Conceptual Design, Engineering Modeling, and 

Experimental Validation of Air Sampling System for 

Chemical Sensor Insertion Into the U.S. Army Research 

Laboratory’s (ARL) Silent Operating Aerial 

Reconnaissance (SOAR) Program 

by Michael L. Nair 

ARL-TR-3105 February 2004 

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Army Research Laboratory 

Aberdeen Proving Ground, MD 21005-5069 

ARL-TR-3105 February 2004 

Conceptual Design, Engineering Modeling, and 

Experimental Validation of Air Sampling System for 

Chemical Sensor Insertion Into the U.S. Army Research 

Laboratory’s (ARL) Silent Operating Aerial 

Reconnaissance (SOAR) Program 

Michael L. Nair 

Weapons and Materials Research Directorate, ARL 

Approved for public release; distribution is unlimited.
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4. TITLE AND SUBTITLE 

 Conceptual Design, Engineering Modeling, and Experimental Validation of Air 

 Sampling System for Chemical Sensor Insertion into the U.S. Army Research 

 Laboratory’s (ARL) Silent Operating Aerial Reconnaissance (SOAR) Program 

5c. PROGRAM ELEMENT NUMBER

5d. PROJECT NUMBER 

 1L162618H80 

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6. AUTHOR(S) 

 Michael L. Nair (ARL) 

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 U.S. Army Research Laboratory 

 Weapons and Materials Research Directorate 

 Aberdeen Proving Ground, MD 21005-5069 

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 ARL-TR-3105 

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14. ABSTRACT 

The U.S. Army Research Laboratory is interested in using a concept unmanned aerial vehicle (UAV) as a chemical weapons 

detection platform. This report details a preliminary effort to determine whether the UAV is capable of sustaining the 

needed air flow into a chemical weapons detector to ensure functionality. 

For this study, it was decided that a minimum volumetric flow rate of 1.4 liters/minute is required to satisfy the minimum 

flow requirement. Basic fluid mechanic equations are used to select the incompressible model for air flow, and then ADINA 

(automatic dynamic incremental nonlinear analysis) is used to create finite element analysis approximations of the flow 

problem. Based on the results of the computer analysis, it was decided that the best concept for acquiring the air flow is to 

use a small blower with an intake and exhaust orifice in the bottom of the UAV. A prototype device has been built and 

tested through the estimated flight profile of the UAV. 

It was found that this simple device leads to volumetric flow rates approximately 10 times higher than the minimum required 

for chemical weapons detection. Based on these results, it is possible to use this UAV as a chemical weapons detector with 

the induced air flow concept.

15. SUBJECT TERMS

 chemical weapons detection; chemical weapons detector; UAV; unmanned aerial vehicle 

16. SECURITY CLASSIFICATION OF: 

19a. NAME OF RESPONSIBLE PERSON

 Michael L. Nair 

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iii 

Contents 

List of Figures iv

List of Tables iv

Acknowledgments v

1. Introduction 1

2. Theory 5

3. Theoretical Analysis 6

4. Materials and Apparatus 12

5. Experimental Procedures 15

6. Results 17

7. Discussion 21

8. Conclusions and Recommendations 22

9. References 23

Appendix A. E-mail Correspondence Regarding Volumetric Flow Rates for Chemical 

 Weapons Detectors 25

Appendix B. Project Proposal From ARL, Received on January 3, 2003 29

Appendix C. Specifications Sheet for DC Motor Used in Prototype 31

Appendix D. Anemometer Calibration 33

Distribution List 37
iv 

List of Figures 

Figure 1. Deployed GLUAV in flight (artist’s conception) from ARL CR-616. ...........................1 

Figure 2. Side view of the GLUAV in folded configuration. .........................................................4 

Figure 3. Front view of GLUAV in folded configuration. .............................................................4 

Figure 4. Outline of GLUAV geometry for ADINA analysis. .......................................................7 

Figure 5. Air flow across bottom surface of GLUAV at 8 m/s, 0 AOA.........................................8 

Figure 6. Air flow across bottom surface of GLUAV at 8 m/s, 20 AOA.......................................9 

Figure 7. Air flow across bottom surface of GLUAV at 20 m/s, 20 AOA.....................................9 

Figure 8. Air flow through inlet at 20 m/s, 20 AOA.....................................................................10 

Figure 9. Air flow through inlet at 8 m/s, 20 AOA.......................................................................11 

Figure 10. Air flow through inlet at 20 m/s, 0 AOA.....................................................................11 

Figure 11. Concept design of blower assembly............................................................................12 

Figure 12. View of concept assembly installed in payload compartment of GLUAV. ................13 

Figure 13. Drawing of the intake housing for prototype. .............................................................13 

Figure 14. Drawing of the exhaust housing for prototype............................................................14 

Figure 15. Assembled prototype on test stand with anemometer holder......................................15 

Figure 16. Schematic of experimental setup.................................................................................16 

Figure 17. Velocity measurements at 0 m/s air flow with regression curve shown. ....................17 

Figure 18. Velocity measurements at 3.5 m/s air flow. ................................................................18 

Figure 19. Regression curve fitted to 3.5 m/s, 5 AOA air flow measurements............................19 

Figure 20. Velocity measurements at 10-m/s air flow..................................................................19 

Figure 21. Velocity measurements at 15-m/s air flow..................................................................20 

Figure 22. Velocity measurements at 20-m/s air flow..................................................................20 

Figure A-1. SOAR vehicle in flight (glider version). .................................................................29 

List of Tables 

Table A-1. Sample set of variables for part 1b. ............................................................................30 
v 

Acknowledgments 

The author would like to thank Dave Lyon, Timothy Vong, Gordon Brown, John Condon, and 

Marshall Childers of the U.S. Army Research Laboratory for their support and assistance in 

providing the opportunity to work on this project. Special thanks to Professor Ernesto Blanco of 

the Massachusetts Institute of Technology (MIT) for his support and guidance throughout this 

project, Steve Haberek of MIT for his assistance in machining, and Dr. Barbara Hughey of MIT 

for the use of her lab equipment. The author would also like to thank Brian Pharris for assisting 

during experimentation, and Michelle Neil for her help in performing the data analysis. 
vi 

INTENTIONALLY LEFT BLANK. 
1 

1. Introduction 

For the past several years, the U.S. Army Research Laboratory (ARL) has been developing a 

concept unmanned aerial vehicle called the gun-launched unmanned aerial vehicle (GLUAV), 

shown in figure 1. The GLUAV is deployed from a specially designed canister inserted within a 

large caliber 105-mm munition that is fired from a mortar or artillery weapon system. 

Figure 1. Deployed GLUAV in flight (artist’s conception) from ARL-CR-515. 

Upon reaching the apogee of the projectile flight path, the GLUAV is expelled and pulled from 

the rear of the canister by means of parachute; it then unfolds its wings and tail fins. The 

parachute is released from the GLUAV and free flight begins. 

The GLUAV is much smaller than typical UAVs in military inventory, such as the Predator or 

Global Hawk. The GLUAV is approximately 25 centimeters long with a wing span of similar 

size. Because of the GLUAV’s small size, its payload capacities are much more limited than 

other such vehicles. As with other UAVs, the GLUAV was originally conceived as a 
2 

reconnaissance platform to carry such equipment as video cameras and transmit the imagery 

back to the operator. 

ARL is now considering the possibility of using the GLUAV as a platform for chemical weapons 

detection. In order to function as a chemical weapons detector platform, the GLUAV must be 

able to successfully carry the sensor as well as satisfy the operational characteristics of a 

chemical weapons detector. 

Chemical weapons detectors function by passing air over a sensing element. This sensing 

element can vary in type, depending on the version and capability of the detector. However, all 

detectors do require a steady flow of air to sample. There is currently no chemical weapons 

detector of which the author is aware that will meet the design criteria for installation in the 

GLUAV. While size and weight are certainly limiting factors, the most challenging factor for 

this application is the survivability of the sensor. As the GLUAV is gun launched, it can 

undergo an initial “set-back” acceleration of approximately 15,000 times the force of gravity 

upon propellant ignition and then experiences an acceleration in the opposite direction (“set 

forward”) of about 10% to 50% (50% being the worst case) of that level as it travels down the 

gun barrel. These forces require a sensor that has been specifically designed to survive such 

loads, usually by the encapsulation of sensitive components. Since this analysis is a first-level 

study to determine the feasibility of using chemical weapons detectors, no work has been 

conducted in designing a detector for this particular application with its unique operational 

requirements. However, if testing is successful and ARL decides to proceed with development, a 

sensor will be designed and tested to meet the requirements for use in the GLUAV. Since weight 

and size of the detector will be determined when a sensor is designed, the critical parameter for 

the functionality of a chemical weapons detector for the GLUAV is the sustainability of 

sufficient air flow to the sensing element. 

To determine the air flow requirements for chemical weapons detectors, three sources were 

contacted. Environics1

, a company based in Finland, produces a small hand-held chemical 

weapons detector called the ChemPro100, which requires a steady flow of 1 liter per minute 

(L/min) of air. BAE Systems2 has recently developed the Chemsonde detector (see 

http://www.nbcindustrygroup.com/baesys.htm), a self-contained sensor designed to be packaged 

in a MJU-10 military flare cartridge. The Chemsonde system requires an air flow of only 

0.1 L/min according to information provided to ARL by BAE Systems. Finally, the U.S. Army 

Soldier and Biological Chemical Command (SBCCOM) was contacted about their experience 

with detectors. They reported that typical military sensors require an air flow of 1.2 to 1.4 L/min 

to function adequately. The correspondence with SBCCOM and Environics is presented in 

appendix A. Since there is some variance in the information reported, it will be assumed that 

greater air flow than 1.4 L/min will be sufficient for chemical weapons detection. 

 1Environics OY, Graanintie 5, P.O. Box 349, FIN-50101 Mikkeli, Finland. 

2BAE Systems, Integrated Defense Solutions, 6500 Tracor Lane, Austin, TX 78725. 
3 

Since the GLUAV is designed as an expendable system, the vehicle will terminate its flight near 

the target area. As a secondary goal of this analysis, it is desired to evaluate the possibility of 

using the GLUAV (after landing) as a stationary chemical weapons detection node. It is 

hypothesized that launching multiple GLAUVs in different directions could provide a sufficient 

number of chemical sensing nodes to relieve the burden of sending American personnel into 

potentially hostile territory to emplace current stand-alone chemical weapons detectors to protect 

U.S. troop concentrations. This functionality would depend on the battery life of the GLUAV 

and the ability to continue adequate air flow to the detector after the GLUAV comes to rest in 

what may be an undesirable orientation. Two assumptions will be assumed for this aspect of the 

analysis: 1) that the GLAUV will have sufficient power to run a chemical weapons detection 

system for some time after landing, and 2) that after landing, the chemical weapons detector’s air 

passages will not be blocked. 

The proposal for this project written by ARL personnel, which contains design requirements and 

guidance, is presented in appendix B. The proposal for this project was outlined in a meeting 

between the author and engineers at ARL in December 2002. While the attached document 

indicates that 1 mL/min is required to show functionality, that figure was based on information 

provided by other sources that have not shown functionality. 

All design information is taken from the SolidWorks3 assembly files for the GLUAV as well as 

ARL reports ARL-CR-466 (2) and ARL-CR-0515 (3). The GLUAV is designed to be launched 

from a conceptual 105-mm mortar or artillery weapons system and as such, the GLUAV is 

limited to a payload diameter of 1.75 inches and a total available payload length of 6 inches. 

ARL has estimated that the chemical weapons detection system should require no more than 3 

inches of that total length. The total payload capacity is initially estimated to be 320 grams. As 

with any other aeronautical application, minimum weight is desired. Any concept that is created 

must have the potential of weighing less than the maximum payload in production form. 

As the GLUAV is deployed from within a canister with its wings folded, any nozzles or other 

protrusions would be required to fit within a 0.1-inch gap between the folded wings. Figure 2 

shows a side view of the GLUAV in its folded configuration as it would sit inside its deployment 

canister. Shown on the bottom of the GLUAV are the wings lying in parallel to the axis of the 

vehicle. 

 

3

SolidWorks Corporation, 300 Baker Avenue, Concord, MA 01742.
4 

Figure 2. Side view of the GLUAV in folded configuration. 

Figure 3 shows a view looking at the front of the GLUAV in the same launch configuration. 

Figure 3. Front view of GLUAV in folded configuration. 

Illustrated in figure 3 is the small 0.1-inch gap formed by the folded wings, which would require 

extensive design of any nozzles protruding from the GLUAV. ARL has also requested that any 

nozzles or protrusions from the GLUAV should be a maximum of 1 inch apart in the axial 

direction of the vehicle. 

Front and Rear Wings Folded Along Bottom of GLUAV

0.1 Inch Gap Between Wings
5 

To determine the sustainability of air flow, it is essential to understand the conditions that the 

GLUAV will experience. In reference (3), the flight conditions of the GLUAV are provided. 

The velocity of the GLUAV ranges from a minimum of about 9 m/s to a maximum of about 

20 m/s. The angle of attack (AOA) experienced throughout the flight profile varies from 

0 degrees to a maximum of about 18 degrees. For the purpose of the study, the velocity range 

will be considered from 0 to 20 m/s and the AOA range from 0 to 20 degrees. 

The GLUAV is designed for use on the tactical battlefield. In order to allow operations 

wherever U.S. Army soldiers are deployed, the chemical sensor must function in a wide range of 

conditions. ARL desired that for the purpose of this study, the examined altitude should range 

from sea level to 1000 feet (304.8 meters) above sea level. No specific temperature range was 

provided by ARL; however, it was decided to ensure operations between -20° C and 50° C. 

Several different schemes were examined in the creation of a feasible design for this test. The 

initial concept relied on the use of the natural flow of air across the bottom surface of the 

GLUAV. It was thought that with proper design of the nozzles, it would be possible to create the 

needed air flow by managing the flow correctly. Because of the gap between the wings, two 

ideas were considered for nozzles. The first was a fixed nozzle that would “mushroom” out 

below the folded wings and be narrow enough at the given position to allow for the small wing 

gap. The second idea was to use an intake nozzle that would be spring loaded and pop out from 

the body of the GLUAV when the wings deployed. This would remove the flow restriction 

imposed by narrowing the nozzle to fit within the 0.1-inch dimension. Finally, an induced flow 

configuration was also considered in which a small blower would be placed inside the GLUAV 

to force air through an orifice and air passage. 

2. Theory 

To analyze the different ideas, the basic assumptions of the model were decided. The key 

decision for the model was whether to use a compressible or incompressible model. The 

incompressible flow model can be used if the fluid flow meets the following condition (1): 

Ma ≤ 0.3 (1) 

in which Ma is the Mach number given by the following equation (1): 

2

2

2

= Ma

a

V (2) 

in which V is the velocity and a is the speed of sound in the given fluid. The speed of sound in a 

fluid is calculated according to the relationship (1) 
6 

( )1/ 2

a ≈ kRT (3) 

in which T is the fluid temperature, R is the gas constant, and k is given as cp/cv.

After one selects the model to use, it is possible to determine the boundary layer effects 

according to the following sequence. To analyze the flow of the fluid over the body of the 

GLUAV, the Reynolds number is found by the equation (1) 

μ

ρVL Re (4) =

in which ρ is the fluid density, V is the fluid velocity, L is the given length, and μ is the 

coefficient of viscosity. Making the assumption that the resulting fluid will be laminar, given by 

Re < 106

, the thickness of the boundary layer is given as (1) 

1/ 2 Re

5.0 ≈ x

δ (5) 

in which δ is the boundary layer thickness and x is the position of the point in question. 

However, this equation only holds for fluid flow parallel to the surface of the body. To verify 

the results of these equations and to study the effects of varying AOAs, finite element analysis 

(FEA) software is used to simulate the flight conditions that the GLUAV would encounter during 

its normal flight profile. 

3. Theoretical Analysis 

To determine whether an incompressible model is acceptable, the Mach number in air must be 

determined. Given the need to use the GLUAV in typical battlefield conditions, it has been 

decided that calculations must be made with the assumption of a minimum operating temperature 

of -20° C and a maximum temperature of 50° C. Although this range does not accommodate 

every possible condition, it will cover the vast majority of potential operating situations. Given 

this temperature range, the speed of sound in air is calculated according to equation 3 and is 

found to vary from 320.19 m/s to 360.34 m/s. Substituting these values into equation 2 along 

with the maximum velocity of the vehicle of 20 m/s, it is found that the Mach number ranges 

from 0.0555 to 0.0625. This range certainly falls well below the threshold for the incompressible 

fluid model given in equation 1. With this information, the rest of the analysis and work 

performed will be conducted with air treated as an incompressible fluid. 

With equations 4 and 5, the maximum boundary layer thickness can be found. The Reynolds 

number must first be determined with the use of GLUAV’s altitude requirements. Since the 

operating parameters of the GLUAV require functionality between 0 and 1000 feet, the density 
7 

of the atmosphere is required at those points. For simplicity, the maximum altitude will be 

assumed to be 500 meters. Therefore, the minimum and maximum for the altitude will be sea 

level and 500 meters—a wider range than required. Density of the standard atmosphere at sea 

level and at 500 m is 1.2255 kg/m3

 and 1.1677 kg/m3, respectively (1). Since the maximum 

boundary layer will occur at the rear of the body of the GLUAV, the characteristic length used is 

the entire length of the vehicle or 25.4 cm. The coefficient of viscosity of air, μ, is 1.8 x 10-5

kg/(m•s) (1). Equation 5 indicates that the boundary layer will be maximized by a low Reynolds 

number. Therefore, the minimum case for the Reynolds number will be found when the velocity 

is at a minimum and the density is at its minimum. With the density at 500 meters and the 

minimum velocity of 8 m/s, Re = 131820.35. With the rear of the vehicle as the position for 

equation 5, the maximum boundary layer is found to be approximately 3.50 x 10-3 meters. The 

maximum Reynolds number is found with equation 4 to be 345863.3, which validates the 

assumption that flow is laminar and that the correct equations were chosen to model the air flow. 

To verify the basic analysis illustrated here, the FEA program ADINA4 (automatic dynamic 

incremental nonlinear analysis) was used to calculate the velocity field across the body of the 

GLUAV. While FEA analysis is very useful to calculate exact solutions, attempts were made to 

keep the model as simple as possible and only use the solutions as predictions of trends. 

Figure 4 shows the geometry created for the analysis surrounded by the surfaces that were 

meshed and analyzed. 

Figure 4. Outline of GLUAV geometry for ADINA analysis. 

 4 ADINA R&D, 71 Elton Avenue, Watertown, MA 02472. 
8 

The bullet-shaped area in the center represents the side profile of the GLUAV as if a crosssectional view were taken along the axis of vehicle. It is understood that this is a twodimensional analysis of a three-dimensional problem, and the answers obtained by ADINA will 

not be exact for the three-dimensional case. However, the ADINA analysis will illustrate trends 

that will occur in the real case and should mirror the answers obtained through equations 4 and 5. 

The geometry was taken from the SolidWorks assembly files of the current design. Although the 

total lengths are all exact, the curve on the nose of the GLUAV is not exact because of 

translation difficulties from the spline curve that defines the nose profile to the ADINA model. 

Although the shape of the nose may not be precise, it is believed that any slight differences will 

not have a significant effect on the analysis of air flow across the bottom surface toward the rear 

of the airframe. Four different analyses were attempted with the minimum and maximum 

velocities, coupled with the minimum and maximum angles of attack. Because of computer 

hardware limitations, the mesh size could not be refined as much as desired; however, the 

analysis conducted did help us understand the air flow conditions through the estimated flight 

range of the GLUAV. 

In figures 5 through 10, the length of the vector arrows indicates the magnitude of the air 

velocity. This magnitude is also represented by the gray scale coloring present. Some figures 

have different color coding, depending on what was chosen to make the figure most readable. 

Figure 5 shows the first analysis run in which there is an 8-m/s ambient flow past the GLUAV at 

0 AOA. The vertical dimension of the meshed area is 0.05 meter; each element in the FEA mesh 

is then 5 mm. ADINA therefore predicts that the boundary layer will actually be closer to 8 or 

9 mm as opposed to the 3.5 mm predicted by equation 5. 

Figure 5. Air flow across bottom surface of GLUAV at 8 m/s, 0 AOA. 
9 

A similar figure is seen for ambient air flow of 20 m/s at a 0-degree AOA. As predicted by 

equation 5, the boundary layer is thinner than at lower speeds. 

Figure 6 shows the velocity field of the air passing by the GLUAV at an air speed of 8 m/s and a 

20-degree AOA. It is shown that as the AOA increases, the boundary layer decreases in size and 

a sizeable portion of the air near the vehicle is actually traveling faster than the ambient air flow. 

Figure 6. Air flow across bottom surface of GLUAV at 8 m/s, 20 AOA. 

Figure 7 shows a similar velocity field for the vehicle as it travels at a 20-degree AOA at 20 m/s. 

It is noted that the velocity field is almost identical in shape as for the 8-m/s flow. 

Figure 7. Air flow across bottom surface of GLUAV at 20 m/s, 20 AOA. 
10 

Given that such different conditions exist, depending on the flight conditions, it was decided that 

it would be difficult to determine exactly the shape of a nozzle to capture the needed air flow for 

all conditions. In addition, any nozzle that was designed would require two 90-degree bends in 

addition to a dramatic constriction needed to fit between the wings of the folded GLUAV. This 

constriction and bends would increase the back pressure and make it more difficult to design a 

nozzle capable of providing the needed air flow. For these reasons, it was decided to investigate 

the possibility of using forced induction to acquire the needed air flow. 

To analyze the flow of air through a possible forced induction system, another simulation was 

run with a simulated inlet channel. This geometry and analysis used in ADINA in no way 

accounts for the blower that would be installed in the GLUAV. Instead, it establishes how the 

ambient air flow and AOA would impact the natural flow of air through the induction 

passageway. Figures 8 and 9 show the air flow across the GLUAV with a mock passageway as 

the vehicle travels at 20 and 8 m/s, respectively, at an AOA of 20 degrees. 

Figure 8. Air flow through inlet at 20 m/s, 20 AOA. 

This analysis shows that the natural flow of air does indeed impact the sensing area of the 

GLUAV while it travels at high AOAs. The effect seen is to increase the total flow through the 

passageway, causing higher air flow than at lower AOAs. Although this effect is noted for both 

minimum and maximum velocities, it happens to a much lesser extent at 8 m/s. This effect is not 

seen at an AOA of 0. In figure 10, we see the velocity plot of air flow as the GLUAV travels at 

20 m/s with a 0 AOA. 
11 

Figure 9. Air flow through inlet at 8 m/s, 20 AOA. 

Figure 10. Air flow through inlet at 20 m/s, 0 AOA. 

When the GLUAV is traveling at 0 AOA, there is negligible air flow through the passageway. 

This would indicate that if a blower were installed, the flow rate through the blower would be 

similar to the flow rate in a zero velocity condition. 
12 

By changing to forced induction, the need for designing nozzles is discarded and the needed flow 

of air is much easier to achieve. With a blower system, there are also other advantages. For 

example, only an intake and exhaust orifice would be needed. This would simplify production 

since the only changes in the airframe would be the drilling of two holes in the bottom surface as 

opposed to making either fixed nozzles or a spring-loaded system. Finally, a forced air induction 

system would assure functionality of the chemical sensor after the GLUAV has landed. Based 

on these preliminary conclusions, it was decided to further pursue the induced flow concept 

through experimentation. 

4. Materials and Apparatus 

Based on the results of the analysis, it was decided that forced induction would alleviate not only 

the analytical difficulties but also guarantee sufficient air flow if designed to be of sufficient size. 

With simple holes instead of nozzles as inlet and exhaust orifices, the size constraint of 0.10 inch 

imposed by the folded wings no longer has an impact on the design, and the size of the holes can 

be enlarged. A conceptual blower system was then designed to serve as a proof of concept. Any 

chemical system that would be designed would be required to fit into a configuration similar to 

that illustrated. However, since this is not intended to be a production level design, dimensions 

could change dramatically, depending on the results of testing. The design, shown in figure 11, 

consists of two plastic pieces that served as the air conduits and structure, a simple direct current 

(DC) motor, coupling shaft, and fan. The concept would mount in the GLUAV as shown in 

figure 12. 

Figure 11. Concept design of blower assembly. 
13 

Figure 12. View of concept assembly installed in payload compartment of GLUAV. 

In order to prove the functionality of such a system, a prototype was built. There was no attempt 

to create a production-ready prototype since any such system would need to be designed for the 

high-g loading experienced by the GLUAV. The intention was to create a prototype that could 

be evaluated to show that the needed air flow could be achieved during the GLAUV flight 

conditions. As such, this prototype is viewed as a proof-of-concept item. 

In order to create a prototype, parts were designed on the basis of the concept shown in figure 12. 

Of the parts illustrated in the concept, only the housing needed to be designed and manufactured. 

The two parts composing the housing are shown in their corresponding drawings in figures 13 

and 14. 

Figure 13. Drawing of the intake housing for prototype. 
14 

Figure 14. Drawing of the exhaust housing for prototype. 

Both pieces were made from 2-inch diameter Delrin5 stock. The stock was cut to shape with 

CNC mills and hand lathes at the Massachusetts Institute of Technology (MIT) Pappalardo 

Laboratory. A small DC motor (catalogue number 273-0223) was purchased at Radio Shack6 for 

use in the device. The motor has a maximum no-load speed of 11,600 rpm and a stall torque of 

53.0 G-cm at 3.0 volts nominal. The data sheet for the motor is presented in appendix C. A fan 

was taken from a small central processing unit cooler in the laboratory. The fan has a large 

central housing where the previous motor was mounted. The fan had a small stub protruding 

from the back side of the hub, which was connected to the DC motor via a coupling shaft made 

from 1/8-inch welding rod. 

In order to facilitate testing, the device was mounted on a Plexiglas7 base plate via 3-inch-long 

machine screws with holes drilled into the side of the device. To record the air velocity, a hotwire anemometer was used. To facilitate accurate measurements, a small stand was made for the 

anemometer with Plexiglas. To ensure repeatability of measurements, a series of holes parallel 

to the axis of the device was drilled in the base plate 0.175 inch apart, which corresponded to a 

pin protruding from the base of the anemometer stand. 

After fabrication and assembly of the pieces, the assembled device appears in figure 15 on a 

mounting plate and stand. 

 5

Delrin® is a registered trademark of E.I. du Pont de Nemours and Company. 

6Radio Shack® is a registered trademark of Tandy Corporation. 

7

Plexiglas® is a registered trademark of Röhm. 
15 

Figure 15. Assembled prototype on test stand with anemometer holder. 

5. Experimental Procedures 

To perform the experimentation, a simple setup and procedure was used. A hot-wire 

anemometer and associated control box was borrowed from the Undergraduate Measurement and 

Instrumentation Laboratory (UMIL) at MIT for the purpose of the experiment. The anemometer 

was calibrated with a manometer according to directions listed in appendix D, also provided by 

the UMIL. Note that the input voltage for tests at 0 and 3.5 m/s air flow was 11.9 volts, and the 

input voltage for the rest of the trials was 12.2 volts. The resulting equation relating fluid flow to 

velocity was entered into an Excel8

 spreadsheet. The anemometer control box was connected to 

a simple digital multimeter to record the voltage signal. The motor on the blower assembly was 

powered by two AA batteries, 2.98 volts, wired with “alligator” clips to the leads on the motor. 

The voltage on the batteries was monitored during testing to ensure that it was constant 

throughout testing. 

With the test stand previously shown, experiments were run at wind speeds of 0, 3.5, 10, 15, and 

20 m/s. For the 3.5-m/s speed, a standard room fan was used to provide the air flow. For higher 

speeds, normal fans were not sufficient and the device was mounted on the roof of an 

 8

Excel is a trademark of Microsoft Corporation. 
16 

automobile. For each wind speed, the flow velocity through the orifice was measured at AOAs 

of 0, 5, 10, 15, and 20 degrees. To measure the air flow, the anemometer wire was oriented in 

the vertical direction, while the movement of the anemometer to sample the air flow was in the 

horizontal direction. For each data point, the voltage of the anemometer was recorded and 

entered into the Excel spreadsheet. Figure 16 shows a top view sketch of the setup showing the 

orientation and configuration of the device and anemometer. 

Figure 16. Schematic of experimental setup. 

To calculate the volumetric flow, the velocity of the air is integrated over the area of the orifice. 

In order to simplify the integration, two assumptions are made: 1) that the air flow is steady state 

and does not change along the length of the passageway, and 2) that air flow is constant along 

the vertical axis. These assumptions allow easy calculation of the volumetric flow rate. To 

determine the volumetric flow rate, the data points are translated to such a position as if the 

orientation of the anemometer mounting holes were perpendicular to the axis of the exhaust 

opening. To translate the data points, the position of each of the holes was multiplied by the 

cosine of 30 degrees, which is the angle formed by the air passageway and the line of holes. 

This new line of data points is perpendicular to the axis of the hole so the opening is then a 

simple circle as opposed to the more complicated ellipse. To determine the flow rate, an 

Hot-wire anemometer

Signal to control box

Holes for mounting anemometer

Direction of airflow

Test Device
17 

equation representing the flow must be determined. This expression is found through the use of 

Excel’s curve fit feature. Because of the unusual shape of the data, a sixth order polynomial was 

selected to model the data. This equation was then multiplied by the equation of the circle, given 

by r2

 = x

2

 + y2, in which r2 is 1.27 cm. This expression is then integrated across the opening of 

the circle and gives the volumetric flow rate. 

6. Results 

The results of the experiment were very well matched to the model’s predicted behavior. 

Figure 17 shows the air flow velocity when the ambient air velocity is zero, along with the 

regression curve. The X-axis shown for all graphs has been normalized so that the middle of the 

air passageway is zero. The device in these figures is oriented 180 degrees from the depictions 

in ADINA so that position -1.15 cm is toward the rear of the device and 1.15 cm is the front. 

Velocity of Airflow at Stagnation

y = -2.543x6 - 0.4539x5 + 4.3094x4 + 0.1197x3 - 1.1268x2 + 1.2785x + 0.9326

R2

 = 1

0

0.5

1

1.5

2

2.5

3

-1.5 -1 -0.5 0 0.5 1 1.5

Position (cm)

Velocity (m/s)

Figure 17. Velocity measurements at 0 m/s air flow with regression curve shown. 

The regression curve given by Excel is shown on the top of the figure with the accompanying R 

value. The limits of integration chosen are from -0.01265 to 0.01265m. Although these 

numbers actually correspond to a smaller orifice than exists, it allowed for the thin layer of 

stagnant air immediately near the surface of the air passageway. The equation for the circle 
18 

representing the orifice area is given as x2

 + y2 = 0.00016129. The flow rate is then calculated to 

be 2.362x10-4 m3

/s. This is then converted to liters per minute and found to be 14.17 L/min. The 

results for the 3.5-m/s ambient flow are given in figure 18. 

Air Velocity in Orifice at 3.5 m/s Airflow

0

0.5

1

1.5

2

2.5

-1.5 -1 -0.5 0 0.5 1 1.5

Position (cm)

Velocity (m/s)

0 AOA

5 AOA

10 AOA

15 AOA

20 AOA

Figure 18. Velocity measurements at 3.5 m/s air flow. 

To determine the volumetric flow rate at 3.5 m/s, only one of the data curves, 5-degree AOA, 

was selected since all the data curves are observed to be similar. As before, a sixth order 

polynomial was used to model the curve, and the curve was then integrated with the same 

parameters as given earlier. The data and the polynomial expression are shown in figure 19. 

With the same process for the 5-degree AOA measurement with an ambient flow of 3.5 m/s, the 

flow rate is calculated to be 25.28 L/min. 

Although the data points for higher velocities were taken, it was noted that the air flow velocities 

were similar to those measured at 3.5 m/s. For this reason, fewer data points were recorded for 

the 10-, 15-, and 20-m/s trials. Additionally, linear regression and exact calculation of the flow 

rate were not performed. The recorded velocities for these trials are shown in figures 20 through 

22. 
19 

Polynomial Curve fit of 3.5 m/s Air Flow

y = 0.93x6 - 0.6998x5 - 1.2053x4 + 0.8294x3 - 0.4855x2 + 0.4667x + 1.6637

R2

 = 1

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

-1.5 -1 -0.5 0 0.5 1 1.5

Position (cm)

Velocity (m/s)

5 AOA

Poly. (5 AOA)

Figure 19. Regression curve fitted to 3.5 m/s, 5 AOA air flow measurements. 

Air Flow Velocities at 10 m/s Ambient

0

1

2

3

4

5

6

7

-1.5 -1 -0.5 0 0.5 1 1.5

Position (cm)

Velocity (m/s)

0 AOA

5 AOA

10 AOA

15 AOA

20 AOA

Figure 20. Velocity measurements at 10-m/s air flow. 
20 

Airflow with 15 m/s Ambient Flow

0

2

4

6

8

10

12

14

16

-1.5 -1 -0.5 0 0.5 1 1.5

Position (cm)

Velocity (m/s)

0 AOA

5 AOA

10 AOA

15 AOA

20 AOA

Figure 21. Velocity measurements at 15-m/s air flow. 

Airflow at 20 m/s Ambient Flow

0

2

4

6

8

10

12

14

16

-1.5 -1 -0.5 0 0.5 1 1.5

Position (cm)

Velocity (m/s)

0 AOA

5 AOA

10 AOA

15 AOA

20 AOA

Figure 22. Velocity measurements at 20-m/s air flow. 
21 

7. Discussion 

The experiment conducted proved that it is quite easy to obtain the needed air flow with induced 

flow. Even at stagnation flow, the simple blower device constructed here forces a volumetric 

flow rate ~10 times greater than what is required for chemical detector functionality. When the 

ambient air flow velocity was increased, the flow rate through the device also increased. 

Although it was not calculated explicitly for the higher velocities, it can be seen that the flow rate 

increases with ambient air velocity through the range measured. For the cases of 10-, 15-, and 

20-m/s ambient flow, the measured orifice air velocity did not drop below 1.8 m/s. As shown by 

the figures, the recorded velocities for these three trials were noticeably higher as a whole than 

the lower speed trials, which indicated that the air flow through the device at these velocities is 

also significantly higher than needed. Since the intent of this experiment was to prove the 

sustainability of the needed air flow, calculation of the volumetric flow rate was not performed 

for the higher ambient air velocities since it can be seen from figures 21 through 23 that the 

volumetric flow rate will be sufficient. It can therefore be concluded that the air flow through 

the orifice at all conditions will easily surpass the required minimum air flow for chemical 

weapons detection. 

As mentioned earlier, an automobile was used to achieve the needed velocities. It is likely that 

these velocities were not exact, since most speedometers are inaccurate by several percent. 

However, given that the device seems to produce a flow many times greater than needed, this 

difference is probably negligible. If it is desired to know the exact flow through the device, then 

more precise equipment would be needed to establish the ambient air flow. 

It is also worth noting that the data seem to strongly support the model given by ADINA. The 

figures printed from ADINA show that with higher AOAs, air will begin to flow naturally 

through the passageway, augmenting the induced flow. Note that ADINA predicts that most of 

the flow will be pressed against the back wall of the exhaust orifice. This effect is seen in the 

changes of the curve from the lower velocity trials to the higher velocity trials. In the lower 

speed trials, the minimum speeds are recorded closest to the back wall of the exhaust, and the 

speed increases as the anemometer was moved toward the front of the device. This increase is 

not as noticeable and there is even a decrease as the probe is moved to the very front of the 

vehicle during the higher velocity trials. This effect indicates the accuracy of the ADINA model 

since it did predict trends that were later detected experimentally. 

Although this demonstration showed that air flow is far beyond what is required, many changes 

could be made to improve the design. Several aspects of this proof-of-concept device lead to 

very significant losses in efficiency. First, the fan used has a large hub in the center. This 

reduces the fan efficiency since a smaller percentage of the passageway was actually being used 

to push air and the hub in the middle adds resistance to the flow of air. In addition, the fan could 
22 

be better sized for the orifice in which it sat. In the case of the device made, there was a sizeable 

gap between the diameter of the passageway and the fan blade. A smaller gap would also lead to 

more efficient flow. As can be seen in the drawings presented earlier, the housing that was 

machined had many sharp corners which led to interrupted air flow, also causing resistance to the 

air flow. Elimination of these corners and a more efficient shape could alleviate this flow 

impediment. 

8. Conclusions and Recommendations 

From the data acquired, it can be conclusively stated that a simple induced air flow system will 

easily achieve the volumetric flow rates needed to ensure operability of a chemical weapons 

detection system in ARL’s GLUAV. This functionality has been predicted analytically, and 

experiments have proved this prediction. 

For a production version of this system, numerous improvements could be made, and many were 

previously mentioned. One key change for production could be shrinking the entire package to 

make it smaller and lighter. Because the air flow in this demonstration device performed beyond 

the minimum air flow requirements without any effort being placed on air flow optimization, it is 

believed that a similar device, optimized for maximum flow rate, could be made significantly 

smaller. This smaller size would allow more usable payload for other items and more flexibility 

in the design of a chemical weapons detector for this system.

Based on the experiment, it appears that induced flow is indeed superior to the use of nozzles to 

duct the flow through the vehicle at the needed speed. With a design similar to that presented 

here, modifications of the GLUAV airframe are minimal and air flow is guaranteed and 

sustainable after landing. 

In order to conduct a demonstration of this added capability for the GLUAV, the chemical sensor 

must demonstrate its ability to survive the extreme acceleration from launch, conform to weight, 

size and power constraints, and have a proven air induction system. Commercially available 

chemical sensors should be procured and then shock tested to determine their suitability for this 

application. Since most commercially available sensors are larger than the form factor required 

for the GLUAV program, a demonstration airframe should be built for this purpose. Through 

dimensional analysis, it will be possible to enlarge the demonstration airframe to allow for 

experimentation with the available sensors and still provide valid data for GLUAV use. 

Combined with computational fluid dynamics, the demonstration airframe would develop and 

demonstrate the feasibility of a functional intake system for use with a currently available 

chemical weapons detector. After functionality of the sensor is demonstrated, the investment can 

be made to miniaturize the chemical sensor to fit into the available space on the GLUAV if it is 

deemed worthwhile. 
23 

9. References 

1. White, F.M. Fluid Mechanics, Boston: WCB McGraw-Hill, 1999, p. 24, 773. 

2. Miralles, C.T. Feasibility Study: Gun-Launched Unmanned Aerial Vehicle Concept; 

ARL-CR-466; U.S. Army Research Laboratory: Aberdeen Proving Ground, MD, 2001. 

3. Miralles, C.T. Gun-Launched Unmanned Aerial Vehicle Flight Demonstration and Concept 

Validation; ARL-CR-515; U.S. Army Research Laboratory: Aberdeen Proving Ground, 

MD, 2003. 
24 

INTENTIONALLY LEFT BLANK. 
25 

Appendix A. E-mail Correspondence Regarding Volumetric Flow Rates for 

Chemical Weapons Detectors 

1. E-mail regarding Environics ChemPro 100. 

From: Mrphillip@aol.com 

Date: Mon, 27 Jan 2003 07:12:55 EST 

Subject: ChemPro Info 

To: nair@mit.edu 

X-Mailer: AOL 8.0 for Windows US sub 230 

X-Spam-Not-Checked: Messages over 100K not checked 

X-Scanned-By: MIMEDefang 2.28 (www . roaringpenguin . com / mimedefang) 

Michael, 

Attached is some info on the ChemPro 100 you requested. The airflow for the system is 1 liter/min. 

 4-Page Brochure-ChemPro - Nov 25 2002.ppt 

2. E-mails from SBCCOM regarding military chemical weapons detectors. 

Here is some info on a specific type of military Chemical Sensor which 

has been incorporated into vehicles. If you have particular questions 

about this specific detector, please contact Mr. Roger Griffin at 

410-436-2112. 

-----Original Message----- 

From: Griffin, Roger L SBCCOM 

Sent: Wednesday, February 19, 2003 2:57 PM 

To: Vigus, Richard A SBCCOM 

Cc: Griffin, Roger L SBCCOM 

Subject: RE: HLD Products and Services Web Page 

Rich- I'll again caution you and Mr. Nair that restricting or assisting 

the airflow into the ACADA introduces numerous questions and cautionsthe major one Dan brought up with the membrane. We have mounted 

M22s/M88s within vehicles and the primary rule is to keep the line short 

and straight (and heated if possible). 

Please have Mr. Nair call me with questions. The spec data on the pump 

assembly follows. Note that the airflow thru the M22 is twice the 

figures listed. We have two pump assemblies per M88 (one for the G 

side and one for the H side). 

3.1.2 Air draw and energy consumption. With 2.25 0.01 V DC applied 

to the pump connector, the air flow rate through each cylinder shall be 

greater than 350 ml/min when the inlet is restricted to produce a 

suction pressure of 5.86 kN/m2 2% (0.85 0.02 psi). The air flow 

rate through each cylinder shall be greater than 500 ml/min when the 

inlet is restricted to produce a suction pressure of less than 0.53 
26 

kN/m2 (0.08 psi). The current consumption of the pump motor under these 

conditions shall be no greater than 130 mA. 

Roger 

-----Original Message----- 

From: Vigus, Richard A SBCCOM 

Sent: Wednesday, February 19, 2003 9:46 AM 

To: Griffin, Roger L SBCCOM 

Subject: FW: HLD Products and Services Web Page 

Roger---Do you know what the power rating for the air pump in the ACADA 

is? We got a question from this guy doing a thesis at ARL related to 

feasability of installing chem sensors in vehicles---and he's asking 

some questions about pressure drop, etc...do you have that info? 

-----Original Message----- 

From: Nowak, Daniel M SBCCOM 

Sent: Wednesday, February 19, 2003 9:37 AM 

To: Vigus, Richard A SBCCOM 

Subject: RE: HLD Products and Services Web Page 

Rich, suggest you contact Engineering (Klein) on the older point 

samplers and Roger Griffin for the M22. They should have the specs for 

the pumps. 

DAN 

-----Original Message----- 

From: Vigus, Richard A SBCCOM 

Sent: Wednesday, February 19, 2003 9:27 AM 

To: Nowak, Daniel M SBCCOM 

Subject: RE: HLD Products and Services Web Page 

okay, so how about the power rating of the air pump? perhaps with that 

info this guy can figure it out. 

-----Original Message----- 

From: Nowak, Daniel M SBCCOM 

Sent: Wednesday, February 19, 2003 9:20 AM 

To: Vigus, Richard A SBCCOM 

Subject: RE: HLD Products and Services Web Page 

I don't know this one. Blowing air into the point sensors is not a good 

idea since some have membranes which could be ruptured. 

DAN 

-----Original Message----- 

From: Vigus, Richard A SBCCOM 

Sent: Wednesday, February 19, 2003 9:13 AM 

To: Nowak, Daniel M SBCCOM 

Subject: FW: HLD Products and Services Web Page 

Dan---Can you help? 

-----Original Message----- 
27 

From: Garcia, Patsy R SBCCOM 

Sent: Wednesday, February 19, 2003 8:47 AM 

To: Vigus, Richard A SBCCOM 

Subject: FW: HLD Products and Services Web Page 

Can you answer this one? 

 

-----Original Message----- 

From: michael nair [mailto:nair@MIT.EDU] 

Sent: Friday, February 14, 2003 4:43 PM 

To: Garcia, Patsy R SBCCOM 

Subject: RE: HLD Products and Services Web Page 

 

Thank you for your response. I had one more question, I was wondering 

if you have any information regarding the pressure loss across the 

sensing element. For example, if I was to blow air through the sensor, 

what kind of a head loss would I have from the flow restriction. If you 

have any information relating to this, I would appreciate any help that 

you would be able to offer, 

Sincerely, 

Michael Nair 

At 10:40 AM 1/28/2003 -0500, you wrote: 

Typical military point sensors for chemical agents operate in the 1.2 

1.4 liters/minute range for flow rate. 

 

-----Original Message----- 

From: michael nair [mailto:nair@MIT.EDU] 

Sent: Wednesday, January 22, 2003 1:06 PM 

To: homeland.defense@sbccom.apgea.army.mil 

Subject: HLD Products and Services Web Page 

 

Dear Sirs: 

I working on my thesis project with the US Army Research Lab in 

Aberdeen, Maryland. The focus of my thesis is the feasibility of 

installing a chemical weapons detector on a concept vehicle. I am in 

need of information regarding typical minimum flow rates for detectors. 

If you have any information regarding this topic I would appreciate any 

help that could be offered. 

Sincerely, 

Michael Nair 

Michael L. Nair 

617-504-1067 

94 Spring Street, Apt. 1 

Cambridge, MA 02141 

Michael L. Nair 

617-504-1067 
28 

94 Spring Street, Apt. 1 

Cambridge, MA 02141 

[] smime.p7s 
29 

Appendix B. Project Proposal From ARL, Received on January 3, 2003 

Senior Design Research Project for Michael Nair, senior ME Student Massachusetts Institute of 

Technology (MIT) 

Title: Conceptual Design, Engineering Modeling, and Experimental Validation of Air Sampling 

System for Chemical Sensor Insertion into the U.S. Army Research Laboratory’s (ARL) Silent 

Operating Aerial Reconnaissance (SOAR) Program. 

Figure A-1. SOAR vehicle 

in flight (glider 

version). 

Major Focus of Project: 

1. Conceptual design and engineering modeling of air sampling system within constraints 

specified by ARL 

a. Use of SolidWorks to perform computer-aided design/engineering from models 

provided by ARL. 

System Constraints: All components must fit within payload tube (fuselage, 

1.75-inch diameter) of SOAR model as defined in SolidWorks files. Length of payload 

tube available for sensor not to exceed 3.0 inches. Ductwork associated with the intake/ 

exhaust manifolds should not exceed 0.1 inch in diameter and should be less than 1.0 inch 

apart. Extension and air capture/exhaust direction will be determined by student during 

the design process. Current estimates indicate that the air sampling system should be able 

to pass ambient air across the detector surface at a minimum flow rate of 1 ml/min. 

Student will conduct a survey of existing hand-held chemical sensors to confirm or modify 

this flow rate. The design of the air sampling system should also take into account the 

expected operating characteristics at an altitude of 0 to 1000 ft above sea level (ASL), 

taking into account the vehicle’s expected flight performance (angle of attack (AoA), 

velocity, bank angle, etc.) as specified in the provided technical reports. Other sampling 

system characteristics such as mass, volume, power requirements, sensitivity, response 

time, etc. will be determined by the student. 
30 

b. Predict the performance of the conceptual design using either analytical or computational fluid dynamics methods. Predict the performance through SOAR operating 

altitudes (1000 ft - ground) and various AoA of min. = 0 to max. = 20 degrees and 

velocities (velocity of min. = 9 to max. = 20 m/s) at CL = 0.8-1.0 (Miralles, 2003) to 

determine the operating AoA range at the CL being analyzed. 

Table A-1. Sample set of variables for part 1b. 

Altitude AoA Max Flow Rate 

 

1000 Ft a = AoA range Ra 

500 Ft a = AoA range Ra 

Ground = 0 Ft a = g (TBD at ground) Rg 

2. Using commercial-off-the-shelf (COTS) items, integrate a surrogate sampling system 

into the SOAR for purposes of wind tunnel experiments. 

3. Experimentation in wind tunnel to validate conceptual design and predictions 

a. Design an experimental setup and test matrix to verify selected data points in part 1b. 

Assistance will be provided for the location of a wind tunnel facility if one is not 

available at MIT. 

b. Use a flow meter as a surrogate chemical sensor to measure the flow rate into the 

SOAR. Student will perform research and purchase/borrow the correct flow meter 

within performance and size constraints using COTS. If a flow meter is not readily 

available that meets the size constraints mentioned, the student can consider placing 

the meter outside and behind the airframe, with the ductwork fixed (but extended) 

within the airframe as originally envisioned. The student can explain differences and 

limitations of this meter setup compared to the ideal meter-within-the-airframe case. 

c. Data will be recorded and saved from the flow meter experiments. 

d. Analyze data to verify the validity of calculations in part 1b. If experimental versus 

calculated data disagree, try to determine why the calculated data do not match. 

4. Document and/or present results 

Final report to include design methodology, description of calculations, experimental data and 

analysis, and mechanical details (SolidWorks model). 
31 

Appendix C. Specifications Sheet for DC Motor Used in Prototype 

SR-3 Motor 

(273-0223) Specifications Faxback Doc. # 19830 

At 1.5 V (Nominal Voltage): 

 Resistance......................................................0.25 Ohm 

 No Load Speed...................................................5700 RPM 

 No Load Current...................................................0.20 A 

At Maximum Efficiency: 

 Speed...........................................................4100 RPM 

 Current Rating....................................................0.55 A 

 Torque..........................................................9.4 G-cm 

 Power Used........................................................0.39 W 

 Efficiency......................................................... 45% 

 Stall Torque...................................................33.5 G-cm 

At 3.0 V (Nominal Voltage): 

 Resistance......................................................0.50 Ohm 

 No Load Speed.................................................11,600 RPM 

 No Load Current...................................................0.25 A 

At Maximum Efficiency: 

 Speed...........................................................8300 RPM 

 Current Rating....................................................0.99 A 

 Torque.........................................................15.0 G-cm 

 Power Used........................................................1.29 W 

 Efficiency...........................................................52% 

 Stall Torque...................................................53.0 G-cm 

Specifications are typical; individual units might vary. Specifications 

are subject to change without notice. 

(IR-02/29/96) 
32 

INTENTIONALLY LEFT BLANK. 
33 

Appendix D. Anemometer Calibration 

(Taken from “Volumetric Fluid Flow Measurement, Fall, 2002” from MIT course 2.671, 

courtesy of Dr. Barbara Hughey) 

3. Calibration of Hot Film Anemometer and Positioner 

A hot film anemometer is a common instrument for measuring gas velocity. Its operation is 

described in appendix B, where it is shown that the relationship between its output voltage Vout 

and fluid velocity ug is

Vout = C + D ug

2

in which C and D are constants. The only practical way to determine C and D is by experiment. 

The Pitot probe (appendix A) allows a simple determination of velocity ug from a pressure 

measurement. As discussed in appendix A, it relies on the Bernoulli relation, in which the only 

physical constants necessary are fluid densities and the gravitational acceleration. A Pitot tube 

may therefore be used as a velocity measuring standard, from which the hot film anemometer 

may be calibrated (i.e., the values of C and D in equation 1 determined). 

3.1 Hot Film Anemometer Setup 

CAUTION: THE HOT FILM SENSOR IS VERY FRAGILE. BE EXTREMELY CAREFUL 

WHEN/IF YOU HANDLE THE PROBE. 

1. Record the serial number and any other information on the anemometer box in your lab 

notebook. The sensor is already installed in a stainless tube that slides in the brass holder 

mounted on the translation stage at the end of the pipe. The clips extending from the brass tube 

are there to protect the sensor wire. 

2. The sensor wire should be oriented vertically and positioned to face the free jet nozzle, which 

is connected to the house compressed air supply. 

3. Turn on the E3610A power supply labeled “15V for Anemometer”. The digital multimeter 

labeled “Anemometer” should read about 3.7 V. Blow gently on the hot film sensor. You 

should see the voltage output increase in magnitude. The hot film anemometer is now ready to 

be calibrated (sect. 3.3). 

3.3 Calibration of Hot Film Anemometer 

Please read appendices A and B before proceeding. 

1. The Pitot tube is mounted on the positioner with thumbscrews and washers and should also be 

facing the free jet nozzle. The Pitot tube must be oriented parallel to the direction of air flow out 

of the nozzle, and the hot film sensor wire must be perpendicular to the flow direction. Make 

sure that both probes are at the same height as the nozzle tip and at the same distance from the 

nozzle. The positioner will be turned by hand (using the black knob at left of the positioner) to 

alternately place each probe at the centerline of the air jet. 
34 

2. The flow is adjusted with the regulator valve on the wall. Open the 1/4 turn shutoff valve on 

the wall by rotating the yellow handle so that it points down. Start with the regulator turned 

counterclockwise until the pressure gauge reads approximately 0 psi. 

3. Record the “no-flow” manometer deflection and the “no-flow” anemometer output (in volts) 

in your lab notebook. Look carefully at the gradations on the manometer scale, to make 

sure you read them correctly. Is the no-flow anemometer output constant? This is a very 

sensitive instrument; what could be causing error in your no-flow reading? If you detect a nonzero manometer deflection, you should subtract its value from your subsequent measurements in 

order to correct for the slight offset of the two manometer columns with no flow. 

4. Increase the pressure by turning the regulator valve clockwise until it reads about 4 psi. Is 

this gauge or absolute pressure? Place the anemometer at the approximate centerline of the air 

nozzle and move the positioner back and forth to find to point of maximum anemometer output 

voltage. Record the anemometer output and positioner output voltage at this location in your lab 

notebook. 

5. Now place the Pitot tube at the approximate centerline of the air nozzle and move the 

positioner back and forth to find to point of maximum manometer deflection. Record the 

manometer deflection and positioner output voltage at this location in your lab notebook. 

6. Without moving the positioner, decrease the pressure to produce an air flow sufficient to 

produce a manometer deflection, hw (defined in figure 7) of about 6 mm or less. Allow time for 

the flow to stabilize. Measure and record this manometer deflection. In your analysis (step 10), 

be sure to correct for the initial manometer reading at zero air flow (if it was not zero). 

7. Using the positioner, move the anemometer probe to the jet centerline, resetting the positioner 

at the location found in step 4, as determined by the pot output voltage. Record this value of 

Vout, as well as the positioner output voltage at this location. You should notice that Vout 

fluctuates while you are moving the positioner, and displays larger voltages while in motion than 

while still. Why does this occur? Which is the correct reading? 

8. Increase the flow until Vout is approximately 7 V. Repeat this measurement sequence (reading 

the hot film probe first this time, then the Pitot probe first in the next sequence) for at total of six 

air flows (including the no-flow point). Reset the positioner for each sensor at the locations 

found in steps 4 and 5, as determined by the pot output voltage. Do not exceed 100 mm 

deflection of the manometer (Vout less than 8.3 V). Record the manometer deflection and Vout for 

each flow rate. 

9. Turn the regulator fully counter clockwise and close the air flow shutoff valve (yellow handle 

horizontal). 

10. Convert the manometer reading hw to velocity ug using the Bernoulli relationship (equation 

14 in appendix A). The manometer angle θ is 30°. Do not forget to include your zero flow 

point. Present the data as a plot of Voutvs ug

2 . The plot should be a straight line. Perform a 

linear regression (you can use the Trendline feature in Excel9

) to determine the constants C and 

D that appear in equation 19 in appendix B. Print the plot (including the equation and the R2 

value) and include in your lab notebook. 

 9

Excel is a trademark of Microsoft Corporation. 
35 

Appendices 

A. Flow Velocity Measurements With a Pitot Probe and Manometer 

The Pitot probe works on the Bernoulli principle, which states that in steady flow of an 

incompressible fluid (or compressible fluid at low Mach number), the change in kinetic energy 

per unit mass between any two positions equals the work done on the mass by pressure, P, and 

gravity forces. For a fluid with flow velocity u and density ρg: constant 

constant

2

2

1 =















+ + gh u P

ρ g

in which g is the acceleration of gravity and h is the fluid height with respect to a reference point. 

For the probe arrangement shown in figure 7, where h2 = h1, we may express equation 7 as 

1

2

2

2

1

1

2

1

2 2 gh u P gh u P

g g

+ + = + +

ρ ρ

The Bernoulli constant at an arbitrary point 1 is unaffected by the presence of the Pitot probe at 

point 2. With u2 = 0 (stagnation point), equation 8 may be solved for u1, which equals the flow 

velocity ug: 

( ) 1 2 1

2 P P

g

u ≡ ug = − ρ

The pressure difference (P2 – P1) can be measured in a number of ways; one method is to use a 

manometer. The manometer is simple but has limited sensitivity, range, and frequency response. 

For this case where the fluid is air, P0 (defined in figure 7) and P1 are related as 

P0 = P1 + ρ g gh1

For the “reclining” manometer (figure 7), with θ measured from the horizontal plane, as 

indicated in figure 7, pressures P3 and P2 are related as 

P3 = P2 + ρ g g( ) h1 + hw sinθ

in which hw is the difference in water column heights measured in the reclined plane. Pressures 

P3 and P0 are also related as 

P3 = P0 + ρ wghw

in which ρw is the density of water. Combining equations 10, 11, and 12 gives 

P2 − P1 = g(ρ w − ρ g )hw sinθ ≅ gρ whw sinθ

because ρw >> ρg. Combining equations 10 and 13, we obtain 

g

w w

g

gh u

ρ

2ρ sinθ =

Note that ug ∝ hw sinθ . Differentiating equation 14 and dividing both sides by ug yields 
36 

w

w

g

g

h

dh

u

du

2 =

That is, the Pitot probe/manometer sensitivity Sm (defined as dhw/dug) is given by 

sinθ

2 g

g

w

g

w

m

u

u

h

du

dh S ≡ = ∝

Equation 16 states that Sm vanishes for small velocities and, for a given ug, increases as q becomes 

small. That is, for the same ug, the ratio of Sm(θ) for a reclining (0° < θ < 90°) manometer and for 

a normal (θ = 90°) manometer is given by 

( )

( ) θ

θ

sin

1

90 = ° ug m

m

S

S

which for θ = 30° as used in the present setup gives a “gain” of 2. 

When calculating the velocity from the manometer readings, you may use the following 

values: 

ρg = 1.2 kg/m3 (at 25°C); ρw = 1.0 × 103 kg/m3; and g = 9.80 m/s2. 

Note that to limit the velocity error to 1%, we should really know the air temperature and pressure 

quite accurately. Specifically, from equation 14 and the gas law p = ρRT, we can see that both air 

temperature and pressure must be known to within 1%. (As may be inferred from equation 14, a 

1% error in each of the two quantities introduces individually a 0.5% error in ρg; together they 

appear as a 1% error in ug.) For example, at near room temperature (~300K), ug is off by 1% from 

the above value if the air temperature differs by 3° C from 25° C. 

B. Hot Film Anemometer 

The hot film anemometer probe operates on the basis of heat transfer. The anemometer electronics 

keep the film resistance constant by maintaining the film temperature constant. The film temperature is kept constant by controlling the electric current through the film so that joule heating 

balances gas cooling. Thus, when a hot film sensor is convectively cooled by a stream of moving 

gas, the electric current through the film is increased to match the cooling. The voltage across the 

film element, Vf, which is directly related to probe output, Vout, thus can be used to indicated flow 

velocity. To relate quantitatively Vf to flow velocity, we use an empirical heat transfer law for the 

sensor element: 

( )( ) g g

f A B u T T

R

V = + −

2

where the left-hand side expresses electric heating in the sensor element and the right-hand side 

expresses gas cooling. R is sensor resistance and T is sensor temperature, ug is gas velocity, and 

Tg is gas temperature. A and B are empirical constants. Because T is kept constant, R is also 

constant, and equation 18 my be expressed as 

Vf = C + D ug

2

Since Vout is proportional to Vf, equation 19 implies that 2 Vf is also related linearly to ug .
37 

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