Lesson 4 - Process Instrumentation
This lesson takes a closer look at process instrumentation. Whilst it explores the basic operations of manual instruments, it primarily focuses on the instruments that can be used in process control. It also investigates in more depth the instrumentation commonly used within the brewery.
Dissolved Gases
Dissolved gases play an important role in the brewery. As a result, they must be measured accurately.
Dissolved oxygen (DO)
Historically, we used to measure dissolved oxygen using a membrane sensor. This required lots of calibration and maintenance. Fortunately, this has now been largely replaced by an optical device that only requires yearly calibration and maintenance.
So how does this work? We use a luminescent dissolved oxygen sensor, covered in a layer of luminophore on a transparent carrier. A blue LED sends out a light pulse, which hits the luminophore. The luminophore gets excited by the light and emits a red light of its own. However, if there are oxygen molecules present, they absorb some of the energy and emit less red light. The sensor also has a red LED, which it uses as a reference signal. This is shown in the figure below:
Figure: Luminescent dissolved oxygen sensor
The same principle is used to check recovered CO2 for the presence of oxygen. The oxygen in the CO2 has the same effect on the sensor as that dissolved in beer.
Carbon dioxide (CO2)
Carbon dioxide is measured in a number of ways.
One way of measuring CO2 utilises Henry’s law, which states that the concentration of a gas in solution is proportional to its partial pressure above the liquid. A sample of liquid is taken into a chamber that is rapidly expanding and forcing the gas out of solution. The pressure and temperature of this gas phase is then measured and used to calculate the CO2 concentration.
Another method uses attenuated total reflection (ATR) technology. Near infrared light is transmitted via a sapphire crystal and reflected off the surface. CO2 absorbs the specific wavelength and the decrease in the reflection’s intensity is used to calculate the CO2.
Traditionally, CO2 in a bottle was measured by piercing the lid – the following article discusses a technology for measuring the CO2 in a bottle without piercing the lid. Note, this is for illustrative purposes only and is not a recommendation or endorsement of the supplier’s product.
https://www.gasworld.com/new-co2-measurement-solution-launched/2004056.article