TDS Meter: Do You Need One?
A Total Dissolved Solids (TDS) meter can be a useful tool in brewing. This meter provides an indirect reading on the ionic content of water which can give you an idea of how the water quality has varied. TDS is the sum of all the various ions (solids) in the water.
For brewers using water supplies that vary, a TDS meter reading can provide an indication of how much the water quality has varied from previous measurements. While the reading can’t tell you which ion concentrations have changed, it does give you a general indication if the water is more or less mineralized than what you normally expect. With enough meter usage and lab testing of the water, a brewer can correlate how their water quality varies with respect to a TDS reading and can better estimate what their starting water quality is and use that information to adjust the starting water profile in Bru’n Water and obtain better treated water predictions.
For brewers using Reverse Osmosis (RO) water, a TDS meter is an important tool for assessing that the system is actually producing high quality (low TDS) water. Over time, the membrane that strips most of the ionic content out of the water can fail. As that occurs, the TDS content of the treated water typically rises over time. By monitoring TDS in the treated water, the brewer can tell when the RO system is failing and will need membrane replacement. This need is particularly critical for homebrewers that purchase RO water from vending machines. Without a TDS meter, it is very difficult to determine if the treated water actually has the low TDS that the brewer expects.
An important factor to be aware of when using a TDS meter is that the temperature of the solution has a large effect on the TDS reading. The higher the solution temperature, the higher the TDS reading will be. It is best to standardize on bringing your water sample to a consistent temperature before taking a reading.
While it would be great to accurately measure what the TDS content of water is, a TDS meter reading is only an approximation. However in brewing, we don’t need to worry about exact values. We are more interested in relative readings (is today’s reading much higher or lower than readings I typically measure?). Small reading differences (<20 ppm) are not a concern. Differences of 50 ppm or more are when it is apparent that the water quality has changed significantly.
A TDS meter is a very simple device and is typically durable and long-lived. Essentially, the device is a pair of electrodes that measure the conductivity of a solution by applying voltage to each electrode and measuring the current produced through the solution. That current reading is converted into a corresponding TDS value. The figure below provides a simplified schematic of a typical TDS circuit and helps illustrate why the meters are inexpensive and durable. While you can find these meters for less than $10, purchasing a name brand such as HM Digital, Oakton, Hanna, Hach, etc is probably a better investment and those meters can be had for $20 to $30.
For brewers using RO water or those dealing with varying water supplies, a TDS meter can be a very useful tool.
