Andrew L. Reehorst, Dean R. Miller, and Colin S. Bidwell

Glenn Research Center, Cleveland, Ohio

Total Water Content Measurements With an 

Isokinetic Sampling Probe

NASA/TM—2010-216217

June 2010
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Andrew L. Reehorst, Dean R. Miller, and Colin S. Bidwell

Glenn Research Center, Cleveland, Ohio

Total Water Content Measurements With an 

Isokinetic Sampling Probe

NASA/TM—2010-216217

June 2010

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135
Acknowledgments

The authors would like to thank Dr. David Brinker and the staff of the NASA Glenn Icing Research Tunnel for their assistance 

in the development and testing of the Isokinetic Probe.

Available from

NASA Center for Aerospace Information

7115 Standard Drive

Hanover, MD 21076–1320

National Technical Information Service

5301 Shawnee Road

Alexandria, VA 22312

Available electronically at http://gltrs.grc.nasa.gov

Level of Review: This material has been technically reviewed by technical management. 
NASA/TM—2010-216217 1 

Total Water Content Measurements With an Isokinetic 

Sampling Probe 

Andrew L. Reehorst, Dean R. Miller, and Colin S. Bidwell 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 

The NASA Glenn Research Center has developed a Total Water Content (TWC) Isokinetic Sampling 

Probe. Since it is not sensitive to cloud water particle phase nor size, it is particularly attractive to support 

super-cooled large droplet and high ice water content aircraft icing studies. The instrument is comprised 

of the Sampling Probe, Sample Flow Control, and Water Vapor Measurement subsystems. Analysis and 

testing have been conducted on the subsystems to ensure their proper function and accuracy. End-to-end 

bench testing has also been conducted to ensure the reliability of the entire instrument system. A Stokes 

Number based collection efficiency correction was developed to correct for probe thickness effects. The 

authors further discuss the need to ensure that no condensation occurs within the instrument plumbing. 

Instrument measurements compared to facility calibrations from testing in the NASA Glenn Icing 

Research Tunnel are presented and discussed. There appears to be liquid water content and droplet size 

effects in the differences between the two measurement techniques. 

Introduction 

The need for a new form of water content measurement instrument became apparent as NASA 

became deeply involved with the study of hazards associated with super-cooled large droplets (SLD) 

flight conditions (Ref. 2). As NASA began experimental studies in SLD conditions, it became apparent 

that the current liquid water content (LWC) instrumentation would be inadequate. Existing 

instrumentation was designed and intended for cloud measurements of cloud droplet size distributions 

with a typical median volumetric diameter (MVD) of <50 μm. When the larger droplets associated with 

SLD conditions impact the existing instrumentation, the measured LWC could be significantly in error 

due to droplet breakup, splashing, and bouncing (Ref. 3). 

In addition to the inadequacies of current LWC instrumentation for SLD conditions, a new need for 

improved water content measurement of mixed-phase and high ice water content (HIWC) conditions has 

also more recently come to the attention of NASA. Cruise altitude engine icing caused by flight into 

HIWC conditions has been shown to be a serious flight hazard (Ref. 4). To date, there is limited data 

available to quantify this environment, in part because of the lack of adequate water content 

instrumentation. Due to its basic design, existing water content instrumentation suffers the effects of ice 

particle bouncing and inadequate heater capacity when tasked to measure HIWC conditions. 

One solution option that eliminates droplet splash, bounce, and breakup effects of SLD conditions 

and ice particle bounce effects for HIWC conditions is a heated isokinetic cloud particle sampler. 

Isokinetic Sampling is defined as “Any technique for collecting airborne particulate matter in which the 

collector is so designed that the air stream entering it has a velocity equal to that of the air passing around 

and outside the collector” (Ref. 5). By its nature, a properly functioning heated isokinetic cloud particle 

sampler should not be influenced by the size or phase of the cloud particle. By sufficiently heating the 

sampling probe, all water particles sampled (ice and liquid) are evaporated into the air sample. An 

additional benefit of this heating is that the exterior of the probe remains free of ice accretion. When all of 

the water is evaporated, the cloud’s total water content (TWC) can be calculated for a known air sample 

flow rate by differencing the measurement of the absolute humidity of the sampled air from the 

measurement of the absolute humidity of the ambient air. 
NASA/TM—2010-216217 2 

Nomenclature 

Ainlet inlet area (m2) 

dref reference distance (m) 

dpart particle diameter (m) 

LWC liquid water content (g/m3

) 

MVD median volumetric diameter (μm) 

Stk Stokes number: τ V/dref (from Ref. 1) 

TWC total water content (g/m3

), water content made up of both liquid and ice particles 

Vexternal ambient external free-stream air velocity (m/s) 

ρexternal ambient external free-stream air density (kg/m3

)

ρdryair dry air density (kg/m3

) 

ρpart water particle density (kg/m3

) 

μ air viscosity (Pa s)

τ relaxation time: ρpart * dpart

2

/18 μ (from Ref. 1) 

Instrument Concept 

The concept behind a total water content (TWC) instrument based on heated isokinetic cloud particle 

sampling is relatively straightforward. This form of TWC instrument can be broken into 3 subsystems: 

(1) Sampling probe (2) Sample flow control, and (3) Water vapor measurement. A block diagram of such 

an instrument is shown in Figure 1. 

Sampling Probe 

The sampling probe must be designed to ensure isokinetic sampling is possible and must provide 

sufficient heat to completely evaporate the ingested moisture. To ensure isokinetic sampling capability, 

the probe would ideally satisfy the thin-wall assumption. Uncorrected isokinetic sampling theory is valid 

for probes with an outer diameter to inner diameter ratio of 1.1 (Ref. 6). This ensures that when the probe 

inlet is properly aligned with the external flow velocity vector and that the probe internal velocity matches 

the external velocity, the flow entering the probe will not have been disturbed. An extreme example of 

this criterion not being satisfied is a sampling orifice in a flat plate. Even if the orifice is aligned with the 

flow and the internal velocity is set equal to the external velocity, the flow about the flat plate will 

influence the incoming streamlines such that a significant number of the particles intended for sampling 

will be deflected away from the probe inlet. In the same situation, a probe satisfying the thin-wall 

assumption would not disturb the incoming streamlines and the probe will ingest all particles within the 

incoming stream-tube. 

Sample Flow Control 

The objective of this kind of instrument is to obtain a sample with the same ratio of constituents as the 

ambient free-stream conditions. If the particles of interest have no mass, then they will always follow the 

air streamlines and a sample with proper particle to air ratio can always be obtained regardless of the inlet 

velocity. However, since we are interested in sampling water particles that can have significant inertia 
NASA/TM—2010-216217 3 

Figure 1.—Basic block diagram 

of sampling total water content 

(TWC) instrument subsystems. 

Figure 2.—Schematic of possible probe inlet flow 

conditions. (a) Isokinetic sampling. (b) Sub-isokinetic 

sampling. (c) Super-isokinetic sampling. 

(such that their paths can deviate from turning air streamlines), we must be careful to properly set the 

probe’s inlet flow conditions. When a probe’s mean inlet face velocity is equal to the external free-stream 

conditions, we have isokinetic sampling (Fig. 2(a)). When the internal flow is too slow, the condition is 

considered to be sub-isokinetic (Fig. 2(b)). When the internal flow is too fast, the condition is considered 

to be super-isokinetic (Fig. 2(c)). 

Sub-isokinetic sampling results in elevated concentration measurements since too little air is being 

ingested relative to the number of particles captured. Super-isokinetic sampling results in low concentration 

measurements since too much air is being ingested relative to the number of particles captured. 

Water Vapor Measurement 

When the sampling probe is properly designed and aligned, and the sample flow rate is properly 

controlled, the sample obtained will have the same constituents as the external conditions. Since our 

probe is assumed to be sufficiently heated, all of the captured sample moisture is evaporated to water 

vapor. Therefore, the determination of TWC with this probe comes down to subtracting the ambient water 

vapor from the sample water vapor and converting the result into the desired water content units. 
NASA/TM—2010-216217 4 

Figure 3.—Detailed block diagram of NASA TWC Isokinetic Probe. 

Apparatus 

The NASA Isokinetic TWC instrument is based upon the concept described above. A detailed block 

diagram of the instrument is seen in Figure 3. While this diagram may seem rather complicated, the 

system can still be broken down to the subsystems described above. 

The forward and backward facing tubes within the tunnel flow in Figure 3 make up the Sample Probe 

subsystem. The forward facing tube collects the ambient air and cloud particle (Sample) and the rear 

facing tube collects only the ambient air (Reference). 

The Filter, Flow Meter, Shop Air source, Pressure Regulator, and Ejector in Figure 3 make up the 

Flow Control subsystem. Airflow through the Sample probe is set by adjusting the amount of suction. 

Introducing a regulated air source to an ejector (Ref. 7) creates the required suction. The suction driven 

flow is filtered and measured with a thermal mass flow meter (Ref. 8). By monitoring the sample flow 

rate, the ejector source air can be adjusted until the target flow rate is achieved. 

The Infrared (IR) Gas Analyzers and the Chilled Mirror hygrometers in Figure 3 make up the 

Humidity Measurement subsystem. Two differential, nondispersive, IR gas analyzers (Ref. 9) are utilized 

to make the primary water vapor concentration measurements for both the sample and the reference air 

sources. This form of gas analyzer measures the difference in infrared absorption between two sampling 

cells. To make an accurate, absolute measurement of its water vapor concentration, the reference is 

compared to dry nitrogen (high precision nitrogen source dried with a magnesium perchlorate dessicant). 

The second water vapor concentration gas analyzer makes an accurate differential measurement between 

the sample and reference air sources. To maximize the systemic accuracy of the water vapor 

measurements, the measured absolute water vapor concentration of the reference source is passed 

electronically to the differential unit. Two chilled mirror hygrometers are used as backups for the water 

vapor measurements. While chilled mirror hygrometers can be quite accurate, their time response is 

relatively poor, and thus not as attractive as the IR gas analyzers. Since the air flow rates through the IR 

gas analyzers and the chilled mirror hygrometers are not as high as the required isokinetic sampling flow 

rate, the sample air source for the water vapor measurements is drawn from the manifold in the suction 

line upstream of the flow meter. Since we are making very accurate flow rate measurements to ensure 

isokinetic sampling, after passing through the water vapor instruments, the sample air is dumped back 
NASA/TM—2010-216217 5 

into the manifold (downstream of where it is drawn) to ensure no leakage. After passing through the water 

vapor instruments, the reference air and the dry nitrogen are vented to the room. 

The installation in the NASA Glenn Icing Research Tunnel (IRT) is shown in Figures 4 and 5. 

Figure 4 shows the sampling probe within the wind tunnel test section and Figure 5 shows the remainder 

of the instrument system in the wind tunnel control room. 

The control and measurements of the TWC Isokinetic Probe are handled with a data acquisition 

program written in the LabVIEW programming environment (Ref. 10), running on a laptop computer. 

Data signal input, control output, and analog-to-digital conversion is performed with an Ethernet-based 

portable data acquisition system (Ref. 11). Some of the calculations made in the LabVIEW program are 

described in the Appendix. 

Figure 4.—TWC Isokinetic Probe in the IRT. 

Figure 5.—NASA TWC Isokinetic System equipment in the IRT. Left, 

on desk, is controlling PC laptop; Center, in tall rack, are the Chilled 

Mirror hygrometers, the data acquisition hardware, and the IR Gas 

Analyzers; Right, on table, power supply that provides current for 

sampling probe heat. 
NASA/TM—2010-216217 6 

Results and Discussion 

Potential Error Assessment 

Early in the development of this instrument system, it became apparent that there were several 

potential sources for error. The potential error sources align with the subsystems described earlier. 

For the Sample Probe subsystem, a concern was violating the thin-wall assumption. As discussed 

earlier, to satisfy the thin-wall assumption the ratio of the outer diameter to the inner diameter of the 

probe inlet needs to be 1.1 or less. For the probe used in this work, that ratio was 1.33. To examine this 

further, a computer analysis was performed using PMARC (Ref. 12) to provide the flow solution and 

LEWICE3D (Ref. 13) to provide the droplet trajectory analysis (Fig. 6). The trajectories calculated by 

LEWICE3D were then used to calculate inlet catch efficiency for a range of droplet size, external flow 

velocity, and probe flow angularity. The inlet catch efficiency data and the approximation used to correct 

the experimental TWC Isokinetic Sampling probe data is shown in Figure 7. To improve the accuracy of 

the instrument this correction is applied, but as one can see, the corrections are quite small for all but the 

smallest droplet sizes (error caused by flow angularity between 0 and 5° was considered negligible, so no 

flow angularity correction is applied). 

Figure 6.—LEWICE3D calculated droplet trajectories for NASA TWC 

Isokinetic Sampling Probe. 

Figure 7.—LEWICE3D calculated TWC Isokinetic Sampling Probe collection 

efficiency and the related Stokes Number formula approximation. 

Collection Efficiency vs Stokes 

Number

0.88

0.9

0.92

0.94

0.96

0.98

1

0.10 1.00 10.00 100.00 1000.00 10000.00

St

LEWICE results Formula Approximation
NASA/TM—2010-216217 7 

For the Humidity Measurement subsystem, the main concern was the accuracy and proper calibration 

of the IR gas analyzers. To help assess the accuracy of the IR gas analyzers, the chilled mirror 

hygrometers were added to the system. And to examine the accuracies of the humidity measurements in a 

controlled manner and to assess various calibration techniques, a bench test was performed. For bench 

testing the system, the probe head was replaced with a chamber that could heat and evaporate very 

controlled amounts of water. The system software was modified to time-integrate the total amount of 

water sampled, thus providing a controlled end-to-end system check of most of the TWC instrument 

(water vapor measurements, the flow control and measure, analog-to-digital signal conversion, data 

acquisition, and system software). Using the bench test, the instrument was found to be very repeatable 

and accurate to within 5 percent. 

Early bench testing pointed to a problem in the Sample Flow Control subsystem. The bench testing 

and further flow meter comparison tests led to the rejection of one of the original two flow meters. While 

the rejected flow meter was designed to be temperature and pressure compensated, it was found to have 

significant error when operating in a suction driven flow. The thermal mass flow meter that is used today 

has been checked against a differential pressure volumetric flow meter with excellent agreement and 

receives annual NIST traceable calibration. 

Another finding of the bench testing is that for this kind of instrumentation, it is very important to 

monitor the sample’s dew point and temperature and the temperature of the tubing and instrument 

surfaces exposed to the sample (the sample path). If the sample’s dew point goes above the temperature of 

the sample path, condensation will take place and the measured water vapor will be incorrect. For the 

NASA TWC instrument system, this is handled by using a heated hose to transport the sample from the 

probe to the individual humidity instruments. Also the tubing length is kept to a minimum. The use of 

tubing that is resistive to water absorption is also obviously very important. 

Icing Wind Tunnel Condensation Cloud 

In the course of the development and testing of the TWC Isokinetic Sampling Probe, the authors 

experienced several instances of apparent elevated icing wind tunnel LWC (above the wind tunnel’s 

calibrated spray LWC). Upon further consideration, it has been theorized that a condensation process, not 

unlike that which causes cap clouds above mountaintops, caused these events. This additional cloud 

generation can occur when the humidity, heat exchanger temperature, and air speed conditions within any 

icing wind tunnel are suitably combined. To help illustrate this formation process, Figure 8 displays a 

conceptual icing wind tunnel layout. Flow, which is from the left, passes through the heat exchanger and 

on through the contraction into the test section. In region A, the airspeed is Va, static temperature is Tsa, 

and the dew point is Dewa. As the flow goes through the heat exchanger, the airspeed remains steady 

(Vb = Va); if the heat exchanger is cooling the airflow, then Tsb will be less than Tsa; and if the heat 

Figure 8.—Conceptual icing wind tunnel layout. 

 
NASA/TM—2010-216217 8 

exchanger temperature is below Dewa, then the flow will be dried (Dewb < Dewa). As the flow 

accelerates through the contraction into the test section, the velocity increases (Vc > Vb) and the static 

temperature drops (Tsc < Tsb). If the static temperature drops below the dew point, the air becomes 

supersaturated and a spontaneous condensation cloud begins to form. As water vapor condenses into 

liquid water, the dew point decreases (Dewc < Dewb). If the dew point in regions B and C are known, 

then the LWC of resultant condensation cloud can be calculated (described in Appendix). The MVD of 

mountaintop cap clouds are on the order of 10 μm with a rather narrow size spectra as a result of direct 

condensation with little or no coalescence (Ref. 14). This paper’s authors theorize that the MVD of an 

icing wind tunnel condensation cloud would be 5 μm or smaller since the growth time within the region 

of supersaturation (from the end of the contraction through the test section) is a fraction of a second. It 

should be noted that droplets such as these with very low inertial parameters would not significantly 

contribute to wing ice accretions nor be detectable by instruments requiring particle impact. Depending 

on the icing wind tunnel conditions, the LWC of a condensation cloud can be significant. Since the TWC 

Isokinetic Sampling Probe would not discriminate between condensation cloud and the normal spray 

cloud of the IRT, the data presented in the next section has been corrected for the presence of 

condensation cloud. 

NASA Glenn Icing Research Tunnel (IRT) Data 

The instrument system as described above was tested in the IRT in January and again in May 2008. 

The test condition matrices for these two tests are found in Tables 1 and 2. The conditions run in the May 

2008 test were selected from the IRT calibration conditions. These conditions were used to ensure that 

there was no calibration interpolation error possible. Because of this, the May 2008 data is considered our 

best set of data and is emphasized here. 

All of the TWC Isokinetic Sampling Probe data presented here has been corrected for collection 

efficiency as described earlier. Each of the TWC data points has been time averaged over a stable period 

of output of at least 1 minute. The data stability depended on both spray and instrument output stability 

(the chilled mirror hygrometers would typically require several seconds to reach a stabilized value 

following spray activation while the IR gas analyzers had no apparent stability issues). All of the IRT 

LWC data presented is the sum of the IRT LWC calibration and the calculated condensation cloud LWC 

(if a condensation cloud was calculated to be present). The condensation term is calculated using a prespray measured facility dew point temperature, and is assumed to be constant throughout the spray. 

Figure 9 presents all of the data gathered in the May 2008 IRT test (see the test matrix in Table 2). 

The linear fit of the IR gas analyzer data results in a slope of 1.1886, and the similar slope for the chilled 

mirror hygrometer data is 1.1485. The chilled mirror TWC data shows better agreement to the 

condensation cloud corrected IRT LWC calibration data. But this is largely because the chilled mirror 

hygrometer data was gathered after it had settled to steady values. If data were recorded from spray-on to 

spray-off, chilled mirror derived TWC data would certainly have degraded accuracy. 

There is a significant amount of spread to the data in Figure 9. As an attempt to understand the 

variations in the TWC to LWC comparisons, a calculation of the TWC to IRT LWC difference as a 

percentage of the IRT LWC was made and plotted versus IRT LWC, spray MVD, and IRT airspeed. 

Figure 10 displays the percent difference versus the IRT calibration plus condensation cloud LWC for the 

May 2008 test data. A linear fit trend line was calculated for the IR gas analyzer and chilled mirror 

hygrometer based TWC measurements. Based on the trend line data, it appears that there is a significant 

dependency of the measured water content differences on the facility LWC. 

Figure 11 presents the percent water content differences versus the spray MVD. Based on the 

calculated trend lines, there appears to be a strong correlation between the measured water content 

differences and the tunnel spray’s MVD. 

The final error examination in Figure 12 is of the percent water content differences versus the test 

section airspeed. There does not appear be any correlation in this data. 

 
NASA/TM—2010-216217 9 

TABLE 1.—JANUARY 2008 IRT TEST MATRIX 

Condition 

number

Airspeed

(m/s)

Total temp

(

o

C)

LWC

(g/m3)

MVD

(μm)

1 51.4 –2.7 0.55 15.9

2 51.4 –2.7 0.99 18.9

3 51.4 –2.7 1.53 21

4 51.4 –2.7 1.01 47.2

5 77.2 –1.0 0.52 18.7

6 77.2 –1.1 1.01 19.4

7 77.2 –1.1 0.52 78.2

8 77.2 –1.0 0.73 191.5

9 90.0 –0.2 1 40

10 77.2 –7.0 1.01 19.4

11 77.2 –12.0 1.01 19.4

12 38.6 –2.9 0.55 15

13 38.6 –3.1 1 19

14 38.6 –2.9 1.5 21

15 51.4 –1.1 0.3 14.4

16 77.2 –1.1 1.22 21.5

17 51.4 –2.2 0.7 20

18 51.4 –9.0 0.3 14.4

19 51.4 –9.1 0.55 15.9

20 51.4 –8.9 0.7 20

21 51.4 –14.0 0.3 14.4

22 51.4 –14.0 0.55 15.9

23 51.4 –14.0 0.7 20

24 51.4 –14.0 1.1 156

TABLE 2.—MAY 2008 IRT TEST MATRIX 

Condition 

number

Airspeed

(m/s)

Total temp.

(

o

C)

LWC

(g/m3)

MVD

(μm)

1 25.7 –10.0 2.48 20.5 

2 38.6 –10.0 1.73 20.5 

3 51.4 –10.0 1.45 20.5 

4 64.3 –10.0 1.3 20.5 

5 77.2 –10.0 1.21 20.5 

6 90.0 –10.0 1.03 20.5 

7 102.9 –10.0 0.98 20.5 

8 115.7 –10.0 0.89 20.5 

9 128.6 –10.0 0.79 20.5 

10 77.2 –10.0 0.55 14.3 

11 77.2 –10.0 1.03 24 

12 77.2 –10.0 1.32 33.9 

13 77.2 –10.0 0.18 13.8 

14 77.2 –10.0 0.92 47.2 

15 77.2 –10.0 0.63 13.2 

16 51.4 –10.0 0.7 14.3 

17 51.4 –10.0 1.46 24 

18 52.5 –10.0 1.67 33.9 

19 129.1 –9.8 0.37 14.3 

20 128.6 –10.0 0.76 24 

21 128.6 –3.4 0.37 14.3 

22 129.1 –5.9 0.37 14.3 
NASA/TM—2010-216217 10 

Figure 9.—Catch corrected TWC measurements versus IRT calibration plus calculated condensation 

cloud LWC for May 2008 test. 

Figure 10.—Water content percent difference versus IRT calibration plus calculated condensation 

cloud LWC, for May 2008 test. 

Catch Corrected Isokinetic TWC vs Total IRT LWC

assumes pre-spray D-corner dewpoint remains constant through spray

y = 1.1886x

y = 1.1485x

0

0.5

1

1.5

2

2.5

3

3.5

0 0.5 1 1.5 2 2.5 3

IRT LWC cal plus calcuated Condensate 

(g/m3

)

1:1 Line Stable Avg LiCor Stable Avg C-M Linear (Stable Avg LiCor) Linear (Stable Avg C-M)

Water Content Percent Difference versus IRT Total LWC

0

5

10

15

20

25

30

35

40

0 0.5 1 1.5 2 2.5 3

IRT Total LWC (cal + condensation) 

(g/m3

)

IR gas analyzer chilled mirror Linear (IR gas analyzer) Linear (chilled mirror)
NASA/TM—2010-216217 11 

Figure 11.—Water content percent difference versus IRT MVD, for May 2008 test. 

Figure 12.—Water content percent difference versus airspeed, for May 2008 test. 

Water Content Percent Difference versus MVD

0

5

10

15

20

25

30

35

40

0 5 10 15 20 25 30 35 40 45 50

MVD 

IR gas analyzer chilled mirror Linear (IR gas analyzer) Linear (chilled mirror)

Water Content Percent Difference versus Airspeed

0

5

10

15

20

25

30

35

40

0 20 40 60 80 100 120 140

Airspeed (m/s)

IR gas analyzer chilled mirror Linear (IR gas analyzer) Linear (chilled mirror)
NASA/TM—2010-216217 12 

Figure 13.—Catch corrected TWC measurements versus IRT calibration plus calculate condensation 

cloud LWC, for both January and May 2008 tests. Data fit for May data is for LWC values <1.5 g/m3

(to match range of January data). 

One final plot (Fig. 13) is included that shows the data from both the January and May IRT tests. The 

data fits for the May data were limited to 1.5 g/m3

 to match the range of the January LWC data. The data 

from the two test entries agree very well (the IR gas analyzer data’s fit matches to 4 decimal places). This 

is particularly interesting since the test matrix for the two tests were significantly different and provides 

an indication of the general repeatability of the measurement system. 

Conclusions 

Overall it appears that the TWC Isokinetic Sampling Probe is working quite well. In bench tests it has 

been able to repeatedly measure evaporated water mass within 5 percent. The probe head has been 

checked with accepted computational tools to have a minimal level of required correction. And the flow 

system has been independently checked against other flow measurement sources. The differences that 

remain are likely the cause of two dramatically different measurement methods measuring somewhat 

different parameters. The TWC Isokinetic Sampling Probe by design should measure all water particles 

(liquid and frozen) within its sample volume. The IRT calibration is based upon “accretable” moisture, 

since it is the timed measurement of ice accretion thicknesses formed on a small radius leading edge 

collector. While actions are taken to make these timed accretion measurements very accurately, the 

accretion process itself is dependent on particle phase, droplet size, and to a lesser degree, cloud LWC. 

And while the IRT calibration method has long been accepted and is very appropriate for the work 

normally performed in the IRT, it is not completely surprising to the authors that there could be a 

difference between the two measurement techniques. Further, it is also very possible that at elevated 

LWC and MVD values that the accretion calibration method is under measuring due to droplet breakup, 

splashing, and bouncing. 

All 2008 Iso Data by Month of Testing

Catch Corrected

y = 1.1472x

y = 1.1472x y = 1.1121x

y = 1.0946x

0

0.5

1

1.5

2

2.5

3

3.5

0 0.5 1 1.5 2 2.5 3 3.5

Total IRT LWC (cal+condensate) (g/m3)

IRGA-Jan Chilled Mirror-Jan IRGA-May Chilled Mirror-May

IRGA-May<1.5 C-M-May<1.5 1:1 Line Linear (IRGA-Jan)

Linear (Chilled Mirror-Jan) Linear (Chilled Mirror-May) Linear (IRGA-May) Linear (IRGA-May<1.5)

Linear (C-M-May<1.5)
NASA/TM—2010-216217 13 

Appendix A.—Calculations 

A.1 Flow Rate Calculations 

To set the probe sampling flow rate to ensure isokinetic conditions, the desired sample air mass flow 

rate is calculated as: 

Target Dry Air Mass Flow Rate (kg/s) = Vexternal (m/s) * Ainlet (m2

) * ρexternal (kg/m3) 

where 

 Vexternal and ρexternal are based upon wind tunnel test section values. 

Since the thermal mass flow meter used to monitor the sample flow rate (Ref. 8) operates in standard 

flow units, we make the following conversion: 

 

Target Dry Airflow Rate (Std L/min) = Target Dry Air Mass Flow Rate (kg/s)/Standard Conditions Air 

Density (kg/L) * 60 (s/min) 

where 

 standard conditions are T = 21.1 °C and Ps = 101.3 kPa 

(Note: These standard conditions are defined by the air flow meter’s manufacturer.) 

By adjusting the shop air pressure entering the ejector, the sample flow rate is adjusted until it 

matches the target isokinetic condition. 

A.2 Water Concentration Calculations 

After the sample flow rate has been set ensuring isokinetic sampling, the TWC can be calculated 

based upon the sample and reference water vapor measurements. 

Cloud water concentration = Sample water vapor measurement – Reference water vapor measurement 

(Note: these measurements are made in mmolwv/moldryair units.) 

Next, the molar based units need to be converted to mass units: 

Cloud mixing ratio (gwv/kgdryair) = Cloud humidity (mmolwv/moldryair) * 18.02/28.97 

where 

 water vapor molar mass = 18.02 g/mol 

 dry air standard conditions molar mass = 28.97 g/mol 

Finally, 

TWC (g/m3) = cloud mixing ratio (gwv/kgdryair) * ρdryair (kg/L) * 1000 (L/m3

) 

Where 

 ρdryair is at wind tunnel test section pressure and temperature 
NASA/TM—2010-216217 14 

A.3 Condensation Cloud LWC 

To calculate the LWC of a condensation cloud in the IRT, first the initial saturation partial pressure of 

water vapor (WVPP) is calculated from the facility dew point measurement (measured just downstream of 

the heat exchanger): 

WVPPi (kPa) = ao + T (a1 + T (a2 + T (a3 + T (a4 + T (a5 + T * a6 )))))/10 

where 

 T dew point temperature in °C 

 ao 6.107799961 

 a1 4.436518521×10-1

 a2 1.428945805×10-2

 a3 2.650648471×10-4

 a4 3.031240396×10-6

 a5 2.034080948×10-8

 a6 6.136820929×10-11

(From Lowe, P.R. and Ficke, J.M., “The computation of saturation vapor pressure,” Tech paper No. 4–74, 

Environmental Prediction Research Facility, Naval Postgraduate School, 1974.) 

Then, with a known air pressure (P), the initial absolute water vapor concentration (WVC) is 

calculated: 

WVCi (mmolwv/moldryair) = WVPP (kPa) / P (kPa) * 1000 

From this term we need to subtract the water vapor concentration from the test section to determine 

the excess moisture (which we assume has all become condensation cloud). This can either be similarly 

calculated from dew point, or we can use the direct output of the IR gas analyzer (in mmolwv/moldryair

units). 

WVCexcess = WVCi – WVCtestsection

We then convert to Mixing Ratio (as we did on the previous page): 

Cloud mixing ratio (gwv/kgdryair) = WVCexcess (mmolwv/moldryair) * 18.02 / 28.97 

where 

 water vapor molar mass = 18.02 g/mol 

 dry air standard conditions molar mass = 28.97 g/mol 

Finally we calculate the condensation cloud LWC: 

 

LWC (g/m3) = cloud mixing ratio (gwv/kgdryair) * ρdryair (kg/L) * 1000 (L/m3

) 

Where 

ρdryair is at wind tunnel test section pressure and temperature 
NASA/TM—2010-216217 15 

References 

1. Hinds, William C., Aerosol Technology: properties, behavior, and measurement of airborne particles, 

Second Edition, John Wiley and Sons, Inc., 1999. 

2. Bond, Thomas H., Miller, Dean R., and Potapczuk, Mark G.,“Overview of SLD Engineering Tools 

Development,” AIAA–2003–386, 2003. 

3. Miller, Dean R., Lynch, Christopher J., and Tate, Peter A.,“Overview of High Speed Close-Up 

Imaging in an Icing Environment,” NASA/TM—2004-212925, AIAA–2004–0407, 2004. 

4. Mason, Jeanne G., Strapp, Walter J., and Chow, Philip, “The Ice Particle Threat to Engines in Flight,” 

AIAA–2006–206, 2006. 

5. Glickman, Todd S., Glossary of Meteorology, Second Edition, American Meteorological Society, 

2000. 

6. Brockman, John E., “Sampling and Transport of Aerosols,” in Aerosol Measurement: Principles, 

Techniques, and Applications, Second Edition, ed. Baron, Paul A., and Willeke, Klaus, WileyInterScience, Inc., pp. 143–195, 2001. 

7. Operating Instructions for VDF Series Variable Flow Vacuum Pumps, Revision A, Vaccon Vacuum 

Products, Inc., 2006. 

8. Model 4040/4043/4045 General Purpose Thermal Mass Flowmeter, Operation and Service Manual, 

Revision E, TSI Incorporated, 2002. 

9. LI–7000 CO2/H2O analyzer Instruction Manual, Third Printing, LI–COR, Inc., 2005. 

10. National Instruments Corporation LabVIEW Home Page, Retrieved 23 Sept. 2008, 

http://www.ni.com/labview/. 

11. IOtech Daqbook/2000 Series, Retrieved 23 Sept 2008, 

http://www.iotech.com/catalog/daq/catdk1.html. 

12. Ashby, D., Dudley, M., and Iguchi, S., “Development and Validation of an Advanced Low-Order 

Panel Method,” NASA TM–101024, 1988. 

13. Bidwell, C. and Potapczuk, M., “Users Manual for the NASA Lewis Three-Dimensional Ice 

Accretion Code (LEWICE3D),” NASA TM–105974, 1993. 

14. Politovich, Marcia K., and Vali, Gabor, “Observations of Liquid Water in Orographic Clouds over 

Elk Mountain,” J. Atmos Sci., 40, 1300–1312, 1983. 
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Total Water Content Measurements With an Isokinetic Sampling Probe 

5a. CONTRACT NUMBER

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Reehorst, Andrew, L.; Miller, Dean, R.; Bidwell, Colin, S. 

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WBS 645846.02.07.03.04.01 

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National Aeronautics and Space Administration 

John H. Glenn Research Center at Lewis Field 

Cleveland, Ohio 44135-3191 

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13. SUPPLEMENTARY NOTES

14. ABSTRACT

The NASA Glenn Research Center has developed a Total Water Content (TWC) Isokinetic Sampling Probe. Since it is not sensitive to cloud 

water particle phase nor size, it is particularly attractive to support super-cooled large droplet and high ice water content aircraft icing 

studies. The instrument is comprised of the Sampling Probe, Sample Flow Control, and Water Vapor Measurement subsystems. Analysis 

and testing have been conducted on the subsystems to ensure their proper function and accuracy. End-to-end bench testing has also been 

conducted to ensure the reliability of the entire instrument system. A Stokes Number based collection efficiency correction was developed to 

correct for probe thickness effects. The authors further discuss the need to ensure that no condensation occurs within the instrument 

plumbing. Instrument measurements compared to facility calibrations from testing in the NASA Glenn Icing Research Tunnel are presented 

and discussed. There appears to be liquid water content and droplet size effects in the differences between the two measurement techniques.

15. SUBJECT TERMS

Aircraft icing; Aircraft safety; Hygrometers; Flowmeters; Wind tunnel calibration 

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