NATIONAL BUREAU OF STANDARDS REPORT

10 105

FIELD TESTS OF AN EXPERIMENTAL MODEL VISIBILITY METER

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
NATIONAL BUREAU OF STANDARDS

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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

2120414 10 105

FIELD TESTS OF AN EXPERIMENTAL MODEL VISIBILITY METER

By

J. W. Simeroth

J. C. Wilkerson

J. E. Davis

For

Naval Air Systems Command

NATIONAL BUREAU OF ST,

for use within the Government,

and review. For this reason, the whole or in part, is not author Bureau of Standards, Washingto

the Report has been specifically

IMPORTANT NOTICE

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST) on October 9, 2015.

;s accounting documents intended

subjected to additional evaluation

listing of this Report, either in

I Office of the Director, National

y the Government agency for which

apies for its own use. <0>

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

FIELD TESTS OF AN EXPERIMENTAL MODEL VISIBILITY METER

By

J. W. Simeroth

J. C. Wilkerson

James E. Davis

This report describes the field testing of and results obtained from

an experimental model visibility meter developed by AGA. The device

operated from the backscatter of infrared radiation by particles in the

atmosphere. The output followed the visibility trends and the average

output varied approximately linearly with the logarithm of the visibility

for ranges greater than one kilometer. The instrument output for a given

transmittance had a wide range of values. Some of the malfunctions

encountered and the limitations of performance and suggestions for improve

ment of the equipment are discussed.

1. INTRODUCTION

Accurate and timely visibility information is very important for

operations at airfields. This information is usually obtained from visual

observations or from the runway visual range (RVR) or runway visibility

(RVV) determined from the transmis some ter measurements. The increase in

amount of traffic at most major airfields and the emphasis on operations

in lower visibility conditions require continuous visibility measurements

from several locations on the field. Other applications in addition to

those of aviation, such as controlling traffic on highways and waterways

and measuring fog density at remote sites and at special locations, are developing. In many locations a single-ended installation, one with all

the field components placed at one site, is needed because terrain or obstructions prevent installing a transmissometer . The recent development of several types of fog detector devices using either the backward

scatter or forward scatter of emitted radiation has possibilities of

making measurements suitable for determining visibility.

AGA Aktiebolag, Signals Division, Lidingo, Sweden, developed and

tested an experimental model of a visibility meter. This visibility meter

has the potential of measurements along a path at angles above, as well

as in, a horizontal direction. AGA agreed to lend this visibility meter

to the National Bureau of Standards (NES) for tests at the Areata (Airport

California, for a period of six months starting in January 1968. Since

this period was not in the main fog season at Areata, the loan period was extended to Octobero

- 2 -

2. DESCRIPTION OF EQUIPMENT

2. 1 System Description . The AGA visibility meter was an experimental

model developed from the RTM-IA fog detector which is based on the backscatter of infrared radiation. The test instrument consisted of an emitter

a receiver, a 60-hertz to 50-hertz power frequency converter, and a scale

adjuster (see figure 1). The emitter and receiver were housed in separate

units which bolted to a common frame. This assembly was mounted atop a pedestal, which permitted the optical axes to be adjusted simultaneously

to any angle from horizontal to nearly vertical. The power frequency

converter and scale adjuster were housed in four small boxes which were

fastened to the sides of the pedestal. The power required for this unit was approximately 200 watts from a 120-volt, 60-hertz source. The

recorder was a 1.0 milliampere d-c instrument which could be located at

any site as long as the resistance of the meter lines did not exceed 200

ohms

The instrument worked on the backscatter principle (see figure 2).

An infrared light beam was projected from the unit and partly scattered

by particles in the atmosphere. The backseat tered light was focused on a photo-diode in the receiver, and the signal generated by the photo-diode

was amplified by an electronic amplifier. A sample of light (pilot beam)

from the emitter was fed into the receiver photo-diode by a fiber-optics

bundle. The intensity of the pilot beam was automatically adjusted to

balance with the atmospheric return signal. A signal corresponding to

the attenuation of the pilot beam fed the indicating element of the

recorder

The optical axes at the emitter and receiver were separated by 1.0

meter and were alined so the axes crossed at 200 meters from the visibility

meter. The visibility meter was sensitive to the backscattered light in th< region from 6 to 200 meters from the instrument. The instrument was intended to operate in visibilities between 0.8 and 20 kilometers. The

apparatus had a built-in protection against sunlight which screened the

photocell when the sun was within three sun diameters of the optical axis.

2.2 Emitter . The emitter utilized a 150-watt, 12-volt, photographic

projection lamp, which was operated at 6 volts for extended life. The

light was projected through a perforated-shutter disk operated by a 50-

hertz synchronous motor, ^^^hich modulated the beam of light at 7 50 flashes

per second. The light beam was filtered by a dark red filter so that the

projected radiation was near infrared. This light then passed through a 200-millimeter diameter lens-window combination and into the atmosphere.

This beam of light was shaped by an auxiliary lens to spread part of the

light towards the axis of the receiver. The intensity distribution of

this spread light was designed to produce an approximately uniform

response independent of distance from the receiver of each section of

the fog volume sampled. The instrument is sometimes referred to as a "tailored-beam" visibility meter. The pilot beam was obtained through

K i;ci:i V i-.K f'.MlTTi'.R

le adj.

connection

scale-drive

Converter 12 V, DC/

/220 V, 50 Hz

Rectifier

*

B-6338/1

Visibility Ueter (experimental model) seen from the rear

Figure I . AGA Visibility Meter seen from rear.

4

Figure

2.

Diagram

of

the

AG

A

visibility

meter.

-5-

a hole in the re f lectorized coating at the side of the lamp. After

modulation at 750 flashes per second in a different phase from that of

the main beam, the pilot-beam signal passed through a neutral-density

filter and fiber-optics bundle to the receiver photo-diode. This neutraldensity filter was a circular -type variable filter with a density range

of 1 to 1000 which was calibrated along the edge with a linear scale from

0.0 to 3.0 and was called the "gray-scale”. The grayscale, servomotor,

and the potentiometer for driving the recorder were mounted in the emitter

housing and are discussed with the receiver.

2.3 Receiver . The receiver portion of the instrument viewed the

atmospheric backscattered light through a lens-window combination similar

to that of the emitter. The receiver photo-diode sensing element (photocell) was mounted near the focal point of the lens and viewed both the

atmospheric return and the pilot-beam signal from the emitter. Because

of the synchronous shutter arrangement, the receiver viewed the atmospheric

return signal and the pilot signal alternately. The sensing element was followed by several tuned amplifier stages and a synchronous detector.

The action of the synchronous detector and associated circuitry was to

compare the amplitude of the atmospheric return signal with that of the

pilot-beam signal. When the signals were not equal or balanced, the

circularly-mounted gray-scale was rotated by a servo-motor towards

balance every five seconds. When the signals were balanced or when the

reflected signal was greater, an indicating lamp visible from outside the

instrument was energized. The instrument output for the recorder was

taken from a one-turn potentiometer connected to the gray-scale shaft.

The output current varied directly with the gray-scale setting. The

output circuit allowed wide adjustment of the gray-scale positions to be

represented by the zero and full scale positions of the recorder.

2.4 Recorder . The recorder furnished with the equipment was a Siemens dotted-line recorder type M734. This recorder had a range of

0 to 1.0 milliamperes d-c and was of the falling-bar type. The paper

speed was 60 millimeters per hour, and each roll lasted approximately one month. There were two hand-produced scales on the recorder, one for fog

and rain, and one for snow. These scales were intended to apply when the

range of the recorder was adjusted for gray-scale settings of 0.5 and 3.0

to agree with recorder readings of 0.0 and 1.0, respectively. The

recorder was powered from a 120-volt, 60-hertz source.

2.5 Power Frequency Converter . The instrument utilized a 60-hertz

to 50-hertz power frequency converter. The 50-hertz power was required

to operate the synchronous shutter-drive motor^ The remaining portion of

the instrument operated from 60 hertz. The input to the converter unit was 120 volts, 60 hertz at 1 ampere. The 60-hertz power was rectified to

12-volts d-c and the 12-volts d-c was converted to 50 hertz at 220 volts.

Sixty-hertz 220-volt power was also provided.

- 6

-

3. INSTALLATION

The site selected for testing of the visibility meter was the east

test site at the Areata Airport. This site is relatively flat and free

of obstructions which interfere with the fog, as is shown in the aerial

view, figure 3. The instrument was mounted on a concrete pad and covered

by an instrument shelter which had two apertures cut in the wall for the

emitter and receiver. This allowed the optical axes to be alined from

0 to 6 degrees above horizontal. The instrument shelter also had a movable

roof to provide the opening when the instrument was aimed more than 6

degrees above the horizontal. The instrument and instrument shelter are shown in figure 4.

The installation of this instrument was near the projector stand

of transmis some ter TL-2. The atmosphere sampled by the visibility meter

was close to and nearly parallel with the 250-foot baseline of the transmissometer TL-2.

The readout-meter leads were extended 2600 feet to the recorder in

the laboratory. A milliammeter was also installed in the instrument

shelter for maintenance and calibration purposes.

4. TEST PROCEDURE

The procedure for the field tests of this visibility meter was to

operate the equipment continuously and to record the output. Except for

some special occasions, the unit was aimed with the optical axes of the

emitter and receiver at six degrees above the horizontal. In several

fog conditions, series of measurements were made with the optical axes aimed at 6, 30, and 60 degrees above the horizontal. For these measurements, the elevation angles were changed manually by raising and lowering

one edge of the frame on which the emitter and receiver were mounted.

At each of these angles, the position was maintained for approximately

four minutes or longer while two sets of readings were recorded. Throughout the field tests, the records of transmittance from a 2 50-f oot -base li ne transmissometer located approximately 20 feet from the visibility meter were obtained for comparison with the visibility meter records.

The calibration of the visibility meter was checked at the initial

installation by the manufacturer's representatives and was checked

periodically during the field tests. These calibrations were made by

placing a diffuse white target approximately ten meters in front of the

unit. Then the gray-scale reading for a balance was determined for

different aperture openings of the receiver. Diaphragms to fit over the

receiver lens were provided with apertures of 4 , 14, and 50 millimeters

diameter which were intended to provide gray-scale readings of 2.7, 1.5,

and 0.3.

I Ih

Figure

3.

Aerial

View

of

the

East

test

site.

Figure 4, AG A Visibility Meter as installed in its shelter.

-9-

Two potentiometers were provided, one for adjusting the recorder

zero to correspond to any desired minimum grayscale setting, and the

second to adjust the recorder range to agree with the desired range of

grayscale values. For the original adjustments, the recorder indication

corresponded to a gray-scale range of 0.5 to 3.0. This range, according

to the manufacturer's calibration for fog and rain, was designed to cover visibilities of one kilometer and above. To obtain data at lower visibility

conditions, the limits were adjusted so the recorder would cover grayscale

ranges of 0.2 to 3.0, 0.0 to 3.0, and 0.0 to 1.0 at various times during

the field testing. The balance position was determined by the density of

the gray-scale filter and was not related to the range of grayscale

covered by the recorder. The recorder indication was linear with the

gray-scale setting for the range covered. The recorded data were converted

from recorder indications to grayscale settings for presentation.

5. RESULTS

5. 1 Results from Continuous Operation. The grayscale settings

(as indicated by the recorder) as a function of t ran smis some ter reading

over a 250-foot baseline are shown in figure 5. The unit was aimed with

the optical axes six degrees above the horizontal for these data. These

data were obtained during the first six weeks of the field testing before

the records indicated changes in the sensitivity of the unit. The average

value and range of gray-scale settings for each transmissometer reading

interval are shown with the number of observations in each average.

Usually these observations were made when the recorder indication was nearly stable for five minutes or longer; however, some measurements of

shorter duration were used to obtain data in transmittance ranges which

seldom have stable periods or at times when the recorder indication

reached temporary peak or minimum values.

5.2 Effect of Changing Elevation Angles . A series of measurements

were made, for several different fog conditions, in which the elevation

angles were at 30 and 60 degrees, as well as at the normal six degrees

above the horizontal. The results from those tests made before the equipment malfunctions affected performance are given in table 1. Most of

these tests were in atmospheric conditions with fairly good visibility

below denser layers of fog. The data obtained before the equipment

malfunctions were too limited to evaluate the effect of changing elevation

angles in all conditions. This limitation is discussed further in section

6.4. The differences of the grayscale settings at balance for 30 and 60

degrees to the gray-scale settings at six degrees show some effects of

angle on the gray-scale setting for associated transmissometer readings.

Federal Aviation Administration (FAA) Flight Service Station official

observations of ceiling and visibility were used to provide ceiling information. The sets of data are arranged in order of reported ceiling heights

and visibilities and not in chronological order.

-10 - >o>

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- 11 -

5.3 Calibration Results . The initial calibration, made immediately

after installation, showed the gray-scale setting at balance to be within

0.05 of the prescribed values for each aperture. A calibration check

after one month of operation gave results which differed greatly from the

initial settings, indicating a malfunction. The emitter lamp was replaced

and the hole in the reflector coating from which the pilot beam was obtained was enlarged. The gray-scale settings at calibration were then within 0.1

of the initial settings. After another month of operation, the results

during a calibration check indicated more malfunctioning. Investigation

showed that the emitter lamp socket was damaged either from excessive

heat or arcing at the contacts. A replacement socket was not available

so the contacts were cleaned and adjusted. The calibration check then gave

grayscale settings 0.1 to 0.3 units higher than the recommended values.

Calibration checks before the equipment was dismantled were intermittant

but none were considered satisfactory. (The manufacturer reported that

calibration checks made after the instruments were returned and after

replacement of the emitter lamp and socket indicated normal performance.)

5.4 Response During Unsatisfactory Operation o The records of operation appeared nearly normal during the period when the calibration check

gave gray-scale settings 0.1 and 0.3 units higher than recommended. To

determine the change in performance indicated by this change in calibration,

the gray scale settings and corresponding transmissometer readings were tabulated for a two-week period. The results are shown in figure 6. Note

that the average results are approximately 0.3 units higher than the

averages in figure 5.

5. 5 Response in Rain . The spread of the gray-scale settings of the

AGA visibility meter for a given transmittance, as shown in figure 5, is

very wide. One possible cause of this wide variation is believed to be

the relation between particle size and the backscatter of light. Since

there was no equipment at Areata for measuring atmospheric particle size,

the grayscale settings during a 48-hour period of rain were tabulated as

a function of transmissometer reading and are presented in figure 7. Heavy rain which reduces transmittance to low values seldom occurs at

Areata. For most of the data used in figure 7, the reduction in transmittance was caused primarily by the fog accompanying the rain but the

larger rain drops would be a contributing factor.

5.6 Daylight Effects . The response of this visibility meter was affected by daylight. At the beginning of the field testing, the grayscale setting was as much as 0.1 gray-scale units lower in daylight than

at night for similar transmittance. The decrease in grayscale setting

in daylight increased with time and in some cases appeared to exceed 0.2

gray-scale units. During the period of the lamp malfunction, when the

filament was out of focal alinement, the daylight effect exceeded 0.5

grayscale units.

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C\J

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«

tc

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Figure

5.

Groy-scole

setting

os

o

function

of

tr

onsmissometer

reading

for

period

before

molfunction.

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6.

Gray-scale

ring

asa

function

of

transmissometer

rending

after

malfunction.

14

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CVJ d

a>

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3

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0.3

0.4

0.5

0.6

0.7

0.0

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Reading

(250*

boseline)

-15-

5.7 Recordei' Operation . The dotted-line record usually had a spread

of 4 to 10 percent of full scale and the chronological order of the dots was not always easy to determine. The instrument output was also recorded

with a continuous-line recorder for many of these tests. This type record was better for reading values. Frequently, the atmospheric conditions

changed rapidly at Areata and the continuous records were more useful for

following these trends than were the dotted-line records.

5.8 Maintenance . Routine maintenance consisted of caring for the

recorder and keeping the lenses clean. A calibration check was made at

approximately one month intervals to determine equipment performance.

As indicated in paragraph 5.3, these calibration checks were more sensitive

to unsatisfactory performance than routine observation of the records. A

number of malfunctions of the components occurred, as was anticipated for an experimental instrument. These malfunctions are discussed in Section 6.

6. DISCUSSION OF RESULTS

6.1 Gray-Scale Setting as a Function of Transmittance . The average

gray-scale setting at balance, as shewn in figure 5, is a smooth function

of transmittance. However, the spread in individual readings within a given increment of transmittance was greater than would be expected from

normal nonuniformity of the atmosphere. Experience at Areata indicates

that two transmissometers separated by distances similar to the distance

between the AGA visibility meter and the transmissometer would seldom

have such a marked spread in readings. Several factors inherent in the

system or equipment may have contributed to the spread. The servomotor

drive at balance appeared to vary by as much as 0. 1 units of the grayscale. The daylight effects described in paragraph 5.6 may have changed

the setting by 0.1 or more units between daylight and night. Perhaps the

major factor in response variation is the effect of the particle size on the backscatter.

6.2 The Effect of Particle Size . The backscatter of light from a particle in the atmosphere is dependent on the size of the particle and

the wavelength of the light. The total scattering of white light from

fog particles is considered to be independent of the particle-size

distribution, but scattering in a specified direction, such as backscatter,

is dependent on the particle size. In previous tests by AGA, two curves

for visual range as a function recorder indication were obtained--one for

water in liquid state and one for water in a crystalline state. The

liquid-water curve was intended to include rain, mist, and fog and perhaps

haze

-16-

The data on the response in rain at Areata are presented in figure 7. The rain data were limited to a 48-hour period from two separate storms.

The rain varied from moderate to very light and most of the time was accompanied by fog. On February 19, except for a short period when dense

fog occurred, the gray-scale setting, as depicted by the closed dots,

showed higher than average gray-scale settings, near the upper limit of

the ranges shown in figure 5. This indicates a low backscatter for a given transmittance. For the one-hour period of dense fog identified by

the open circles, the gray-scale settings indicate a very high backscatter.

Although light rain continued to fall, the transmis some ter records indicated

that the surface air mass had characteristics which differed from those of

the surface air mass earlier and later. The data for February 20, identified by the +'s, were obtained primarily in rain and indicate low backscatter. The data for February 21, for rain accompanied by fog, indicate

the backscatter was moderate to high. Further investigation, with

measurements of particle size and distribution, is needed to determine the

effect of particle size on the response of this equipment. Although the

data in rain do not definitely show the effect of particle size on backscatter, they do show that the gray-scale setting for a given transmittance

changes with a change of air mass.

6,3 Correlation of Gray-Scale Setting with Visibility . The visibility

curve for liquid particles furnished with the recorder indicated that the

gray-scale setting varies linearly with the logarithm of the visibility.

Figure 8 shows this recorder -scale calibration provided by AGA and the

visibility for day and night as determined from the transmissome ter readings as a function of gray-scale settings.

Visibilities derived from the transmissometer readings for day are computed from the relation

T

V /250

250

° = e where

T^^q is the transmittance over a 250-foot baseline,

V is the visibility (of a large dark object) in feet, and

o

£ is the contrast threshold. A value of 0.55 is used for ^ •

Visibilities at night are determined from the relation

IT

250

V /250

n S V

V n where

I is the intensity of the light

25 candelas

source observed

assumed to be

T250

is defined as above

visibility

(stotute

miles)

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Figure 8

-18-

V is the visibility (of the light) in feet, and

n

S is the illuminance threshold of the observer, assumed to be

V

1.58 X 10 ^ lumens per square foot.

The gray-scale settings corresponding to transmissometer readings

were obtained from the curve in figure 5. Note that the curve for daytime visibilities obtained from transmittances is nearly a straight line

down to about one-half mile (one kilometer) or 0.8 transmittance. For

lower visibilities, the gray-scale settings decreased very slowly. At

the high visibilities, the departure from linearity may be caused partly

by small errors in the 100 percent setting of the transmissometer or the error in averaging the AGA visibility meter recorder readings near the

top limit. Because of the set upper limit on the recorder and servometer

drive, departures from the average reading near full scale can be greater

downward than upward, and the mean reading will tend to be low. The

representatives from AGA stated that satisfactory operation was expected

down to gray-scale settings of 0.2, If satisfactory operation to this

level were obtained, this would extend useful range in daytime visibility

to approximately one-fourth mile.

6.4 Effects of Elevation Angle . The data obtained with the axes of

the emitter and receiver more than six degrees above horizontal were inadequate to evaluate accurately the instrument's suitability for determining slant visibility. Additional tests of changing elevation angles were not made because the data were not reliable after the equipment

malfunctions. The tests which were performed were in advection type fogs

or low stratus cloud conditions only. Under these conditions the visibility

is greater near the ground than at higher elevations in the fog or inside

the stratus clouds. Thus the gray-scale settings were expected to be lc«i?er as the elevation angle was increased. Note that the differences in table 1 are nearly all negative at both 30 and 60 degrees for reported ceilings up

to 800 feet. At these elevation angles, the backscatter into the receiver

from particles more than 200 or 300 feet above the ground should be negligible. The differences obtained indicate that transmittance was lower for

several hundred feet below the bases of clouds or fog layers than at ground

level. No data were obtained in radiation type fog, which occurs infrequently at Areata, In shallow radiation type fogs, differences of the type

reported in table 1 might be positive. Further investigation will be

required to accurately determine slant visual ranges from these measurementt

-19-

As is evident from figure 8, the calibration curve of the visibility

meter is of the form

log V = aS + b

where V is the visibility, S is the grayscale setting and a and b are constants. Therefore, the difference between two grayscale settings is

proportional to the logarithm of the ratio of the corresponding visibilities

that is

logCV^/V^) = a(S^ - S2)

From figure 8 it is seen that a is approximately 0.6. Thus

log(V^/vp = 0.6(S^ - sp

The ratios of the visibilities indicated by the 30^ and the 60'' elevations to the visibility indicated by the 6° elevation have been

computed using this relation and are included in table 1. Note that the

instrument is designed to use a fog sample approximately 300 feet long.

Thus, if the fog density is uniform horizontally but varies with height,

measurements made at elevations of 30° and 60° should be indicative of

the slant visibility from heights of 150 and 300 feet respectively.

6.5 The Effectiveness of the Calibration Checks . Despite the

capability of the calibration checks producing useful check information,

there was a lack of confidence in the results of these checks^ This lack

of confidence was caused primarily by inconsistency of results when the

checks were repeated. Repeated checks sometimes differed by 0.5 units of

the gray-scale setting. However, the inconsistency in results seemed

greater when the equipment was malfunctioning; thus the variations may

have been real rather than inconsistencies in check procedure. The

inability of the operators to see the emitted infrared radiation for

checking target alinement reduced confidence in the alineraent. An infrared sensitive sighting device for the emitter would aid in target location.

A more definite indication of an erroneous calibration than the 0,1 to

0.3 gray-scale units would be desirable because of lack of confidence in

the reading, although the change in operating response as shown in figure 6 was of the same order as the change in calibration.

6.6 Error from Daylight Effects . The daylight effect produced a serious error in response which should be eliminated. Since the daylight

effect seemed to be increased by malfunctioning components, it is possible

that the effect may be eliminated or reduced to an acceptable level.

Further investigation is needed.

6.7 Discussion of Equipment Malfunctions . The several malfunctions

which developed in this equipment were not catastrophic but seriously

affected the performance of the instrument. The more important of these

malfunctions will be discussed individually.

- 20 -

6.7.1 Emitter Lamp Failures . The photographic lamp used as the

emitter source failed after operating less than two months. This trouble was caused by the failure of the cement, allowing the bulb to come loose

from the lamp base. The bulb was hanging by the lead-in wires and the

lamp still functioned. (The manufacturer reported that the replacement

lamp had failed similarly when he inspected the unit after the unit was returned to him.) Rearranging the mounting to provide a base -down

installation for the lamp would reduce the heat on the cement. Since

the emitted radiation is not in the visible range, the effects of this

lamp failure were not visible and were not detected until an attempted

calibration showed the instrument performance unsatisfactory.

After lamp replacement, performance continued unsatisfactory. The

manufacturer suggested enlarging the opening through the reflecting surface

at the side of the lamp from which the pilot-beam signal was obtained.

This helped but a fully satisfactory calibration was not obtained.

6.7.2 Lamp Socket Failure . The lamp socket was damaged either by

the heat from the lamp or from arcing at the contacts. A replacement

socket of a suitable type was not available. Adjustments were attempted

which appeared to correct the troubles, but these may not have been fully

satisfactory *• The calibration checks after the trouble was originally

discovered never gave grayscale settings as close to the specified

values as desired.

6.7.3 Output-Voltage Potentiometer Failure . Shortly after the

equipment was put in operation, intermittent output was noted and traced

to poor contact in the potentiometer supplying the output voltage corre- sponding to the grayscale position. A replacement potentiometer was

installed, which corrected this problem.

6.7.4 Backlash in the Servodrive . After replacement of the outputvoltage potentiometer, excessive backlash was noted in the coupling to the

servomotor drive. The couplings and connections were tightened. This

eliminated some of the backlash effect, but at times the drive seemed to

hesitate or overdrive by 0.1 or sometimes 0.2 units of the gray-scale. (This

intermittent functioning may have resulted from failure to respond correctly

to signal and not to mechanical backlash.)

*The manufacturer reported this socket was defective when he inspected the

equipment after its return.

- 21 -

7. CONCLUSIONS AND RECOMMENDATIONS

7.1 ConclusionS o

7.1.1 Instrument Performance . For visibilities greater than one kilometer (day scale), the average gray-scale setting varied approximately linearly with the logarithm of the visibility. For Icwer visibilities, the gray-scale setting approached a minimum value with very little

change below 0.5 kilometer. The spread of gray-scale settings for a given

transmittance was so great that individual measurements would be of limited

usefulness. The spread of data is wider than would be expected from

comparison of the measurements from two parallel transmissometer installations.

7.1.2 Slant Visibility Measurements . More investigation will be

needed to determine the capability of the instrument as a slant visibility

meter. These tests could not be conducted during the time at Areata

because of instrument malfunctions.

7.2 Recommendations .

7.2.1 Minimum Response . Improve the sensitivity of the device so that the gray-scale setting will continue to decrease with transmittance

down to or below a gray-scale setting of 0.2.

7.2.2 Daylight Effect . Eliminate the effect of daylight on the

response or reduce the change in response to less than 0.02 gray-scale

units.

7 .2.3 Lamp and Lamp Socket . Correct the lamp and lamp socket

troubles

USCOMM-NBS-DC

III
I