ions has conductivity higher. Organic contaminants, such as from fish and plants don’t have much impact on TDS readings. It is the inorganic ions from fertilizers, rocks and substrate that increase TDS values.

FAQ

Increased conductivity reads as lower resistance?

Yes. To make readings easier to understand the value is calculated as 1 / resistance. Resistance comes in Ohm and conductivity in Siemens. A TDS meter gives readings as conductivity in Siemens S. Purer water registers as smaller value.

What readings should I expect?

Pure water readings come as lower number and contaminated water as higher number. Reading examples:

0.01 µS DI cati / ani ion exchanger demineralized water

0.1 µS distilled water

1.0 µS good RO reverse osmosis water

10 µS regular RO reverse osmosis water

100 µS very soft water

300 µS medium hardness water

600 µS hard water

1200 µS very hard water

Why do I need a TDS meter and where to get one?

TDS meters are not like pH testers which are costly, require periodic calibrations to maintain accuracy and have a limited lifespan.

TDS meters cost as little as $5 on eBay and function forever. Readings are easy and take only seconds. Calibration is not needed because these meters are used as comparison tools. The main objective is to compare the difference between two liquids. One is the source water and the other is aquarium water. Greater the difference, more dissolved substances aquarium water contains. In order to maintain consistent water parameters this differential value will indicate when and how large water changes are needed.

TDS meters do not read single mineral details, though they read what test kits cannot. Once you get to know the routine you will not need any test kits.

TDS meters are manufactured for specific applications. Usually they can read kitchen salt NaCl ppm, KCl ppm, “442 ppm” or µS. Laboratories and water treatment plants use mostly µS. General public stores carry unit calibrated for NaCl ppm readings. Please see conversion table below.

What is the practical use?

What we are going to see in a newly setup aquarium is a gradual increase in water conductivity over time so we can determine the overall water deterioration and take action before problems become visible. For example, one PPS-Pro dose increases TDS 6 µS, at the end of the day the 6 µS dose is removed by plant uptake. Another example is some substrates dissolve minerals so by water changes we remove the extra elements before water becomes too problematic for plants. Basically the TDS meter is a maintenance tool.

Can I use test kits instead?

Yes, but TDS meter is cheaper, easier, and very fast in seconds and works for ever.

Can TDS meter replace test kits?

Yes and no, it depends on what the final objective is. If we need to locate an exact problem (KH, GH, Ca, Mg) we need to use test kits. Once we know what the long term repeated routine is we can use the TDS meter to simplify the process.

My TDS meter uses KCl ppm, NaCl ppm, 442 ppm, what is it?

The true TDS reading is micro Siemens µS. Some companies make TDS meters calibrated in equivalents of specific salt concentrations. Let’s say x amount of NaCl in y amount of water gives z value of ppm. This makes it difficult to compare the values with other TDS meters, but because the principle is the same among all TDS meters you can still use it to compare the overall water deterioration rate.

How to calibrate or verify TDS & Conductivity meters?

Calibration & verification of TDS and Conductivity meters can be done with kitchen salt.

Read RO water

Dissolve 3.0 grams of kitchen salt in 1000 ml RO

Take 100 ml

Add 900 ml RO water into the 100 ml

Read salt solution

Result = salt solution – RO water

Expected readings

300 ppm as NaCl

318 ppm as KCl

419 ppm as “442”

614 µS

Test can be done with kitchen iodized salt, pickling salt or sea salt dissolved in distilled or RO water.

How do I convert ppm TDS to µS TDS?

The conversion factors may not be linear, only estimated and dependent on the actual TDS meter mode and manufacturer.

TDS meter mode:

NaCl 1.47 to 1.50 [ppm], (1ppm or 1mg / L table salt)

442 1.65 to 1.85 [ppm], (442™ Natural water, 40% sodium sulfate, 40% sodium bicarbonate, and 20% sodium chloride.)

KCl 1.50 to 1.57 [ppm]

µS TDS ≈ ppm TDS * 1.50 (for NaCl mode)

Example

150 µS TDS ≈ 100 ppm TDS * 1.50

Where can I read more about it?

There is an excellent article at http://reefkeeping.com/issues/2004-04/rhf/feature/index.php

How do I use a TDS Tester with PPS-Pro?

This is the best combo. The TDS tester will tell you when and how large water changes are needed to maintain water column consistency.

This is how it works. RO water is ~10µS, one daily PPS-Pro #1 dose is 1 ppm of NO3 representing ~6µS. So, to limit NO3 to 15 ppm, we need to make sure that the water column TDS reading is not more than 100µS (10+15*6=100). If it is more than we need to change water to bring the TDS reading back to 100µS.

For tap water, let’s say 300µS tap, we have 300+15*6=390µS limit. So if the aquarium water column is more than 390µS we need to change water to bring it back to 390µS.

What if my TDS meter reads ppm instead of µS?

There is a variety of TDS meter modes and units, KCl ppm, “442” ppm, NaCl ppm, µS microS and converting between them is impractical. In order to overcome this we can use the following,

Take three readings,

tap

tap with 10 drops of solution #1 macros in 1000 ml tap

aquarium

then,

water change IF [ aquarium ] > [ tap with 10 drops of solution #1 macros in 1000 ml ]

The first two readings will not change, so only one reading will be needed to determine water changes. The aquarium readings.

How do I find daily TDS increase due to PPS-Pro dose?

If you want to know the daily fertilizer TDS addition, then add 10 drops of PPS-Pro macros and 1 drop of PPS-Pro micros in 1000 ml water. Then,

TDS daily fertilizer = ( TDS 1000 ml water with fertilizers – TDS 1000 ml water without fertilizers ) / 20

This is for solution #1 macros at 1 ml per 10 gallon or 40L and #2 micros at 1/10th of 1 ml per 10 gallon or 40L daily addition of 1 ppm NO3, 0.1 ppm PO4, 1.33 ppm K, 0.1 ppm Mg and 0.01 ppm TE(Fe).

Additionally, this option applies to µS and ppm of any model.