What to look for in a pH or chlorine sensor

Date: 14/03/19 | In: Water Quality

You know you need to monitor pH, so you do some research and find oodles of pH analysers and instruments on google. How do you sift through and make a shortlist to find the right one? What should you look for in a pH sensor, or any other online instrument?

There are 5 things you should consider:

1) Accuracy

How close is the measurement to the actual value?

The accuracy of most instruments will be dependent on the device used for calibration. So at the time of calibration, the sensor will be 100% accurate. Yet how accurate was the calibration device?

We would not advise you to consider accuracy if the device requires calibration.

2) Precision

You may think that accuracy and precision are related. They are not. Precision refers to the spread of multiple readings and how close they are to one another. For example a sensor that can distinguish between 0.01 and 0.02 is more precise than one that distinguishes between 0.1 and 0.2.

3) Resolution

This is the smallest change a sensor can detect, so the precision. The reason we have distinguished them is because when displays were analogue, that measurement was often determined by the resolution of the the display screen. With new digital displays, that is no longer a problem. However, beware, as some manufacturers may state a higher resolution of say 0.001 (related to the display screen resolution) when the actual sensor’s precision or resolution is 0.1.

If you are confident that the sensor resolution you are looking at is accurate, this is a good measurement of what to look for in a pH sensor.

4) Drift

How much will the signal change over a given period, for example over a month?

This is a really good indicator of how good a sensor is. As we’ve mentioned earlier, a sensor will be 100% accurate when its just been calibrated. But how much will that change over the course of a month? 1%?, 10%?

When you’re looking at drift, make sure you have all the information to make a decision and check what it refers to. The current reading or the full scale?

5) Repeatability

This refers to how easy it would be for someone to replicate. For our purpose it will depend on whether the monitor is a continuous or continual online instrument. With continuous online instruments the sample is analysed all the time, therefore repeatability does not apply. However, for continual online instruments, a series of samples are analysed continually, making it suitable for repeatability.

Repeatability can be a useful measure, especially if there will be 2 or more sensors on the application that are calibrated at the same time.

You can use this checklist to help choosing most online instruments such as pH, chlorine, turbidity, conductivity or chlorine dioxide.