Balancing accuracy and speed
In the mid-1930s, Arnold Beckman created one of the first commercial pH meters— made originally to measure the acidity of lemons. Then an assistant professor of chemistry at the California Institute of Technology, he started selling his device as the Model G acidimeter, later known simply as the Model G pH meter. This work led him to start Beckman Instruments, known today as Beckman Coulter, which continues to make pH meters. Today, though, pH meters provide many features beyond those of the Model G.
In part, the expanded features in pH meters reflect the broad use of this technology. Researchers use pH meters in a wide range of research fields—including biological and chemical, agricultural and environmental, and more—and virtually all kinds of manufacturing. As Jon Bormann, global product manager for HQd series meters at Hach (Loveland, CO), says, “You’ll find a pH meter in almost any kind of lab you can imagine.” He adds, “pH is the most commonly measured parameter.”
A meter is only one part of a pH system, because an appropriate electrode is crucial to the application. For example, if you are measuring samples with high solids or samples that are low in ionic strength, the pH electrode design will influence the overall system accuracy, maintenance schedule, and the expected life of the electrode.
My own days in a “lab”—like those of many other scientists—started with a chemistry set that included pH paper, better known then as litmus paper. Given the time to use that process, dipping the paper in a solution and then comparing the strip’s color to a reference chart, it seems as though any real pH meter would have seemed fast by comparison, but I did not always find that to be the case. I remember sometimes waiting and waiting in real labs over the years for the pH meter to settle on a reading for some solutions. In some cases, a fairly wide pH range would work, so speed meant more to my research than precision did.
Scanning the selection
In general, today’s pH meters fall into one of three general categories. So-called testers come at a low cost—typically a couple hundred dollars—and provide only limited features, such as a compact size. The next class is usually called “portable,” and they come with a probe on a cable that can be from three to 100 feet long. Then there are benchtop pH meters, which typically stay in a lab.
“In the benchtop and portable split,” says Taylor Sundby, life cycle product manager at Hach, “some of the meters on both sides are more high-end.” For example, some users—such as R&D or pharmaceutical researchers—might need a higher-precision pH meter than other users. “Typically, both meters are equivalent in terms of precision and accuracy,” says Sundby. “Benchtop pH meter users, however, require greater flexibility to allow such precision.”
A fourth category of pH meters could be called process devices. “A process meter could be a panel, maybe in a brewery, that connects to a probe inside a pipe,” says Bormann, “and it takes pH measurements in real time.”
Using the measurements
Process pH meters tend to be used to control a manufacturing process. For example, in a petrochemical process, a change in pH might indicate a problem that needs to be addressed or a parameter that should be adjusted.
Other pH meters tend to be used to calibrate process meters. “In some manufacturing processes,” says Sundby, “most will take ‘grab samples’ to either take back to the lab—typically with a benchtop pH meter— or test on-site with a portable pH meter.” When using an in-line process meter, many users want easier ways to calibrate it. “They often use a benchtop meter to calibrate an in-line meter,” says Bormann. “They’d like to have them talking to each other.”
Sundby points out that some users also want more control over the measurement itself. “Most users are not terribly concerned about controlling how the instrument works, and they just use the value on the screen,” says Sundby. “Some people, though, have a very precise application, and they want to be able to adjust the criteria for accuracy or the speed of the measurement.”
When making such adjustments to how a pH meter works, researchers might also want to set it up just so and then password-protect any other changes. “Maybe in pharma or even food processing, you might have R&D where you need to dial in a meter and then make sure it stays that way,” says Sundby.
Distributing the data
Just getting a pH reading on a meter is not always enough. “For some applications,” says Bormann, “people want getting the data off the meter to be an easier process.” He notes that some users just look at the meter and then write down the pH. “People collecting the information electronically, though, want easier ways to do that, such as using Bluetooth or hooking to Wi-Fi somehow.”
For example, a manufacturer probably uses a laboratory information management system (LIMS) and would like the pH data automatically feeding into the LIMS. “We’re seeing a demand to make that as intuitive as possible,” says Bormann. “They want it more like an iPhone so they don’t need to look at any manuals.”
How a pH meter is used determines what features matter the most. For some, getting the data into a LIMS is a high priority. For others, accuracy matters the most. In some cases, the time required to get a stable reading is important. The latter two features, though, tend to be a trade-off. “In many cases, the response time is five seconds to a couple of minutes,” says Bormann. “Anytime you can take that down, it’s a welcome reduction.” He adds, “The faster you go, though, the more you sacrifice accuracy.” In the end, a pH system must balance accuracy and speed.
Purchasing trends in pH meter types:
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