Fluke 975 Application Note

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Application Note

Fluke 975 Air Meter:

Environmental carbon

dioxide analysis

From the Fluke Digital Library @ www.fluke.com/library

Carbon dioxide (CO2) is a naturally

occurring, colorless, odorless, non-

combustible gas that is ever present

in the atmosphere, usually ranging in

concentrations from 300-600 ppm

(parts per million).

In nature, CO2 is used in plants for

photosynthesis (CO2 in, O2 out), released

through the respiration of aerobic

organisms (O2 in, CO2 out), released

by decaying life forms and outgassing

volcanoes, and is a product of the

complete oxidation (combustion) of

carbon based compounds. CO2 is also

commercially produced and has many

commercial and industrial uses from

beverage carbonation to “dry ice” to

fire extinguishers. How, then, is CO2 a

significant factor in environmental Indoor

air quality?

Processes that produce CO2 in a

typical occupied space include

human and animal respiration,

decaying plant and animal life,

such as garbage in a kitchen,

and combustion processes of

fossil fuel burning equipment.

Most fossil fuel burning equip-

ment, such as furnaces, boilers,

water heaters and fireplaces, has

provisions to vent the combus-

tion products directly to the out-

doors—but venting systems can

become inhibited or even fail.

Stoves, ovens and space heat-

ers may not utilize a vent sys-

tem, relying instead on exhaust

hoods, building ventilation, or

in the case of many residential

applications, natural ventilation

through stack effect and struc-

tural leakage.

CO2 standards

ANSI/ASHRAE Standard 62.1-

2004 Ventilation for Acceptable

Indoor Air Quality addresses

minimum requirements for ven-

tilation and indoor air quality for

typical indoor breathing spaces

that will be acceptable to occu-

pants. The requirements are

“intended to minimize the poten-

tial for adverse health effects.”

The standard is applicable to

indoor occupied spaces except

when other usages or standards

require greater amounts of ven-

tilation. Generally speaking,

ventilation rates are selected for

contaminant and odor control

based on the maximum expected

occupant density and activity

level.

• When these ventilation rates

are maintained, CO2 levels

created by human respiration

should always be at accept-

able, healthy levels.

• When the occupied space is at

maximum expected occupancy

with the recommended venti-

lation rate, CO2 levels will gen-

erally be, depending on space

usage, about 350-1,000 ppm

above outside air CO2 levels.

This is well below the OSHA

workplace threshold of 5,000

ppm CO2. NIOSH and ACGIH fur-

ther stipulate a maximum expo-

sure rate of 30,000 ppm CO2 for

15 minutes. CO2 is an asphyxi-

ant, and at 50,000 ppm CO2 is

considered to be an immediate

threat to life.

Measuring air flow at a supply register.

2 Fluke Corporation Fluke 975 Air Meter: Environmental carbon dioxide analysis

Proper ventilation must be

maintained in occupied spaces,

but ventilation is not free. HVAC

equipment must be sized, not

only for the building load, but

the ventilation load as well.

Outside air used for ventila-

tion must be heated, humidified

and cleaned in the winter, and

cooled, dehumidified and cleaned

in the summer. A building may

not have maximum occupancy in

all spaces at all times. Since ven-

tilation rates are based on maxi-

mum occupancy, over-ventilation

will occur when spaces are not

at maximum occupancy. When

ventilation is more than required,

energy is wasted.

CO2 monitoring and

demand controlled

ventilation

CO2 can be used as a gauge for

determining the effective ven-

tilation rate of occupied spaces

and as a warning signal for

some processes that may have

gone wrong. It’s a predictable

indicator of occupancy. When

CO2 monitoring is used to con-

trol building ventilation rate, it’s

called demand controlled venti-

lation (DCV).

The primary purpose of DCV

is to avoid over-ventilation and

thereby reduce energy costs

when spaces are not at full occu-

pancy. One or more CO2 sensors

are used to control the position

of the ventilation air dampers

based on the CO2 level within

the occupied space.

The maximum open position of

the outdoor air dampers is based

on the ventilation rate, as though

DCV were not being used. The

minimum open position during

the occupied period is normally

set at 20 % of the maximum rate

regardless of the CO2 content of

the occupied space, however,

depending on the age and usage

of the building, this base ventila-

tion rate could be anywhere from

15 to 50 %. This minimum posi-

tion is required for basic build-

ing function ventilation, such as

materials off-gassing.

To qualify for DCV, a time

lag requirement must be met.

To meet it, proper CO2 levels

must be achieved within a time

period, and that time period

depends on the total cfm/person

ventilation rate and the volume

of the space to be ventilated.

The data logging function of the

Fluke 975 AirMeter™ test tool can

be used to ensure the time lag

conditions are being met.

The three most common DCV

control schemes are proportional

control, PI or PID control, or set-

point control. With proportional

control, the ventilation rate is

directly proportional to the rising

and falling indoor CO2 level. PI

or PID (Proportional+Integral+De

rivative) control compensates or

compensates and anticipates for

rising and falling CO2 levels by

accelerating the ventilation rate

according to the indoor CO2 rate

of change.

For spaces that are typically

either empty or fill rapidly, like a

classroom, setpoint control goes

to maximum ventilation as soon

as an increase in CO2 is sensed.

Since CO2 can vary widely

depending on environment (300-

600 ppm), DCV target ventilation

rates are based on the differ-

ential between outdoors and

indoors. The outdoor air damper

“start open” setpoint should be

set at the minimum outdoor CO2

level unless CO2 differential is

used. CO2 differential uses an

outdoor CO2 sensor to compare

actual outdoor verses indoor CO2

levels for more accurate control.

Mysterious CO2

Fossil fuel burning appliances

such as stoves, ovens, furnaces,

boilers and water heaters pro-

duce high concentrations of

CO2 during complete combus-

tion and are not always directly

coupled to a vent system. While

a properly ventilated occupied

space may have CO2 levels of

1,500 ppm, the combustion

products of a fossil fuel burning

appliance has CO2 levels ranging

from 70,000-120,000 ppm, with

30-70 ppm CO as well if the

appliance is clean and properly

set up.

Stoves and ovens may be

unvented in residential applica-

tions, or rely on exhaust hoods

in commercial applications to

vent the combustion products to

the outdoors. Improperly applied

kitchen exhaust systems can

ventilate the kitchen, but not the

appliance. This can be checked

by monitoring CO2 levels in the

kitchen and adjoining spaces.

Until recent years, gas fur-

naces, boilers and water heaters

were decoupled from the vent

system (chimney) by a draft

hood. Vents that produced exces-

sive draft would ventilate the

equipment room, but not neces-

sarily the gas appliance because

of the “cold curtain” that was

created at the draft hood. The

result of this was flue products

spilling into the equipment room.

DCV differential target levels are based on the ventilation cfm requirement

per person. Combined with those figures, the 975 AirMeter™ can be used

to determine whether the space is properly ventilated, over-ventilated, or

under-ventilated. And with that kind of information, technicians can venti-

late the space at the lowest possible heating and cooling cost.

10 cfm/person = 1,000 ppm CO2 differential between indoors and outdoors

15 cfm/person = 700 ppm CO2 differential between indoors and outdoors

20 cfm/person = 500 ppm CO2 differential between indoors and outdoors

25 cfm/person = 420 ppm CO2 differential between indoors and outdoors

30 cfm/person = 350 ppm CO2 differential between indoors and outdoors

3 Fluke Corporation Fluke 975 Air Meter: Environmental carbon dioxide analysis

Since CO2 is heavier than air,

the CO2 would settle at the floor

where the water heater burner

is typically located, depriv-

ing it of oxygen, and resulting

in incomplete combustion and

elevated CO. As this process con-

tinued, the concentration of CO

increased creating a more dan-

gerous situation.

Monitoring CO2 levels in the

equipment room is likely to alert

occupants to potentially haz-

ardous situations more quickly,

before the process gone wrong

can accelerate the production of

CO. (CO2 is heavier than air and

tends to settle, CO is lighter than

air and tends to rise.) Since the

combustion process of fossil fuels

produces 1.3 to 2 times as much

water as CO2, an unexplained

increase in humidity levels, or

condensation on cold surfaces,

can also alert one to a venting

problem.

When an occupied space

is adjacent to a warehouse or

parking garage, simultaneous

increases in both CO2 and CO

can indicate ventilation system

problems. Garages and ware-

houses should be independently

exhausted and the absolute

pressure of the occupied space

should be maintained above the

absolute pressure of the adjoin-

ing garage or warehouse.

Fluke Corporation

PO Box 9090, Everett, WA USA 98206

Fluke Europe B.V.

PO Box 1186, 5602 BD

Eindhoven, The Netherlands

For more information call:

In the U.S.A. (800) 443-5853 or

Fax (425) 446-5116

In Europe/M-East/Africa +31 (0) 40 2675 200 or

Fax +31 (0) 40 2675 222

In Canada (800)-36-FLUKE or

Fax (905) 890-6866

From other countries +1 (425) 446-5500 or

Fax +1 (425) 446-5116

Web access: http://www.fluke.com

©2006 Fluke Corporation. All rights reserved.

Printed in U.S.A. 10/2006 2786485 A-EN-N Rev A

Fluke. Keeping your world

up and running.™

Conclusion

Whether one wishes to:

• spot check areas of concern

• use the MIN MAX feature on

the Fluke 975 AirMeter™ to

narrow in on problem sources

• use the “technical detective”

data logging feature to analyze

a space over an extended time

period to solve elusive prob-

lems

the Fluke 975 AirMeter™ is a

tool that can save time, money,

and even lives.

Reading CO2 levels in an office space.