Frequently Asked Questions About Visibility Sensors

Q2

What is extinction coefficient?

The American Meteorological Society Glossary of Meteorology defines “extinction coefficient” as the fractional depletion of radiance per unit path length. The extinction coefficient, σ, is the sum of the absorption coefficient and the scattering coefficient. Since absorption is usually small, the extinction coefficient is approximately equal to the scatter coefficient. Visibility sensors use formulas to convert the measured extinction coefficient into a measurement of visibility.

Q3

How are visibility and extinction related?

Conversion from extinction coefficient to visibility involves different algorithms, one for daytime and one for night. Daytime visibility is related to the viewing of dark objects against a light sky as previously mentioned. For measurement of visibility in the daytime, Koschmieder’s Law is used: 

V = 3/σ

 Where V is the visibility and σ is the extinction coefficient

 Nighttime visibility is related to the distance at which a point source of light of known intensity can be seen. For measurement of visibility at night, Allard’s Law is used:

 V = e^-σ*V/0.00336

 Where V is the visibility and σ is the extinction coefficient.

 Most applications will use only the Koschmieder’s (V = 3/σ) formula. The aviation community typically uses a photometer to measure the day/night condition and applies both formulas depending on the ambient background light.

Q4

Does the Sentry™ measure runway visual range (RVR)?

Runway visual range (RVR) is an instrumentally derived value that represents the maximum distance at which the runway, or the specified lights or markers delineating it, can be seen from a position above a specified point on its centerline of the runway. 1-3 visibility sensors located alongside and higher than the centerline of the runway normally determine this value. RVR is calculated from extinction coefficient, ambient light level, and runway light intensity. The Sentry™ can provide extinction coefficient and the optional P/N 20007 Photometer can provide the ambient light level. The system integrator usually provides runway light intensity and the RVR calculations.

Q5

How does the Sentry™ measure visibility?

The Sentry™ uses the principle of forward scattering. The optical system is designed such that the infrared light projected from the transmitter (TX) intersects the field of view of the receiver (RX) with a forward angle of 42º. The area of intersection is known as the sample volume. The 42º forward angle ensures performance over a wide range of particle sizes in the sample volume including smoke, dust, haze, fog, rain and snow.   

When the air is clear, very little light is scattered since there are few particles in the sample volume resulting in a small signal received by the sensor. As the number of particles in the sample volume increases, the amount of light detected by the receiver also increases.  In other words, the received signal strength is inversely proportional to the visibility.

Q6

If the Sentry™ measurement range is 16 km, can it really see 16 km away?

Forward scatter type sensors like the Sentry™ measure a sample volume of air close to the sensor, and using the assumption of air homogeneity, calculate the visibility which could be as far as 16 km depending on the clarity of the air. So, forward scatter sensors can not measure what is happening 16 km away but must assume that if the air between the sensor and distance is uniform, than an observer at the sensor can see a range of 16 km.

Q7

 Why not use backscatter instead of forward scatter as the detection method?

Back Scatter – Backscatter sensors collect light scattered in a reverse or backward direction. A typical sensor projects light outward that collides with particles in the air (fog, snow, dust, etc). These particles scatter light in all directions (primarily forward) but the backscatter sensor relies on the weaker energy scattered backwards toward a receiver that collects the light. The technique was popular in the early 1970s before the development of forward scatter sensors. The advantage of the backscatter technique is a small, compact sensor but this is outweighed by the fact that they severely underestimate visibility in snow and are prone to false scattering due to bright light and clogging from blowing snow. 

Forward Scatter – Forward scatter sensors collect light scattered in a forward direction. A typical sensor projects light outward that collides with particles in the air (fog, snow, dust, etc). These particles scatter light in all directions (primarily forward) and this sensor utilizes the stronger energy scattered forwards toward a receiver that collects the light. The FAA has determined that an angle of 42 degrees in the forward direction provides the best response in all weather conditions. The National Weather service (NWS), Federal Aviation Administration (FAA), and World Meteorological Organization (WMO) all recognize forward scatter the as preferred technique. As such, the forward scatter sensors now dominate the market.

Q8

Can the Sentry™ be powered with DC?

Yes, the Sentry™ can be powered using either AC or DC power depending on what option was ordered. The DC power option accepts DC voltages from 10-36 VDC power @ 6W nominal power consumption. The AC power option accepts 50/60 Hz AC voltages from 100-240 VAC power @ 24W nominal power consumption.

Q9

What are the Sentry™ output options?

The Sentry™ is offered with a wide range of output options. The standard output from the sensor is 0-5 or 0-10 VDC analog output. The voltage range is selected t the factory based on the customer’s order. We also offer a variety of other outputs including 4-20 ma current loop, control relays, diagnostic relays, as well as serial RS-232, RS-422, and RS-485. See the Sentry Options Brochure for more information on the Sentry™ options.

Q10

Are external hood heaters available?

Yes, this option helps keep a build-up of snow from blocking the Sentry™ optical path. The heaters are installed under each hood and draw a total of 50 W for AC powered sensors and 12 W for 12 VDC powered sensors. They are thermostatically controlled to turn on at ~3^o C and off at ~ 8^o C.

Q11

Is a photometer available?

Yes, the optional P/N 20007 Photometer is available to determine the day/night condition. It is used primarily for aviation purposes and allows the Sentry™ to switch between Koschmeider’s law (daytime) and Allard’s law (nighttime) in the conversion of extinction coefficient to visibility. It must be used with the Sentry™ Microprocessor option.

Q12

Can I use the Sentry™ in hazardous environments?

No, the Model SVS1 Sentry™ cannot be used in hazardous atmospheres. For this special application we offer the SVSEEx Sentry that is in compliance with ATEX Ex II 2GD EEx d IIB T5/T6 for use in areas of potentially explosive atmospheres.

Q13

How do I calibrate the Sentry™?

Calibration is recommended every 6 months using the P/N 20004 Calibration Fixture. The procedure requires less than 30 minutes and is very simple to complete. For more information about calibration, see the Sentry™ User’s Guide, Section 4 or contact EnviroTech Sensors, Inc.

Q14

Do I need to purchase a calibration fixture for each Sentry™?

This question has 2 answers. For the standard SVS1 Sentry, the P/N 20004 Calibration Fixture may be shared between sensors. But please note that at least one Calibration Fixture is required to successfully install and maintain the sensor - it is not optional! For the Hazardous Area SVSEEx Sentry, the P/N 20104 Calibration Fixture is mandatory and it is automatically included when you place an order for the Ex Sentry.

Q15

The factory testing of each Sentry™ consists of detailed optical alignments, electrical adjustments, noise testing, as well as a factory calibration using the EnviroTech standard calibration fixture. These procedures result in excellent operational consistency between sensors. A typical graph of the results of a test between the EnviroTech standard sensor and a production sensor may be viewed here.

 Q16

How often do the optics need to be cleaned?

The local environment determines the cleaning frequency. Typically, the lenses should be cleaned every 3-6 months using a soft cloth and glass cleaner.

 Q17

Can the Sentry™ be used to measure visibility in a tunnel?

Yes, the SVS1-T Tunnel Sentry™ is being used to measure tunnel visibility in several countries including Spain, Saudi Arabia, the Netherlands & the United Arab Emirates. Click here for a Road & Rail Tunnel Application Brochure.

 Q18

Where should I install the sensor?

Site selection is key for the successful performance of your Sentry™ Visibility Sensor. Simply put, if you do not choose a good location for the sensor or fail to install it correctly, it will not measure data that is representative of the visibility in the area. For more information about your specific application, see the Sentry™ User’s Guide, Section 2.1 or contact EnviroTech Sensors, Inc.

 Q19

Do I have to worry about power & signal line surges?

No, the Sentry™ is protected on both power & signal lines with EMI and surge protection. We recommend that the sensor housing be grounded to a low resistance earth ground rod per the instructions in the User’s Guide.

 Q20

What is the warranty period?

EnviroTech Sensors offers an 18-month warranty on the Hazardous Area SVSEEx Sentry and a 24-month warranty on the standard SVS1 Sentry.