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