pH sensor calibration
Welcome back, my friends! Let’s do some calibration!
I know, I know. But you must calibrate/adjust your pH sensor sooner or later, depending on how accurately you want it to measure. If you need those measurements, then you need this calibration. It’s not so bad, really. But it is necessary.
Let’s first sort out what calibration and adjustment mean.
If you calibrate a pH sensor, then you measure the value you receive in a known solution, usually a pH buffer, and control the difference.
If you adjust a pH electrode, then you correct its current measurement value to a reference value, usually the one printed on the buffer bottle.
Simple, right? We commonly do the second point, adjustment, but we call it calibration.
Before you 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, you 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
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 you use acids, washing liquid, or alkali, choose a solution appropriate for your process and the contamination.
Next, flush your sensor with distilled water. Do this, even if you 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. Don’t rub; you might charge or damage the sensor.
Fill a beaker with your first buffer solution, then dunk your electrode. I know it’s tempting to just drop the sensor directly into the buffer bottle, but you’ll avoid contamination and extend the life of your buffer if you use the beaker.
Now you can start your calibration/adjustment. Keep an eye on the stability of the value; an old sensor might react sluggishly. When the value stabilizes, set the device to accept this calibration/adjustment point.
Rinse and repeat
Flush the sensor with distilled water again, then immerse it in another clean beaker with the second buffer solution.
How often you need to adjust the sensor depends on the following:
- Accuracy your process needs
- Stress your process conditions put on the sensor
- Sensor’s ability to withstand that stress
In a drinking water application, you can expect stable conditions, so you may only need to calibrate once a month. A measuring point with a high temperature or high pH might need a weekly tweak. Use your experience – or borrow someone else’s – and pay attention to the diagnostics your 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, you can bring the sensor into a lab or workshop, connect it to a suitable device, and perform your adjustment. Then you can either reinstall it or set it aside as a nicely adjusted backup.
A good adjustment/calibration will not only improve your process accuracy but also save time and money. Cleaning and adjusting your sensor properly will prolong its life, especially if you can do it in the workshop instead of in the field. Hope this guide helps those of you with pH sensors!