TDS testing

TDS stands for Total Dissolved Solids. It's an indicator of the overall mineral content of water, although it does not determine what minerals are present. This measurement 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 minerality because it can vary over time. 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).

TDS meters are easy to use. Dissolved salts (also called electrolytes) increase the electrical conductivity of water. A TDS meter works by measuring water's electrical conductivity (EC) and then it uses an equation to approximate the amount of dissolved solids. 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

There are a variety on TDS meters available. The budget options tend to lack important features, and will likely suffer from poor accuracy. The Apera EC60 has all the bells and whistles, and is the editor's pick, still coming in under $100.

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%
Calibration
EC display	(μS/cm)	(μS/cm)		(mS/cm)	(µS/cm or mS/cm)	(µS/cm or mS/cm)
Backlight					?
ATC	?
Waterproof
Notes	Battery is not replaceable.	None	Magnetic body. Digital 1-point calibration with 342 ppm NaCl.	Digital 1-point calibration with EC 2.77 (2770 μS/cm). Displays icon for 30 days after calibration. Low battery indicator.	Digital calibration (1-point) with any calibration solution. Double-range for TDS and 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. Triple-range for TDS and 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.

All of these meters also feature:

• Temperature display
• Hold function
• Automatic power-off

Inline:

• Inline TDS meter on MoreBeer - Dual probes for monitoring input and output of RO system.

Usage

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 (probably around 1–2 minutes).
2. Put the meter's probe into the sample and wait for the reading to stabilize. If you use a sample container, it needs to be clean and free of any mineral contamination. You can rinse the sample container two or three times with the water you intend to sample.
3. After use, rinse the probe with purified water and/or dry it with a clean cloth.

Calibration

It's always wise to check that a measuring device is actually working properly. 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.

You can make a calibration 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. 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 not reasonably close to 20 ppm or 40 μS/cm, then the device isn't working properly.

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

If you are using your TDS meter to monitor tap water, you can follow the same instructions above, but with a different amount of salt to make a calibration solution with TDS closer to your water's mineral level:

Calibration solutions

TDS	μS/cm	NaCl to make concentrate solution
20	40	1.87 g
50	100	4.74 g
100	200	9.52 g
200	400	19.2 g
500	1000	49.1 g

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Calibration solutions are available to purchase if you prefer not to make it yourself, although options are limited:

• 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

Cleaning

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. This process removes both organic and inorganic (mineral) residue. See Cleaning for more information.

See also

• RO systems
• Water
• Water report