By Scott Manley, La Marzocco
The TDS meter has been the hot topic of late. So, as this month was supposed to be entitled "Using testing equipment, such as portable meters, dip strips, titration kits and collecting samples for off-site laboratory testing"... it feels like a deep dive on the TDS meter is the perfect place to start.
It’s exciting to see so much interest in the testing and reporting of water quality among technicians these days. I believe the continued growth in the specialty coffee industry is driving demand for competent technical services, including water quality testing. I am witnessing technicians and service companies looking for specific solutions to provide to their customers. This is further evidenced by the popularity of my recent “Water Testing for Espresso” presentation during the Specialty Coffee Expo, where we expected 45 people to attend and ended up hosting more than 50. With water testing being all the rage among the cool kids, the number one topic (and also the starting point in most tech’s water education) is the TDS meter. TDS stands for Total Dissolved Solids. I've been answering questions about its purpose on Slack, the phone, and by email almost every day. Additionally, I’ve been providing assistance in selection, sourcing and reviewing options with techs from all over the country. It is a tool that both provides us useful information about our water and also the most confusion about the meaning and weight of its measurement. This month, I’ll focus in on the TDS meter and hope to provide some useful information for selecting, using, and interpreting the metrics of this very important tool.
A good place to start is with a description of what a TDS meter is, how it functions, and what it measures. All TDS meters are really electro conductivity or EC meters. The meter has two electrodes that pass a very small electrical current between them to measure the conductance of an aqueous solution. Pure water – H2O with no additional ions – is an insulator, meaning it doesn’t conduct electricity. If you were to use a EC/TDS meter in pure water, there would be no conductivity, and therefore no results.
This is a convenient property of water. It means that when we do get a measurement value from using a EC/TDS meter, it’s telling us there is something in the water other than the water: an impurity. EC meters typically measure conductance in microsiemens, an SI (metric system) unit of measurement. A TDS meter is an EC meter that converts the unit of microsiemens via a factor to a useful measurement value (usually parts per million [ppm]) without us having to do the math each time. There are many types of TDS meters; the differences in function between them are primarily the factor used to arrive at a given result. Additionally, some meters have adjustable/selectable factors, ranging from 0.2 to 1.0. This is the most confusing thing about these tools and is also the most overlooked part in selecting a proper tool. Selecting a meter with the proper factoring is important in order to get an accurate result.
I received my first TDS meter from my employer. The TDS meter was my first water quality measurement tool. I used it to verify the permeate water from RO systems during PM visits and to determine if a water vendor was needed. At the time, I did not know about factoring or anything mentioned in the previous paragraph. I just knew that, when submerged in water, it displayed a number and I was told that if that number was outside of a standard range, that some further investigation would be needed. I would say that I got lucky because my employer had already researched the proper tool to purchase. They based their selection on the SCA water quality standard, which has a specific measurement for TDS. The SCA TDS measurement standard is called 442 Natural Water™ Standard Solution, as developed by the Myron L Company. Myron L’s website says:
"'442' refers to the combination of salts mixed with deionized water to comprise this standard: 40% sodium sulfate, 40% sodium bicarbonate, 20% sodium chloride. A combination of standard salts is necessary since natural water salt type and concentration can vary greatly by location.It is the best choice when measuring boiler and cooling water samples, city water supply, lakes, wells, etc.” (1)
This is the meter we should be using when testing water for coffee. A 442™ meter uses a factor of 0.71.
When I talk with technicians and cafes about their TDS, the primary tool most folks are using is another type of meter, a salinity meter – which confusingly is also marketed as a TDS meter. These meters are calibrated to a different factor – namely NaCl, or sodium chloride, also known as table salt. They are primarily used in industry to test sea water, aquariums, and brackish water. They are also inexpensive and more common – especially on Amazon. This is not the meter we want to be using for testing water for coffee. An NaCl meter uses a factor of 0.5.
In the range of potable water that we typically test, an NaCl meter will generally read about 30% less than a 442™ calibrated meter. And the 1:2 relationship between NaCl and conductance is non-linear, meaning the higher the TDS we measure, the greater the error to natural water. So if you think you are hitting your TDS standard using a NaCl meter reading 132 ppm, your actual TDS is 187 ppm and that’s why you are calling me about your machine scaling up.
A TDS meter won’t tell us what’s in our water, only how much of something may be in our water. We use other testing methods to determine specific components. A TDS meter is an important part of water testing in that it acts as a check against those other tests. All of the other tests we perform for dissolved solids should add up to the Total Dissolved Solids measurement from our TDS meter. I get water test results from customers nearly every day as the water quality specialist at La Marzocco. The most common error I see in submitted water reports is when the sum of the individual components, mainly total hardness, alkalinity or chloride, are greater than the measured TDS. This usually indicates an NaCl meter was used to measure TDS. Conversely, when a 442 meter is used and the sum of the individual components is less than the measured TDS, we know that there is an unaccounted for component in our water. This may indicate that there is a problem in the testing or an additional test is needed to account for the difference between the sum of the TDS and the individual results. Having an accurate TDS measurement means we can rely on the other measurements we take.
So, if you already have a TDS meter, inspect it and verify that it is calibrated to 442. If you are in the market for a TDS, you should be looking for a 442 calibrated meter. And if you have additional questions or need further clarification, please reach out to me on Slack. I am greatly enthused by all the technicians I interact with in water conversations; it is great to see the attention and focus techs are paying to this important subject. Thank you all.
(1) - Source: http://www.myronl.com/applications/solutionsapp.htm