IC 9482

INFORMATION CIRCULAR/2005

Nitrogen Dioxide Calibration

Standards for Portable Monitors

 Department of Health and Human Services

 Centers for Disease Control and Prevention

 National Institute for Occupational Safety and Health
 Information Circular 9482

Nitrogen Dioxide Calibration Standards for Portable Monitors

By Joseph E. Chilton, Robert J. Timko, and Edward J. Chuhta

DEPARTMENT OF HEALTH AND HUMAN SERVICES

Centers for Disease Control and Prevention

National Institute for Occupational Safety and Health

Pittsburgh Research Laboratory

Pittsburgh, PA

December 2005
This document is the public domain and may be freely copied or reprinted.

Disclaimer: Mention of any company or product does not constitute endorsement by the

National Institute for Occupational Safety and Health (NIOSH). In addition, citations to

Web sites external to NIOSH do not constitute NIOSH endorsement of the sponsoring

organizations or their programs or products. Furthermore, NIOSH is not responsible for

the content of these Web sites.

ORDERING INFORMATION

Copies of National Institute for Occupational Safety and Health (NIOSH)

documents and information

about occupational safety and health are available from

NIOSH–Publications Dissemination

4676 Columbia Parkway

Cincinnati, OH 45226–1998

 FAX: 513–533–8573

 Telephone: 1–800–35–NIOSH

(1–800–356–4674)

 E-mail: pubstaft@cdc.gov

 Web site: www.cdc.gov/niosh

DHHS (NIOSH) Publication No. 2006–104
CONTENTS

Page

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Introduction ................................................................................... 2

Calibration anomalies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Methods ...................................................................................... 3

Electrochemical NO2 detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Creating NO2 standards using gas permeation systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Constant permeation rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Instrument response to NO2 ..................................................................... 5

NO2 cylinder tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Discussion and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

ILLUSTRATIONS

1. Graph of TMX412 monitor response to NO2-nitrogen and NO2-air mixtures as a function of flow rate

 with curve-fit results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2. Crowcon NO2 monitor graphic results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

TABLES

1. TMX412 NO2 monitor responses using NO2-nitrogen and NO2-air mixtures at selected flow rates . . . . . . . . . . . 5

2. Crowcon NO2 monitor responses using NO2-nitrogen and NO2-air mixtures at selected flow rates . . . . . . . . . . . 5

3. TMX412 indicated NO2 concentrations multiplied by flow rates in either nitrogen or air carrier gas ......... 5

4. Parts per million comparisons of permeation tube NO2 to measured NO2 for cylinders balanced

 with nitrogen and air .......................................................... 7
UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT

cm centimeter L liter

cm2

/min square centimeter per minute min minute

cm3 cubic centimeter mL/min milliliter per minute

g gram mm millimeter

g/cm3 gram per cubic centimeter mm Hg millimeter of mercury

g/hr gram per hour ng/min/cm nanogram per minute per centimeter

g/L gram per liter ppm part per million

gm-mol gram mole sec second

gm-mol/cm3 gram mol per cubic centimeter μA microampere

hr hour μg microgram

K kelvin °C degree Celsius
NITROGEN DIOXIDE CALIBRATION STANDARDS FOR PORTABLE MONITORS

By Joseph E. Chilton,1 Robert J. Timko,2 and Edward J. Chuhta3

ABSTRACT

 Mine operators and Mine Safety and Health Administration (MSHA) inspectors use portable gas monitors

in underground mines to measure worker exposure to various gases such as methane, carbon monoxide, and

nitrogen dioxide (NO2). Even in relatively small concentrations, NO2 can produce harmful side effects in

underground workers. Mines using equipment powered by diesel engines, carrying out explosive blasting

operations, and performing extensive arc welding and/or cutting work will have measurable quantities of NO2

released by these processes. Workers in these areas often use gas monitors to warn of excessive gas concentrations and to help them determine when to increase ventilation air that will dilute these gases, thus

minimizing exposures to this gas. Portable gas monitors, used to measure NO2 levels, must be calibrated before

each use. Calibration is typically performed using a small portable pressurized cylinder containing a certified

gas mixture. The certified NO2 standard mixture, usually 10 ppm NO2 concentration, is diluted with either air

or nitrogen gas.

To determine the stability of commercially available portable gas standards, two different cylinders

containing NO2 in nominal 10-ppm concentrations were examined. One cylinder was diluted in air, the other,

in nitrogen. Baseline NO2 mixtures were generated using a standard liquid NO2 permeation tube operated at

a constant temperature. Final dilutions of the test gas mixtures were made with air or with nitrogen using a

mass flow-controlled gas dilution system. This study showed that, when a permeation-type NO2 generator is

used, the electrochemical sensors in portable NO2 monitors provide identical results for gas mixtures whether

they are diluted with air or with nitrogen.

Two different gas monitors, one with a two-electrode sensor and one with a three-electrode sensor, were used

to measure gas concentrations in the two commercially available cylinders. One month after the cylinders were

delivered, the NO2-in-nitrogen cylinder, stated to contain 10 ppm NO2, measured 8.13 ppm. The NO2-in-air

cylinder, also stated to hold 10 ppm NO2, measured 6.88 ppm. Therefore, before using NO2 gas cylinders to

calibrate monitors, it is recommended that their concentrations be verified either by manufacturer recertification

or by comparing the stated cylinder gas concentrations to a laboratory-based permeation tube-generated

standard.

1

Research chemist, Pittsburgh Research Laboratory, National Institute for Occupational Safety and Health, Pittsburgh, PA. 2

Supervisory physical scientist, Pittsburgh Research Laboratory, National Institute for Occupational Safety and Health, Pittsburgh, PA. 3

Engineering technician, Pittsburgh Safety and Health Technology Center, Mine Safety and Health Administration, Pittsburgh, PA.
2

INTRODUCTION

Mine operators and Mine Safety and Health Administration (MSHA) inspectors use portable gas monitors to

measure underground worker exposure to nitrogen dioxide

(NO2). Mines that have equipment powered by diesel engines,

perform explosive blasting operations, and/or do extensive arc

welding or cutting work will have measurable quantities of NO2

released into the mine air. To dilute high NO2 concentrations

and thus limit worker exposures, mines must aggressively

ventilate working sections located downstream of these

operations.

The threshold limit value (TLV) time-weighted average

(TWA) health limit for NO2 exposure is 3 ppm for a

conventional 8-hr workday and a 40-hr workweek. The allowed

ceiling level and the short-term exposure limit (STEL)—a limit

determined with instantaneous monitors—are both 5 ppm. The

STEL values are taken over a 15-min TWA exposure that cannot be exceeded at any time during the workday even if the 8-hr

average TWA is within specifications.4

 Gas concentrations up

to 5 ppm NO2 in air may cause irritation to the eyes and

pulmonary tract. Concentrations between 10 and 20 ppm NO2

will cause coughing and burning in the throat and induce slowly

evolving pulmonary edema. This typically occurs over a 48-hr

period. The NO2 concentration that is immediately dangerous to

life or health (IDLH) is 20 ppm for an exposure time up to

30 min. Serious complications, including death, can occur from

extended exposures at levels of 100 to 200 ppm [NIOSH 1989,

1994].

MSHA regulations require maintenance of instruments used

to obtain NO2 samples. 30 CFR5

 75.320(c) states: "Handheld

devices that contain electrical components and that are used for

measuring air velocity, carbon monoxide, oxides of nitrogen,

and other gases shall be approved and maintained in permissible

and proper operating condition." Comments on this rule

conclude that "proper operating condition" implies calibration

of the instrument. 30 CFR 70, subpart T, addresses the

measurement of exhaust gases from diesel-powered equipment.

Under this subpart, 30 CFR 70.1900(b)(2) states that NO2 shall

be "collected and analyzed by appropriate instrumentation

which has been maintained and calibrated in accordance with

the manufacturer's recommendations." The inference here is

that portable NO2 gas monitors, when used in enforcement

work, should be calibrated before each health survey.

Field calibration of working gas monitors involves several

steps. Individuals who calibrate monitors use specific concentrations of certified NO2 mixtures that are contained in

portable pressurized cylinders. A flow regulator on the

calibration cylinder sets the flow of the certified mixture to

500 mL/min. The gas is then introduced to the monitor through

a hose connected to a manufacturer-provided calibration fixture.

Once the instrument readout has stabilized, differences between

the stated cylinder concentration and the readout are corrected

by either adjusting the readout value by software modification

or by manually adjusting the gain resistor on the instrument until

the readout is equal to the cylinder's stated calibration

concentration.

The purpose of this study was to compare calibration gas

cylinders having balance gases of nitrogen or air that are

commonly used to calibrate portable NO2 gas monitors.

Because even low concentrations of NO2 can have an extremely

adverse effect on individuals working in areas where NO2

emissions exist, it is imperative that portable NO2 monitors be

accurately calibrated. The importance of this work was noted

when a portable calibration gas cylinder, labeled to contain

2 ppm NO2, was found by using a permeation system-calibrated

monitor to contain only 0.9 ppm NO2. If this calibration

cylinder were used to calibrate a field NO2 monitor, the resulting

measurements would be over 100% too high, and a mine could

be improperly cited for excessive NO2 exposure. The accuracy

limits for this cylinder over time or the mixture stability were

not addressed.

CALIBRATION ANOMALIES

To determine actual NO2 concentrations in commercially

available calibration cylinders, we obtained certified calibration

mixtures of NO2-in-nitrogen and NO2-in-air from local

suppliers. Labels on both calibration cylinders stated that the

NO2 concentration in each was 10 ppm. However, when the

monitors calibrated with the NO2-in-nitrogen mixture were

compared to those calibrated with the NO2-in-air mixture, the

4

The STEL and TWA values cited are those found in the American

Conference of Governmental Industrial Hygienists (ACGIH) 1972 TLVs used

by MSHA for coal mines and the ACGIH 1973 TLVs used by MSHA for metal

and nonmetal mines. Current NIOSH-recommended STEL limits are 1 ppm.

Occupational Safety and Health Administration regulations cite 5 ppm as the

ceiling value. 5

Code of Federal Regulations. See CFR in references.

NO2-in-air mixture results were 20% lower and thus out of

compliance. We developed three hypotheses for these observed

results:

1. Assuming the two calibration mixtures have equal

as-advertised NO2 concentrations, the sensor response is greater

in nitrogen mixtures than in air mixtures.

2. If, as filled initially, the two calibration mixtures had

equal NO2 concentrations, then the oxygen and the NO2 in the

NO2-in-air mixture may be reacting in the cylinder to reduce the

NO2 concentration in the cylinder over time.

3. The air-diluted cylinder as initially filled had a smaller

NO2 concentration than the nitrogen-diluted cylinder.
3

METHODS

ELECTROCHEMICAL NO2 DETECTORS

Direct and continuous measurement of NO2 concentrations

can be made using either color-indicating passive chemical

sensors or direct-reading handheld instantaneous monitors that

contain electrochemical sensors. The electrochemical sensor is

either membrane diffusion-limited or capillary space diffusionlimited and uses electrochemical cells that have two or three

independent electrodes. The two-electrode sensor consists of

one anode and one cathode. The three-electrode sensor consists

of a working electrode, the cathode, a counterelectrode, the

anode, and a reference electrode. Both sets of electrodes are

held in an electrolyte solution of 2M sulfuric acid in water.

Electronic circuitry for the three-electrode cell maintains the

ratio of reference to working electrode potential at a fixed

voltage and measures the current flowing between the working

and counterelectrodes. The two- electrode sensor has an

external fixed voltage across the electrodes. The measured cell

current is converted to a voltage and amplified. During

calibration procedures, the output voltage is adjusted with the

zero set potentiometer initially so that the meter display with

exposure to pure air is equal to zero. With exposure to

calibration gas, the meter display is adjusted with the span set

potentiometer to equal the NO2 concentration stated on the

calibration cylinder.

City Technology Ltd., Portsmouth, U.K., produces one

example of the three-electrode type of electrochemical NO2

sensor. The sensitivity of its model 4ND sensor is 0.6 μA/ppm

NO2. This sensor, with an electronic amplifier, has a resolution

of 0.1 ppm, a response time of 24 sec for 90% of final signal,

and a range of 0 to 20 ppm. Handheld portable gas monitors

using these types of electrochemical NO2 sensors are available

from several different manufacturers, including Biosystems,

Middletown, CT; Draeger Safety, Inc., Pittsburgh, PA;

Industrial Scientific Corp., Oakdale, PA; Mine Safety

Appliances Co., Pittsburgh, PA; Crowcon Corp., Milford, OH;

and Thermo Electron Corp., Waltham, MA.

CREATING NO2 STANDARDS USING

GAS PERMEATION SYSTEMS

Standard mixtures of gases are obtained typically in one of

three ways: from cylinders containing the pressurized standard

gas mixture, by mixing pure gases from cylinders with diluent

gases using precision gas flow controllers, or from the dilution

of gases generated at a fixed rate by a permeation device. The

gas permeation system works best for generating mixtures that

are reactive with cylinder wall materials or when making lowconcentration mixtures with standard gases requiring low total

airflows.

To determine the accuracy of an NO2 monitor in either

nitrogen-diluted or air-diluted carrier mixtures, we created

known concentrations of NO2 diluted in air and in nitrogen. The

description that follows also demonstrates that creating accurate

gas mixtures using gas permeation systems requires careful

procedures and proper consideration of the variables involved.

A permeation system produced concentrated NO2 gas using an

NO2 permeation tube made by VICI Metronics of Poulsbo, WA.

This permeation tube is a cylindrical FEP6

 Teflon® plastic tube

with a 4.80-mm outside diameter and a 0.80-mm-thick wall.

The gas diffusion length is 10 cm; the full permeation tube

length including seals is 13.5 cm. The manufacturer states that

with two-phase (liquid and gas) equilibrium at a constant

temperature, the gas permeation rate will be constant over the

life of the tube [VICI Metronics 2004]. At 30 °C, the manufacturer's stated NO2 permeation rate is 1,715 ± 10 ng/min/cm

of tube exposed. Chemical longevity in a standard tube

operating at 30 °C with a 75% initial tube fill is calculated as

follows:

(1)

where L ' tube lifetime, months;

p ' permeation rate;

and DL ' liquid density, g/cm3

.

As an example, assuming a liquid NO2 density of 1.4 g/cm3

and a stated permeation rate of 1,715 ng/min/cm (at 30 °C), the

lifetime of a 10-cm tube would be 1.14 months. Lifetimes can

be extended by operating the permeation tubes at 20 °C with an

initial measured permeation rate of 4.51 × 10&4

 g/hr, which

equals a 759 ng/min/cm loss rate. The calculated permeation

tube lifetime then becomes 2.6 months.

The rate of permeation of a gas through the walls of a porous

cylindrical tube is written as

PR ' (6.283(L × D)(Ci & Co))/ In(R1/R2), (2)

where PR ' rate of gas leaving the tube per minute, gm-mol;

L ' tube length, cm;

D ' effective diffusion coefficient through the

 plastic wall, cm2

/min;

Ci

, Co ' inner and outer gas concentrations, gm-mol/cm3;

and R1, R2 ' inner and outer cylinder radii, cm.

Since the dry nitrogen flowing over the tube continually

ventilates the permeation tube, the outer gas concentration Co

was set to zero. Thus, by Equation 2, the gas evolution rate was

constant when the inner concentration was constant. With

liquid-vapor equilibrium from both gas and liquid present at a

constant temperature, the vapor pressure was also constant. At

6

Fluorinated ethylene propylene.

L

p L = × 1,400

ρ ,
4

constant temperature and pressure, the gas concentration was

also constant. The liquid level decreased during the life of the

tube, so the inner tube area that was exposed to gas increased as

the area exposed to liquid decreased. Since the liquid

concentration at the liquid-tube interface was 1,448 g/L (a very

high value compared to the gas concentration of 3.4 g/L) and the

permeation rate did not change greatly over the life of the tube,

the permeation mechanism had to be primarily that of gas

permeating through the FEP Teflon® plastic tube.

The NO2 gas permeation tube was held in a Pyrex® glass

U-tube in a constant 20 °C water bath kept at a tolerance of

%0.1 °C and &0.1 °C. The inlet side of the U-tube was filled

with glass beads to promote temperature stabilization with the

dry nitrogen carrier gas. The outlet side of the U-tube held the

permeation tube, and Teflon®-lined plastic screw tops sealed

the tops of the U-tube. Dry nitrogen carrier gas constantly

flowed through the U-tube and over the permeation tube to

remove the permeated gas. Stainless steel fittings and tubing

connected the U-tube to 6.4-mm inlet and outlet glass tubes.

A pressurized nitrogen cylinder contained dry nitrogen carrier

gas, which was fed through a molecular sieve-drying column,

through a secondary pressure regulator, and through a precision

needle valve to maintain the constant dilution gas flow over the

permeation tube at 100 mL/min.

Once or twice each week throughout the life of the

permeation tube, it was removed from the U-tube holder and

weighed on a Mettler-Toledo H51AR semimicro analytical

balance having a sensitivity of 10 μg. The tube masses and the

measurement times and dates were recorded. From these

repeated weight measurements, the rate of NO2 tube loss was

calculated in grams per hour. At the 20 °C bath temperature

over a period of several months, the weight loss rate varied from

an initial value of 4.5 × 10&4

 g/hr to a final value of 3.9 × 10&4

g/hr. Since the weight loss of the tube equaled the gaseous NO2

delivered to the test device, the NO2 gas concentration was

calculated from the rate of tube weight loss and the measured

total gas dilution flow.

CONSTANT PERMEATION RATES

Although VICI Metronics (the manufacturer) states that

during the life of the tube at a 30 °C temperature, the permeation

rate is relatively constant (within ±2%), this has not been our

experience while working at 20 °C. In our tests, the measured

permeation rates decreased as the tube aged, almost in

proportion to the change in tube liquid level. The boiling point

of the liquid in the NO2 tube is stated to be 21.2 °C. At 20 °C,

the liquid was primarily colorless dinitrogen tetroxide (N2O4)

with a small amount of dissolved brown NO2. The gas phase

was primarily reddish-brown NO2. In light of the demonstrated

nonconstant permeation rate, we measured, at least weekly, the

actual NO2 permeation rate at 20 °C over the life of the tube.

This was done to determine the current loss rate that we used for

later calculation of the NO2 concentration at a given time with

a given total airflow.

The manufacturer stated that trace water accumulation,

during both the tube run and the weighing operations, might

explain part of the reduced permeation rate for the NO2 system.

Therefore, to reduce the water content of our nitrogen purge gas,

a 22.9-cm-long and 2.5-cm outside diameter molecular sieve

water trap from Scott Specialty Gases of Plumsteadville, PA,

was placed after the regulator of the pressurized gas cylinder.

The generation of standard NO2 mixtures could conveniently be

done if the weight loss of the tube was continually measured

over the life of the permeation system. Accuracy of the weight

measurement required the use of a semimicrobalance with at

least an 8-g capacity. In addition, accurate airflow measurements with a bubble transit time gas flow system, along with

reliable temperature and atmospheric pressure instruments, were

used to calculate the actual test gas concentrations.

To form a desired concentration for calibration, we mixed the

permeated NO2-in-nitrogen carrier with additional nitrogen or

with dilution air. We controlled the quantity of additional

nitrogen- or air-dilution mixtures with a control system using

model FC–261 mass flow controllers from Tylan Corp. of

Carson, CA. This flow system was assembled under a previous

research effort by the U.S. Bureau of Mines [Carpenter et al.

1983]. For each specific gas concentration desired, the dilution

gas flows were set by adjusting mass flow controller voltage and

then measured with a Gilibrator wet-test bubble meter made by

Gilian Sensidyne, Clearwater, FL. The total gas flow was the

sum of the nitrogen flow over the tube and the diluent gas flow.

The equation for the diluted gas concentration is

C wt ( ) (3) TG

FP F = / ,

where CF ' diluted gas concentration, ppm;

w/t ' weight loss rate, g/hr;

T ' absolute temperature, K;

F ' total gas flow rate, mL/min;

P ' pressure (usually atmospheric), mm Hg;

and G ' constant containing standard temperature,

 pressure, and molecular weight values

 22.6079 × 106

.

As an example, at a measured permeation loss rate of 4.5 ×

10&4

 g/hr, a test temperature of 294 K (21 °C), a pressure of

737 mm Hg, and a total gas flow of 1,000 mL/min, the NO2

concentration was 4.07 ppm. The tube permeation rate at 293 K

(20 °C) was sufficient to generate NO2 concentrations from

1 ppm at a total combined gas flow of 3,600 mL/min to 7 ppm

at a total combined gas flow of 500 mL/min.

The final test-gas humidity was adjusted by varying the ratio

of two mass-controlled airflows, one dry and one wet, to form

the final airflow. Wet air was generated by running the air

through a water bubbler. The humidified diluent gas was added

to the nitrogen and NO2 mixture so that the gas phase NO2 did

not contact liquid water. If it had, the gaseous NO2 would have

been lost by dissolution in the liquid water.
5

RESULTS

INSTRUMENT RESPONSE TO NO2

Once specific gas mixtures in nitrogen and air carriers were

created, the next step was to examine the responses of a

commercially available NO2 monitor in both nitrogen and air

carrier gases. In these tests, over 10-min intervals, an Industrial

Scientific Corp. TMX412 monitor measured the response to

permeation-derived NO2 in both air-diluted and nitrogen-diluted

gas mixtures. Test results are shown in Table 1. The TMX412

monitor included the City Technology 4NT sensor. It gave

equal indicated ppm results for NO2 mixtures whether in air or

in nitrogen diluents. The resulting coefficient of determination

R2

 and power regression equations are shown in Figure 1.

These tests were repeated using a two-electrode membrane

limited- diffusion NO2 sensor made by Crowcon Detection

Instruments Ltd., Milford, OH. The results of these tests are

shown in Table 2. As in the previous tests, the Crowcon sensor

indicated that the measured concentrations were essentially the

same whether in nitrogen or in air mixtures. The resulting

coefficient of determination R2

 and power regression equations

for the Crowcon are shown in Figure 2.

Table 1.—TMX412 NO2 monitor responses using NO2-nitrogen

and NO2-air mixtures at selected flow rates

Test

NO2 in nitrogen NO2 in air

Flow, mL/min NO2, ppm Flow, mL/min NO2,

ppm

1 . . . . 1,851 2.2 1,843 2.2

2 . . . . 943 4.3 928 4.4

3 . . . . 630 6.2 618 6.3

4 . . . . 403 9.4 378 10.0

Table 2.—Crowcon NO2 monitor responses using NO2-nitrogen

and NO2-air mixtures at selected flow rates

Test

NO2 in nitrogen NO2 in air

Flow, mL/min NO2, ppm Flow, mL/min NO2,

ppm

1 . . . . 1,845 1.86 1,845 1.82

2 . . . . 935 3.59 935 3.57

3 . . . . 625 5.39 625 5.43

4 . . . . 402 8.38 402 9.02

The generated diluted NO2 gas ppm concentration was

calculated from the Equation 3 gas flow rate. To simplify,

Equation 3 can be rewritten as follows:

QA × F ' C, (4)

where C ' constant.

For specific instruments, a calibration factor can be incorporated into the constant of Equation 4 by using the indicated

NO2 value rather than the value obtained mathematically in

Equation 3, as follows:

QI

 × F ' CN, (5)

where QI ' indicated NO2 concentration, ppm;

and CN ' C multiplied by a calibration factor for the

 specific instrument being evaluated.

Equation 5 is used to generate the data in Table 3, which

shows the TMX412 indicated NO2. The measured instrument

gas flow found in Table 1 was multiplied by the indicated

concentration. For these tests the values were almost constant,

averaging 3,948 with a standard deviation of 122. It should be

emphasized that these permeation tube tests involved all dry-gas

mixtures. Test data have been found to change with the

moisture content of the atmosphere in which the samples are

obtained. The water dependence of the anode reaction indicated

that the TMX412 NO2 sensor is humidity-dependent. The

average product value was 4,500 for humidified test mixtures

having a room relative humidity of 25%. This value was again

obtained by using Equation 5, multiplying the indicated

concentration by the airflow. Comparing this higher CN value to

dry gas mixtures, an increased sensitivity is indicated.

These tests show that there is no inherent difference in

indicated NO2 values when diluted by equally humidified

mixtures of either nitrogen or air, at least for the TMX412

electrochemical NO2 sensor. Thus, hypothesis 1 that the

balance or dilution gas species either nitrogen or air containing

oxygen alone caused gas values to differ is proven false.

Table 3.—TMX412 indicated NO2 concentrations multiplied 

by flow rates in either nitrogen or air carrier gas

NO2 in nitrogen,

ppm × mL/min

NO2 in air,

ppm × mL/min

4,072 4,054

4,055 4,038

3,906 3,893

3,788 3,780

NO2 CYLINDER TESTS

As previously stated, two cylinders of NO2 calibration gases

were obtained from local distributors. Each cylinder was

labeled to contain 10 ppm of NO2. One was diluted with air; the

other, with nitrogen gas. Both cylinders were constructed of

aluminum and had an aluminum-alloy, G–35-type on-off control

valve, with a C–10 outlet fitting for coupling to a flow control

regulator. Pressure in the cylinders was about 33.1 bars. The

gas volume at 21 °C and one atmosphere pressure was 73.6 L.

Cylinder No. 1, diluted with nitrogen, had an expiration date

1 year after its issue date, whereas cylinder No. 2, diluted with

air, had a stated useful life of 6 months.
6

Air

y = 2880 x-0.953

R2 = 0.999

Nitrogen

y = 2850 x-0.951

R2 = 0.999

0

2

4

6

8

10

200 400 600 800 1000 1200 1400 1600 1800 2000

Flow rate, mL/min

ppm

Nitrogen Air

Figure 1.—Graph of TMX412 monitor response to NO2-nitrogen and NO2-air mixtures as a function of flow rate with

curve-fit results.

Air

y = 4660 x-1.046

R2 = 0.999

Nitrogen

y = 3130 x-0.988

R2 = 0.999

0

2

4

6

8

10

200 400 600 800 1000 1200 1400 1600 1800 2000

Flow rate, mL/min

ppm

Nitrogen Air

Figure 2.—Crowcon NO2 monitor graphic results.
7

A handheld TMX412 monitor and a Crowcon fixed-point

monitor were used to measure NO2 concentrations in the two

cylinders. In addition, both monitors measured a concentration

of permeation-tube-generated NO2 mixed with air. The actual

cylinder concentrations were based on a calculation of each

monitor's measurement. The derived proportion is expressed as

C M (6) P

M C

P

= × ,

where C ' cylinder calculated concentration, ppm;

MC ' NO2 monitor cylinder measurement, ppm;

P ' permeation tube calculated concentration,

 ppm;

and MP ' NO2 monitor permeation tube measurement,

 ppm.

The results are summarized in Table 4.

When the cylinder concentrations were measured in

reference to a standard permeation tube-generated gas mixture,

the average NO2 concentrations of both calibration gas cylinders

were less than the advertised 10-ppm value. The calculated

average NO2-in-nitrogen mixture value was 8.13 ppm

(18% below the labeled value), and the NO2-in-air mixture value

was 6.88 ppm (31% below the labeled value). In comparing the

average NO2-in-nitrogen concentration to the NO2-in-air

concentration, the nitrogen carrier gas was found to be 22%

higher. Based on these results, it now seems that hypothesis 2,

the mixture is not stable in the air diluent, is correct and that

hypothesis 3, initial concentrations were not the same, could be

correct.

Table 4.—Parts per million comparisons of permeation tube NO2 to measured NO2

for cylinders balanced with nitrogen and air

Monitor

NO2 concentration, ppm

Cylinder No. 1

10 in nitrogen (label)

Cylinder No. 2

10 in air (label)

Permeation tube

4.20 (calculated)

TMX412 (measured) . . . . . . . . . . . 9.50 8.10 5.10

TMX calculated cylinder . . . . . . . . 7.82 6.67 —

Crowcon (measured) . . . . . . . . . . 6.97 5.86 3.47

Crowcon calculated cylinder . . . . . 8.44 7.09 —

Cylinder average . . . . . . . . . . . . . 8.13 6.88 —

DISCUSSION AND CONCLUSIONS

Portable calibration gas cylinders are normally used to

calibrate monitors for in-mine measuring of various gases. If

one sets the response of a portable gas monitor to the cylinder

stated value, for example 10 ppm, when the actual NO2

concentration in the cylinder is lower, say, 5 ppm, then the

in-mine measurements obtained would be 100% high. Thus, if

one used this NO2 gas cylinder to calibrate a field monitor,

a mine site could be improperly cited for a STEL of 6 ppm NO2

when the true exposure was actually only 3 ppm. To avoid such

problems, it is necessary to use calibration gases with known

values to calibrate monitors used in mine testing.

The differences in portable monitor responses to NO2

calibration gases made either in air or in nitrogen were believed

to be the result of one or more of three different causes. Hypothesis 1 suggested that there was a difference in response

sensitivity of the sensors caused by the presence of either air or

nitrogen as the carrier gas. This work found that portable NO2

monitors with electrochemical sensors are similarly sensitive to

equal concentration NO2 mixtures diluted in either nitrogen

or air.

A survey of the chemical reactions that occur within the

sensor cell also suggests that the carrier gas either air or nitrogen

should not have a significant effect on NO2 measurements. The

chemical reactions occurring in the sensor cell are shown below.

The half-cell reaction at the cathode (the working electrode) of

the NO2 sensor is the reduction of NO2 to form nitric oxide (NO)

and water (H2O) in the sulfuric acid electrolyte, expressed as

follows:

NO2 % 2H% ' NO % H2O & 2e (A)

The half-cell reaction at the anode (the counterelectrode) is the

oxidation of water (H2O) to form oxygen (O2) and hydrogen

ions (H%

):

2H2O ' O2 % 4H% % 4e (B)

The net reaction for the total cell is the sum of the two half

reactions, the reduction of NO2, and the production of nitric

oxide and oxygen:

2NO2 ' 2NO % O2 (C)
8

The two electrode reactions require only NO2 and water. Thus,

the NO2 sensor should work equally well in nitrogen or in air

calibration gas mixtures since this cell does not use oxygen as

a reactant in the total electrode reaction.

Hypothesis 2 suggested that, if the initial gas concentrations

were equal in both cylinders, the gas mixture with air decays

more rapidly than the gas mixture with nitrogen. This was

found to be true for 1-month-old gas cylinders if both mixtures

had been originally filled to the same initial gas concentrations.

We would suspect that both mixtures were originally designed

to be made up to the same concentration since both were labeled

as initial 10 ppm. There is no reason to suggest that gas

mixtures of NO2 in air with oxygen should not be stable, for

NO2 does not decompose to NO at room temperatures. Instead,

if the air in the cylinder contained water, then the NO2 could

react with the metallic cylinder walls to form N2, which is a

more plausible reason for loss of NO2 concentration in the

aluminum alloy cylinder.

Hypothesis 3 suggested that the initial mixes of the

calibration gases were not made to equal concentrations. Since

we do not have access to the procedures used by the manufacturer nor specific data on the preparation of cylinders,

we cannot prove this hypothesis.

In general, the NO2-in-air calibration system is preferred for

field use since multigas monitors containing in addition an oxygen

sensor may be used at worksites together with the NO2 sensor. If

calibration of the NO2-sensing portion of a multigas instrument is

performed in the nitrogen carrier system, the oxygen sensor

display will indicate 0%, causing the instrument to alarm with

both audible signals and flashing lights. The operation of these

alarms requiring battery power during calibration may

significantly reduce the charged capacity of the instrument battery

and thus limit the subsequent monitor use time.

The measurements reported here are for only one commercial

source of cylinder calibration gas. These findings should be

confirmed with other calibration gas cylinder sources. Actual

cylinder NO2 concentrations should be calculated by checking

the advertised gas concentration against either a primary

certified gas mixture, possibly obtained from the National

Institute of Standards and Technology or from a permeation tube

source maintained at a laboratory site. In addition, we

recommend that the actual gas concentration of portable NO2

calibration cylinders be verified periodically or at least monthly.

As previously explained, this pre- and post-calibration requirement becomes crucial prior to using NO2 monitors for

enforcement work.

REFERENCES

Carpenter CR, Chilton JE, Schnakenberg GH, Jr. [1983]. A dynamic gasmixing system. Pittsburgh, PA: U.S. Department of the Interior, Bureau of

Mines, IC 8934. NTIS No. PB 84–101450.

CFR. Code of Federal regulations. Washington, DC: U.S. Government

Printing Office, Office of the Federal Register.

NIOSH [1989]. Nitrogen dioxide. OSHA comments from the January 19,

1989, final rule on Air Contaminants Project extracted from 54 FR 2332 et seq.

[http://www.cdc.gov/niosh/pel88/10102-44.html]. Date accessed: July 2005.

NIOSH [1994]. Nitrogen dioxide: IDLH documentation. [http://

www.cdc.gov/niosh/idlh/10102440.html]. Date accessed: July 2005.

VICI Metronics [2004]. Generating calibration gas standards with

Dynacal® permeation devices. Technical note 1001. [http://www.vici.com/

support/tn/tn1001.pdf]. Date accessed: July 2005.
Delivering on the Nation’s promise:

Safety and health at work for all people

through research and prevention

For information about occupational safety and health topics contact NIOSH at:

1–800–35–NIOSH (1–800–356–4674)

Fax: 513–533–8573

E-mail: pubstaft@cdc.gov

www.cdc.gov/niosh

SAFER • HEALTHIER • PEOPLETM

DHHS (NIOSH) Publication No. 2006–104