A look at a range of devices for watching TDS.
One major question a potential customer may ask before they purchase a point-of-use/point-of-entry (POU/POE) water treatment system is, “How often will I need to change the filter?”
Maintenance and service are directly related to two of their major concerns: money and health. People don’t want to spend money unnecessarily, yet they also want to ensure that their system is operating efficiently and providing clean water.
With a reverse osmosis (RO) system, the three factors that are central to answering the customer’s change-frequency question are: 1) time, 2) flow, and 3) total dissolved solids (TDS) levels.
TDS level is an excellent indicator of RO system performance, but with so many varieties of meters, monitors and controllers on the market, how do you know which is right for you and your customers?
When to change?
Let’s consider the three possible factors when deciding whether to change an RO filter cartridge or membrane:
Time: While certainly a simple concept, it’s also the least reliable. What happens if someone has a guest in town? Or goes away for a month? A customer could be changing an RO system too late or too soon.
Flow: Though more accurate, flow is also not the best gauge of RO system performance. TDS levels of municipal and well water supplies can fluctuate weekly or even daily, and TDS requirements of an RO system can vary widely between brands and filter media. Measuring flow requires the installation of a flow meter, along with diligent monitoring.
TDS: Monitoring the TDS levels of an RO system is a very accurate, effective method of determining when the system needs servicing. New technology allows an inline monitor to be installed quickly and easily. Obtaining TDS measurements can be as simple as touching a button.
Many RO salespeople now include a TDS meter or monitor free of charge with their systems. This approach can help increase sales of replacement filters and membranes, increase the perceived value of the RO system, and enhance the “trusted advisor” role of the RO salesperson.
Choosing the right device for a system is of utmost importance.
The biggest advantage of a handheld meter is its portability and versatility. A customer can use a handheld meter not only in their own home for the RO system, but for a variety of water sources and even on that family trip to Cancun.
The majority of handheld meters on the market have wide ranges, such as 0-9,999 parts per million (ppm); long-lasting batteries; and in some cases a color-coded chart illustrating TDS values. They are simple to use with a push of a button. A handheld meter is usually the least expensive way to go.
Limits of handhelds
Handheld meters to be used by customers have their limitations. As examples, the customer may lose the meter, or may not want to take the time to use it.
To accurately gauge the effectiveness of their RO system, a customer needs to know that system’s minimum effective percent rejection. The customer will have to obtain the TDS of a sample of tap water that bypasses the RO system (or pre-RO feed water), obtain the TDS level for a sample of RO product water, and then determine the percent rejection.
Percent rejection is obtained by subtracting RO product water TDS from the tap water (feedwater) TDS level, dividing that result by the tap water TDS and then multiplying by 100.
For example, if the tap water measurement is 352 ppm and the RO water is 18 ppm, the percent rejection of the RO system would be 94.9 percent.
Since the vast majority of consumers have never heard of TDS, let alone percent rejection of an RO system, it’s critical to educate the customer on what TDS is, as well as what the minimum operating percent rejection of their RO system should be.
Maximum acceptable TDS
An alternative to having a customer determine percent rejection would be to inform them of a maximum TDS level and tell them that if the TDS in the RO product water rises above that point, it’s time to call the salesperson.
This can be an effective way to keep a customer’s RO system running properly, but it may not be entirely accurate, because, as noted earlier, municipal or well water TDS can fluctuate and thus the percent rejection may not be truly represented by a single TDS value.
The advantage of an inline monitor is its ease of use. The monitor can be installed on the unit before a sale and, once the unit is sold and installed, the customer can be instructed in its use in a matter of minutes.
The majority of inline monitors are simple to turn on with the push of a button, possibly followed by a flip of a switch to obtain both the readings of the feedwater and the product water.
Monitors are typically battery-powered and attached directly to the RO system. They can be used at any time, so a customer need only obtain the two TDS levels and figure out the percent rejection.
Most inline monitors fit into the system by having the first (also known as the “in”) TDS sensor connected to the feedwater line via a T-fitting, and the second (a.k.a. the “out”) TDS sensor connected to the product water line, also with a T-fitting.
This is an effective configuration for determining the overall quality of an RO system, but a monitor can be used in a number of different configurations.
For example, the first sensor could be connected after a carbon filter and the second sensor could be connected after the membrane. When used in conjunction with a handheld meter to determine the tap water TDS value, this could be an effective setup for determining which stage of the RO system needs servicing.
Similarly one could install two or even three monitors for systems with more stages.
The primary disadvantage of an inline monitor is the installation requirement. However, with a quick-connect T-fitting, this can be done quite simply.
Inline monitors are more expensive than handheld units, and since all monitors currently on the market only monitor a maximum of two lines, a multiple-monitor configuration could involve some expense.
Controllers are ideal for industrial or large commercial RO systems.
Besides monitoring TDS continuously, controllers also have a set-point. When the TDS levels rise above this pre-programmed level, the controller will perform an action, such as sounding an alarm; turning on a flashing light; or sending an analog signal, a 4-20 mA output signal or, for more advanced controllers, a digital signal to a control station.
Controllers are always AC-powered, so viewing TDS levels is as simple as taking a look. Controllers are typically more accurate, with better sensors, and can control one or two lines.
The biggest disadvantage of a controller is the price. At the very least, a controller will be five times the price of a monitor and ten times or more the price of a handheld unit, which is why they should only be installed on a need basis.
Monitoring TDS levels is virtually essential for any RO system, and can often be done at nominal cost.
In recommending or specifying the type of monitor or control to be used, the water treatment professional will need to do realistic assessments of the customer’s capabilities and needs, as well as balance the costs of monitoring/control equipment and installation with total system size, cost and performance requirements.