Hi! Let’s hop in the pH pool and talk about the different types of pH sensors on the market. First, a pH sensor converts the millivolt signal it receives from an electrode into a pH value. As we’ll learn later, your solution temperature can factor in here, both for the measurement and the electrode. Therefore, your pH sensor should also have a place where you can connect a temperature probe. In most cases, your display will show your temperature values
Which type of pH sensor should I buy?
Whoa, slow down! We’ll get there, don’t worry. Right now, we have more to review, like sensing elements. Two types dominate the market, the traditional, with a pH-sensitive glass membrane, and the ion-sensitive field-effect transistor (ISFET), with a pH-sensitive integrated circuit. A few others exist, but we’ll skip them; they take up such a tiny corner of the market that you probably won’t need those.
When we talk about sensor materials, we need to distinguish between the material of the sensing element and the material that makes up the rest of the sensor, mostly the shaft. Vendors typically stick to three materials for the shaft – glass, polyetheretherketone (PEEK), or plastic. Which would you choose? At first glance, you might go for plastic or PEEK because they seem more robust than glass.
Vendors disagree and go for glass. Despite its fragility, this material is very chemical- and temperature-resistant. And look at it from the construction side. You can fix a traditional membrane to a glass shaft with simple molding, which makes the attachment robust. Not to mention the glass shaft offers high physical stability because it avoids the inner glass tube for the wire brakes. This can become an issue with sensors longer than 120 millimeters.
Now, for ISFETs, you may want PEEK, because you can affix the ISFET chip to the PEEK shaft. PEEK is also chemically stable, and you can use it too for sensors over 120 millimeters long.
Plastic works best in less-demanding processes. Why? Because it doesn’t offer as much physical stability for the membrane. Nor does it offer as much resistance to pressure, chemicals, or temperature for the sensor. Yep, glass makes a better material than you think, with PEEK coming in second and plastic dead last. It still has its uses, though. Read on!
How do I scale out a pH sensor for my process?
Good question! Let’s dig in.
If you work in the food-and-beverage industry, then you need to do everything you can to secure your process safety and avoid contamination risks from a damaged pH sensor. Here you might choose an ISFET with a PEEK shaft, nice and stable and almost unbreakable. Just take care to choose one that can handle your cleaning-in-place cycles, okay?
If you want an easy installation in a low-demand site like a swimming pool or fish farm, then a sensor with a plastic shaft should work just fine. And you’ll probably save a little money, to boot.
For a process that hits temps above 80 degrees Celsius, you’ll want a glass sensor and a reference cell with a robust design. If you go with something else, then you’ll probably wind up changing your sensor a lot.
If you have tiny particles swimming around in your product that can block the contact between the reference and your product, then you’d better pick a sensor with an open hole and a fixed polymer reference system. You could also consider a Teflon diaphragm as an alternative.
So there you have it! That should help you narrow down your choices in the myriad products on the market. Speaking of which, we have a few suggestions there too! For this part, we’re gonna borrow Bo Ottersten’s list from his Coca-Cola article. It covers the same ground. In fact, you can pop over and read that one too if you want to know more about pH. Enjoy!
What type of sensor you should use depends on your application and how much money and effort you want to spend on maintenance and installation. You can find top three vendor lists for pH devices the same way you’ll find top three lists for trucks, blenders, or any other product. These vendors may differ depending on the process application and geographical area. Check the links, and find the right option for you.
In random order, you’ll find manufacturers of pH electrodes and instruments below:
Vendors for laboratory sensors and instruments are often different. Some manufacturers supply both, which is great for one-stop shopping and easy support for troubleshooting.
Check out these suppliers: