A customer recently approached us with what they thought was a potential problem with the new Shel Lab water-jacket CO2 incubator they purchased.
They observed that within moments of opening the incubator's inner glass door the display showing the relative humidity (RH) inside the CO2 incubator plunged from 95% to just about 48% and then took the better part of thirty-minutes to recover.
This PI suspected that maybe the "culprit" was a poorly placed RH sensor that needed to be moved from the front of the unit to the back.
We passed this question to the technical team at Shel Lab and received a "tour-de-force" answer that we felt would be valuable reading for anyone wanting to understand how relative humidity is measured, what impacts relative humidity, and how Shel Lab CO2 incubators monitor and maintain control over this critical variable.
Here is what they wrote:
We use a fast-acting thin-film capacitance humidity sensor in our SCO5W CO2 incubators which is able to rapidly and accurately sense changes in relative humidity, so that’s partly why you see such a quick drop in RH and slow recovery.
Mostly, though, the act of opening the chamber to exchange room air will reduce the chamber humidity rapidly due to the “relative” part of Relative Humidity – the fact that warm air has a higher water capacity than cool air, and heating a volume of cold air will automatically cause its %RH to drop even if the total amount of water vapor in the system remains the same.
Let us explain the science:
Incubator air is typically warmer then room air, and warm air can hold more water vapor than cool air. For instance, the air in a room at 30C and 100% Relative Humidity has almost twice as much water in it than the air in a room at 20C and 100% Relative Humidity.
If you exchange your warm, humid incubator air with cool room air, even if they are both initially at the same %RH for their respective temperatures, the incoming air has less total water in it and as it warms-up its “relative humidity” will go down since it’ll become capable of holding more water.
Our water-jacketed incubators will quickly bring the chamber temperature back up to your desired setpoint.
The chamber air’s %RH will drop rapidly as the air rapidly gets warmer because it’ll warm-up faster than the water in the humidity pan can evaporate.
Here are ways to address your concern with slow-recovery time:
One way to help with getting the %RH back up faster is to raise the height of your bottom-most shelf to get more air circulation around the water pan, and also making sure it’s full keeps the water level up closer to the moving air stream.
If your chamber has our patented copper-caged HEPA filters, then you might want to make sure your filters are changed regularly (depending on your indoor air quality) to make sure they don’t get clogged and reduce air flow.
The humidity sensor is located in the upper air duct of the incubator near the IR CO2 sensor and circulation fan. This is a good area for sensing the incubator conditions since it gets good airflow.
One final point about the difference between CO2 recovery and relative humidity recovery
The reason why you note that CO2 can recover very rapidly is because there is a microprocessor-controlled injection solenoid and a fast-responding dual band IR sensor to inject enough CO2 to recover quickly but not overshoot the desired setpoint.
Also, the %CO2 air can hold is not affected much by temperature and even that small difference is rapidly compensated-for by the microprocessor.
Stellar Scientific is a platinum level dealer for Shel Lab. We've received this distinction for exceptional customer service and care.
A CO2 incubator is a major lab purchase and one you will need to live with for many years. Trust Stellar Scientific and Shel Lab to provide you with the highest quality lab incubator and the know how to keep it running and performing in top condition.