Calibration depends on the required measurements as well as application areas concerned. It is important to calibrate/adjust a pH meter (technically speaking to calibrate the pH electrode in the pH meter’s sensor) sooner or later, depending on how accurately we want it to measure. The article explains the process of calibrating a pH meter and which sensor should we use depending on the application required.  

For example, the accuracy of the measurement is very critical in industries like Life Science. As such, here, it is required to measure pH at regular intervals. On the other hand, in a wastewater treatment plant, since the pH of water in applications doesn´t change at a rapid rate when compared to the Life Science industry, calibration is not that critical.

If we calibrate a pH meter, then we measure the value you receive in a known solution, usually a pH buffer, and control the difference. If we adjust a pH electrode, then we correct its current measurement value to a reference value, usually the one printed on the buffer bottle.

We commonly do the second point, adjustment, but we call it calibration. Before we start, make sure the transmitter signal won’t cause issues in your process when you disconnect it. Also, make sure you program your transmitter for the buffer solution(s) you use.

Speaking of which, we should have the following supplies:

• Cleaning solution
  
• Distilled water
• Clean beakers
• Buffer solution that you trust (two is better if you have another)
• Paper towels

1. Examine the pH electrode

First, check the pH electrode for contamination or damage. If it’s damaged, then fix it or toss it. If it’s just dirty, then use the cleaning solution according to its directions. Whether we use acids, washing liquid, or alkali, choose a solution appropriate for your process and the contamination.

2. Flush the pH sensor

Next, flush your sensor with distilled water. We can do this, even if we didn’t have to clean it, to rinse away anything that may contaminate the buffer solution you’ll use in Step 3. After flushing, dab or pat away excess water with the paper towels. We should not generally rub it more: you might charge or damage the sensor.

3. Immerse the pH electrode

Fill a beaker with your first buffer solution, then immerse your electrode in it. It’s easy to just drop the sensor directly into the buffer bottle, but we can avoid contamination and extend the life of a buffer if we use the beaker.

4. Calibrate the pH meter

Now we can start your calibration/adjustment. Keep an eye on the stability of the value; an old pH meter might react sluggishly. When the value stabilizes, set the device to accept this calibration/adjustment point.

5. Rinse the pH sensor and repeat

Flush the pH sensor with distilled water again, then immerse it in another clean beaker with the second buffer solution.

How often we need to adjust the meter depends on the following:

• Accuracy required by the process
  
• Stress the process conditions put on the sensor
• Sensor’s ability to withstand that stress
  

In a drinking water application, we can expect stable conditions, so we may only need to calibrate once a month. A measuring point with a high temperature or high pH might need a weekly tweak. We can use experience – or borrow someone else’s – and pay attention to the diagnostics the system provides.

Adjustments in the field sometimes come with environmental challenges. However, digital sensors make maintenance much easier. In many cases, the microprocessor that converts the signal can do more, like store the adjustment values in its memory. That way, we can bring the sensor into a lab or workshop, connect it to a suitable device, and perform an adjustment. Then we can either reinstall it or set it aside as a nicely adjusted backup.

A pH sensor converts the millivolt signal, then it receives from an electrode into a pH value. The required solution temperature can factor in here, both for the measurement and electrode. Therefore, a pH sensor should also have a place where we can connect a temperature probe. 

Which type of pH meter should we buy?

There are two types of pH meter in the market: the traditional with a pH-sensitive glass membrane, and the ion-sensitive field-effect transistor (ISFET), with a pH-sensitive integrated circuit. 

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.

So which one would we choose? At first glance, wee 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 site. You can fix a traditional membrane to a glass shaft with simple moulding, 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 millimetres.

Now, for ISFETs, we may want PEEK, because we can affix the ISFET chip to the PEEK shaft. PEEK is also chemically stable, and we can use it too for sensors over 120 millimetres 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. Yes, glass makes a better material than we think, with PEEK coming in second and plastic dead last. It still has its uses, though.

How do we scale out a pH meter for our processes?

If we work in the food-and-beverage industry, then we need to do everything we can to secure our process safety and avoid contamination risks from a damaged pH sensor. Here, we might choose an ISFET with a PEEK shaft, nice and stable and almost unbreakable.  

If we 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 we’ll probably save a little money, to boot.

For a process that hits temps above 80 degrees Celsius, we’ll want a glass sensor and a reference cell with a robust design. If we go with something else, then we’ll probably wind up changing the sensor a lot.

If we have tiny particles swimming around in a product that can block the contact between the reference and your product, then we’d better pick a sensor with an open hole and a fixed polymer reference system. In this case, we could also consider a Teflon diaphragm as an alternative.