Controlling Relative Humidity and Condensation in a CO2 Incubator
Introduction
Deborah ThibodeauxScience Director
For more than 60 years, Baker has helped advance science, discovery and clinical care by pioneering innovative solutions for air containment and controlled environments. Our solutions include:• Biological safety cabinets• Clean benches• Fume hoods• CO2 Incubators
• Controlled-oxygen workstations• Anaerobic chambers• Media conditioning systems
Cheri GaudetMarketing Specialist
Manufacturer Claim
“Our CO2 incubator provides you with the optimal conditions for culture
growth!”
As scientists, it is our duty to investigate the truth of such claims.
Define “Optimal”
• 37°C – 5% CO2 – 95% RH– “Optimal” conditions for many, but not all,
applications– “Optimal” is contextual
• “Optimal” depends upon the requirements of the system being studied– Psychrotrophic / thermophilic bacteria have different
temperature requirements
• “Optimal” depends upon the aim of your research– Healthy vs. stressed systems (e.g., to mimic
dehydration, manipulation of RH is required)
What You Will Learn
How to tell if your CO2 incubator provides active humidity control
How to prevent condensation, even at high humidity
Whether you can trust that your CO2 incubator really provides 95% RH
DOES YOUR CO2 INCUBATOR PROVIDE ACTIVE HUMIDITY CONTROL?
Types of CO2 Incubator Controls
• Open Loop Controller–Without programmable RH–With programmable RH
• Closed Loop Controller (programmable RH is standard)
Do any of these provideactive humidity control?
Open Loop ControllerWithout Programmable RH
Control Programmi
ng (ONE set point)
Heat Water Pan
Humidity Delivered
to Chamber
× One humidity set point determined by manufacturer, not user
× Microprocessor uses mathematical formula based on static (not real-world) conditions
× Uncertainty – is system delivering promised RH?
Control Programming (ONE set
point)
Heat Water Pan
Humidity Delivered
to Chamber
Open Loop ControllerWithout Programmable RH
Does it provide active humidity control?
× No control over relative humidity whatsoever
Open Loop ControllerWithout Programmable RH
“Dial in” to set point
Heat Water Pan
Humidity
Delivered to
Chamber
Open Loop ControllerWith Programmable RH
User-defined humidity set point
×Microprocessor uses mathematical formula based on static (not real-world) conditions
×Uncertainty – is system delivering promised RH?
“Dial in” to set point
Heat Water Pan
Humidity
Delivered to
Chamber
Open Loop ControllerWith Programmable RH
Does it provide active humidity control?
×Only gives the illusion of control
Open Loop ControllerWith Programmable RH
Open Loop Controllers –The Problem
No System Feedback
Closed Loop Controller
Heat water pan
Humidity delivered to
chamber
RH sensor samples
atmosphere
RH measurement informs
control algorithm
Control algorithm adjusts temp of heating
elements
“Dial in” to set point
Closed Loop Controller
Heat water pan
Humidity delivere
d to chamber
RH sensor
samples atmosph
ere
RH measure
ment informs control algorith
m
Control algorith
m adjusts temp of heating element
s
“Dial in” to set point
User-defined humidity set point
Control algorithm adapts to real-world conditions
Certainty – system maintains RH to set point
Closed Loop Controller
Does it provide active humidity control?
Yes
HOW CAN YOU PREVENT CONDENSATION, EVEN AT HIGH HUMIDITY?
Is Condensation Completely Avoidable?
×No
Some condensation is unavoidable, such as when cooling the chamber interior
Is Condensation Preventable?
Yes
During normal operation, condensation is preventable, even at high humidity
What Causes Condensation?
Water condenses from the air onto a surface when the temperature of the
surface falls below the dew point
What Causes Condensation in aCO2 Incubator?
• Fluctuation in environmental parameters (e.g., door opens)
• Interior surface temperature gradient changes– Surface temperature gradient: Variance in
temperature across the interior surface, expressed as a range of temperatures
• Portion of the surface temperature gradient falls below dew point
• Condensation occurs
Technologies that Prevent Condensation?
×No prevention above 90% RH×Designated “cold spot” in incubator cooled
to below dew point to “divert” (not prevent) condensate– Does not prevent condensation from occurring
on other surfaces that are at or below the dew point
• Closed-loop controllers provide better control over air temperature & RH, which directly affect dew point, but this is not enough
Muddled Marketing Claims
Incubator suppliers are not being untruthful when representing the level of control they
are able to provide over RH and condensation…
…but they are not telling you the whole story.
The most pervasive claim about CO2 incubators may be the least understood.
DOES YOUR INCUBATOR REALLY PROVIDE 95% RH?
RH Sensor Accuracy
• Closed-loop controllers in CO2 incubators typically use a capacitive RH sensor to provide feedback to system
• Like all instruments, subject to some degree of inaccuracy
• How accurately the incubator control algorithm maintains RH set point directly depends on the accuracy of the RH sensor
RH Sensor Accuracy
Accuracy of capacitive RH sensors depends on:• Temperature• RH• Length of time in continuous operation• Age of instrument
×Most incubator manufacturers do not disclose this information, let alone
account for it in the control algorithm
Typical RH Sensor Accuracy
Conditions RH Sensor Accuracy
RH is between 10-100% @ 25°C ± 1.8%
RH is greater than 80% for 60 hours Additional + 3%
Temperature is between 10°C and 35°C Additional ± 0-1%
Temperature is greater than 35°C Additional ± 1-2%
Yearly Drift Additional < + 0.5%
Real-World Implications
• An RH sensor performs least accurately under the most common conditions
• An incubator in its first year of use, in continuous operation at 37°C and 95% RH may be delivering humidity levels as low as 88.2% and as high as 98.8%.
Real-World Implications
The verdict?× Uncertainty – impossible to tell whether the system
is maintaining anywhere near the RH set point
Unless sensor accuracy is factored in to the control algorithm
TECHNOLOGIES TO LOOK FOR AND INSIST UPON
Humidity & Condensation Control Technologies
Closed-loop control algorithm • Allows user-defined, active humidity control, reducing experimental error and uncertainty
• Helps prevent condensation by providing better parameter control
Two humidity set point ranges1. “Up to 90%” where user can
“dial in” to a desired RH2. “90%-95%” where a “sweet
spot” of >90% RH can be delivered consistently
• Prevents condensation and the risk of contamination that goes along with it
• Significantly reduces uncertainty pertaining to RH accuracy
Thank You
For more information on this topic, download our white paper:
http://info.bakerco.com/co2-incubator-humidity-control