Analytical sensors – Why switch from analog to digital sensor?
Let’s park pH for a minute to learn more about digital analytical sensors. The industry clearly favors the digital sensor trend, as they’ve become more common over the past decade. But why?
Vendors claim ease of use and better reliability. Digital signals remain unaffected by analog issues such as electrical interference, moisture, and cable length. But does this argument cover the markup on these things? If not, then what else do they offer? Let’s back up a bit and find out how a digital sensor actually works.
The sensor itself remains analog, regardless of parameter – pH, dissolved oxygen, conductivity turbidity, or whatever. But a digital sensor has an extra bit, a microprocessor with enough memory to store data from your sensor and process. Thus, the sensor will process the analytical signal and store this value in its memory.
Often, the sensor must process and store several signals at the same time multiple times a minute. Furthermore, to improve the sensor’s reliability, it processes a large number of values to calculate its average and store that in its memory. You’ll need a fast and powerful processor to avoid slowing down the sensor’s response.
A digital sensor will require a digital transmitter or converter or instrument, whatever you want to call it. You can’t install a digital sensor into your current system unless your transmitter can accept a digital sensor. Many vendors provide sensors and transmitters with compatible tech, so do your research and see what they have to offer.
Once your sensor and transmitter agree, the transmitter should read the data in the sensor’s memory at the same speed that the sensor stores it. The transmitter must take into account several signals to provide reliable measurements. For example, let’s say the sensor needs a temperature compensation. This means that the transmitter needs to read several stored values from the sensor, then process and convert them into values you can read on the display.
We all drift from our original positions, some of us faster than others. How fast a sensor drifts depends on process conditions and manufacturing quality. To correct this drift, we perform sensor adjustments, using at least one standard solution with a known value. The sensor measures the value in your solution(s), and if the displayed value doesn’t match the solution’s value, either the transmitter automatically corrects or you can do a manual correction.
Analog sensors store their corrections in the transmitter. Digital sensors can store their own adjustment values, including times and dates. That way you can adjust a sensor in one location, like a lab or workshop, then install the sensor in a different location, like in the field. The instrument reads the adjustment values stored in the sensor and applies them to the process.
Beyond sensors and transmitters, several suppliers provide software that enable communication between digital sensors and standard PCs or laptops. Smartphones and tablets are next, I’m sure. Besides measuring and adjustment, you get the ability to document, save, and print your sensor data- not just sensor ID with adjustments and timestamps but also operator IDs, solution batch numbers, and more. With this software, you can save and review the complete histories of all current and past sensors.
With digital analytical sensors, you receive 100 percent signal integrity, end of story. Unlike an analog loop, you never risk signal loss between the sensor and the display. Either you have a measurement without signal loss or no measurement at all.
Take a pH measurement, for example. Humidity, oxidation, or dirt can create an electrical resistance that will lower signal values. Worst of all, you may get intermediate drops, signal variations that come and go depending on things like environmental shifts. These issues can give you unreliable signals that you may not recognize right away as bad. Digital sensors will either send you perfect signals or nothing, and nothing means you need to fix something.
Diagnostics and traceability
We love diagnostics, as they can improve process reliability and safety. Knowing when a sensor needs care means just as much as knowing when it gives good signal. Most digital sensors use their histories to calculate maintenance – how many hours it has worked in the process, how many batches it has processed, how much exposure it has to process conditions. And by comparing a sensor’s current values to the saved factory values, you receive a good platform for sensor management decisions. Should you use it again after maintenance or replace it? A digital sensor will tell you!
As for traceability, a digital sensor carries its identity and history as digital data. The transmitter can read this data from the sensor, show it on the display, and with a suitable protocol, send it to the control system. You as a process operator now have that data in the control room to monitor your process and the sensor itself. And if you use the software mentioned earlier, you can get a report with the sensor ID and history, including its current and past adjustment values.
Of course you can keep your analog sensor, for a while, anyway. But as more companies hop on the digital trend and start producing faster and more cheaply, you’ll probably get left behind. Why not give a digital sensor a try? You may even like it.