TDS testing

From Brewing Forward

TDS stands for Total Dissolved Solids. It's an indicator of the overall mineral content of water; it does not provide information about the individual minerals present.[1] TDS testing is mainly useful for monitoring the effectiveness of a RO purification system (whether the system is at home or in a store). It can also useful for monitoring changes in tap water because mineral levels can vary over time.[2][3][4][5] For tap water, a water report is generally required to properly adjust it for brewing since the quantities of specific minerals needs to be known. TDS is usually expressed as parts-per-million (ppm), which is practically the same as milligrams per liter (mg/L).[6]

TDS testing is conducted with a TDS meter, which is easy to use. It works indirectly: Dissolved salts (electrolytes) increase the electrical conductivity (EC) of water.[6] A TDS meter measures the EC, and then uses an equation to estimate the amount of dissolved solids.[5][6][3][7] This estimation is adequate for the purpose of monitoring a RO system, and determining whether the RO water is appropriately low in minerals to be used as a base for building a mineral profile.

TDS meters[edit]

There are a variety of TDS meters available. The cheaper options tend to lack important features, and will likely suffer from poor accuracy.

Handheld TDS meters on Amazon
Product Dumsamker Inkbird HM AP-1 Bluelab Extech EC150 Apera EC60
TDS range 0-9990 ppm ±2% 0-4995 ppm ±2% 0-5000 ppm ±2% 0-5000 ppm ±2% Variable range ±1% Variable range ±1%
ATC1 No Yes Yes Yes Yes Yes
Calibration No No Yes Yes Yes Yes
EC display Yes (μS/cm) Yes (μS/cm) No Yes (mS/cm) Yes (µS/cm or mS/cm) Yes (µS/cm or mS/cm)
Backlight Yes Yes No Yes No Yes
Waterproof No No No Yes Yes No
Notes Battery is not replaceable. Dumsaker provides zero customer service; seller doesn't respond to messages. This meter is marketed under other cheapo brands as well. None Digital 1-point calibration with 342 ppm NaCl. Magnetic body. Digital 1-point calibration with EC 2.77 (2770 μS/cm). Displays icon for 30 days after calibration. Low battery indicator. Digital 1-point calibration with any calibration solution. Dual-range TDS & EC, which improves accuracy (can read TDS up to 19,990). Customizable TDS conversion factor. Adjustable ATC settings. Low battery indicator. Probe requires soaking before reading if it hasn't been used in a month. Digital calibration (up to 3-point with 1413 µS, 12.88 mS, 84 µS). The 1413 µS & 12.88 mS calibration solutions are included. Tri-range TDS & EC, which improves accuracy (can read TDS up to 10,000). Fancy platinum black conductivity sensor improves accuracy and performance. Customizable TDS conversion factor. Adjustable ATC settings. Replaceable probe.

1 ATC is Automatic Temperature Compensation. Conductivity is affected by temperature, and meters with ATC automatically adjust the reading to provide a standardized value (25°C). If the meter does not have ATC, the sample must be at 25°C (77°F) in order to provide an accurate reading.

All of these meters also feature:

  • Temperature display
  • Hold function
  • Automatic power-off


Inline TDS probes for monitoring input and output of RO systems exist, but there's no reason to believe they are accurate or worthwhile.

Usage[edit]

Using a TDS meter is simple:

  1. Let the system run for a sufficient period of time for the TDS to stabilize before taking a sample (typically around 1–2 minutes).
  2. Put the meter's probe into the sample, swirl gently to remove bubbles, and wait for the reading to stabilize.
    • If you use a sample container, it needs to be clean and free of any mineral contamination.[8] You can rinse the sample container two or three times with the water you intend to sample.[6]
    • If your meter does not have ATC, the sample must be at 77°F (25°C).
  3. After use, rinse the probe with purified water and/or dry it with a clean cloth.

Calibration[edit]

TDS meter reading 20ppm
Reading a 20ppm test solution to verify accuracy

It's always wise to check that a measuring device is actually working as expected rather than taking the results just on faith.[6] TDS meters are known to drift and give inaccurate readings. Even if your meter isn't able to be calibrated, you can check the reading against a solution with known conductivity (a calibration solution) to verify that it is giving accurate results. Generally you should use a calibration solution with TDS near the range that you want to measure, although some meters require a specific TDS calibration fluid.

Buying a calibration solution

Calibration solutions are available to purchase, although options are limited. Here are a few:

  • 84 μS/cm solution available on Amazon
  • 100 μS/cm and 1413 μS/cm solutions available on Amazon
  • 700 μS/cm solution available on Amazon
Making a calibration solution

You can make a calibration/testing solution yourself. For monitoring RO systems, we suggest using 18.7 mg/L NaCl (sodium chloride, table salt), which reads as 20 ppm TDS or 40 μS/cm.[1]

  1. Using a high resolution scale, carefully weigh 1.87 g NaCl and add it to enough distilled water to make 1 L of a "concentrated" NaCl solution.
  2. Mix well to fully dissolve the salt.
  3. Add 10 mL of the concentrated NaCl solution to 990 mL distilled water to make a 20 ppm TDS solution. Mix well. Discard the remaining concentrated solution.
  4. Take a TDS reading of the solution. If the reading is reasonably close to 20 ppm or 40 μS/cm, then the device is working correctly. Otherwise, the device should be calibrated per manufacturer instructions, if possible.
    • Note: If the meter does not have ATC, the sample must be at 25°C (77°F).

The calibration solution can be stored in a sealed container for later use.

You can follow the same instructions above, but with a different amount of salt to make a calibration solution with a TDS tailored to your needs:

Calibration solutions[1]
TDS μS/cm NaCl to make concentrate solution
20 40 1.87 g
42 84 4.00 g (for Apera EC60 meter)
50 100 4.74 g
100 200 9.52 g
200 400 19.2 g
351 701 34.2 g (for HM AP-1 meter)
500 1000 49.1 g
1385 2770 142 g (for Bluelab meter)

Cleaning[edit]

If the probe surface is dirty, soak it in water with a mild detergent (e.g. dish soap) for 15 minutes. Then soak it in a dilute acid solution (such as citric acid or acetic acid) for another 15 minutes. Then rinse it well with distilled water.[1] This process removes both organic and inorganic (mineral) residue. See Cleaning for more information.

See also[edit]

References[edit]

  1. ↑ a b c d Conductivity probe. Forston Labs. 2010.
  2. ↑ Briggs DE, Boulton CA, Brookes PA, Stevens R. Brewing Science and Practice. Woodhead Publishing Limited and CRC Press LLC; 2004.
  3. ↑ a b Palmer J, Kaminski C. Water: A Comprehensive Guide for Brewers. Brewers Publications; 2013.
  4. ↑ Taylor DG. Water. In: Stewart GG, Russell I, Anstruther A, eds. Handbook of Brewing. 3rd ed. CRC Press; 2017.
  5. ↑ a b Brungard M. TDS meter: Do you need one? Bru'n Water website. Accessed May 2022.
  6. ↑ a b c d e Using electrical conductivity and total dissolved solids meters to field test water quality. North Dakota State University website. 2019. Accessed May 2022.
  7. ↑ Feminella V. Pure brewing water using reverse osmosis. The Screwy Brewer website. 2014. Accessed May 2022.
  8. ↑ FAQ. Buckeye Hydro website. Accessed May 2022.
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