The most crucial thing to remember when measuring the content of any gas in a process pipe is to make sure that all of the gas you want to measure is in solution. Here are the main points:
- There can be no CO2 bubbles. The liquid must be clear. If there are CO2 bubbles then your dO2 or CO2 results will be lower than your actual value.
- Measurement points need to be as far as possible from any source of cavitation, turbulence, or gas injection. When measuring after a carbonator or a wort oxygenation source, place your probes as far as you can from the injection point.
- Measurements in tanks and in process lines are not the same. What you measure in a process line should reflect what is in the pipe where the sensor is located. Once your beer is in the tank, several other factors may come into play and cause your instruments to show either higher or lower values than what you see in your pipe.
Lets talk for a minute about this last point. For example, how could a value go lower? In the case of wort or beer, you could lose gas in the tank (O2 for wort, CO2 for beer) if the head pressure on the tank is less than the pressure in the process pipe. It’s not always possible to keep some gas from escaping, so it’s good to be aware of this and not immediately assume there’s a problem with your instruments.
Here are some things we recommend with regard to in-line measurements.
- Never place a sensor in a descending pipe. Liquids in descending pipes can degas if the line does not have adequate pressure. If this happens it pulls a vacuum on the probe, causing it to read too low.
- Place sensors in horizontal or ascending pipes.
- All sensors should be as far from pipe bends as possible, but never closer than five pipe diameters from any elbow.
- Sensors should never be on the suction side of a pipe.
- Place probes as far from pumps as possible. Placing them right after pumps can cause false low readings.
- When placing a sensor before a rotary filler, keep it out of the drip line of the filler.
My final thought about in-line measuring is that you’ll always do best with an installation if you think in terms of maximizing the ability of all of the gases to stay in the liquid.