Transmissometer LT31

Features

• Single baseline for the MORmeasurement range10 … 10 000 m (33 … 32 800 ft)

• State-of-the-art white light source

• Automatic alignment with qualitycontrol

• Automatic calibration

• Internal back-up battery

• Compatible with VaisalaTransmissometer MITRAS andSKOPOGRAPH II Flamingo

• Window contaminationcompensation

• Meets the ICAO and WMOrequirements for RVR and visibility

• Based on decades of fieldexperience

• Sophisticated self-diagnostics

Vaisala Transmissometer LT31 enables accurate and reliable single baselinemeasurement for CATIIIb category airports.

Accurate and reliable determination ofrunway visual range (RVR) reducesairport downtime and improves safety ofoperations. Vaisala Transmissometer LT31is the qualified observer of RVRproviding correct visibility data withoutinterruptions and with minimalmaintenance.

Measurement Rangefrom 10 to 10 000 m(33 to 32 800 ft)LT31 provides the most accurate andreliable solution for the automaticmeasurement of RVR.The meteorological optical range (MOR)

measurement range is from 10 to 10 000meters (33 to 32 808 ft), which coversthe full required RVR range(CATI … CATIIIb), as well as the rangerequired for aeronautical visibility(the ICAO defined visibility). LT31accuracy meets the ICAO and WMOrequirements.

This full measurement range is achievedwith a single baseline system, whichmakes measuring easier and moreeconomical.

White LED Provides WideSpectrum Light SourceLT31 incorporates a white LED as a lightsource. White light is needed for the bestaccuracy in transmittance measurement.The WMO recommends the use of a widespectrum (white) light sources fortransmissometers as narrow spectrumlight source (for example, lasers orcolored LEDs) will cause measurementerror with some weather phenomena.

Automatic Systems

Automatic Calibration withIntegrated Forward ScatterSensorCalibration of a transmissometer istraditionally based on humanobservations. A reliable and accuratecalibration has required very highvisibility, stable conditions, and skilledand well trained personnel.

The Vaisala-patented automaticcalibration method for transmissometersis based on an integrated forward scattersensor/present weather sensor.The system automatically detects drift

and adjusts the sensor settingsaccordingly. Weather conditions do notneed to be as good as for manualcalibration; LT31 automaticallyrecognizes suitable conditions.

Automatic Fine AlignmentOne of the major error sources intransmittance measurement is alignmentdrift. Checking and adjustingthe alignment has also required skilledand trained personnel.

To maintain the measurement accuracy,LT31 performs an automatic optimizationof the alignment. The alignment qualityis also continuously evaluated withoutany human intervention.

The automatic fine alignment alsoprovides easy handling of LT31 duringinstallation. The alignment quality duringharsh weather conditions is ensured bya double mast construction. The outertube works as wind and solar radiationshield. The inner support structure isseparated from thermal and mechanicalstress caused by solar radiation and windload.

Contamination ReductionIn general, precipitation leads toan increased amount of windowcontamination. LT31 has long and narrowweather protection hoods that reducethe amount of window contaminationcaused by precipitation.

For wind-driven precipitation or dust,the instrument is equipped witha powerful blower. The blower createsan air curtain in front of the instrumentwindow. The air curtain is specificallydesigned not to disturbthe measurement path and causethe measurement errors that have beena problem with conventional blowerdesigns.

The integrated forward scatter sensor

Up-to-date Data

True window contamination compensation based onV-shaped windows

Automatic WindowContamination CompensationWindow contamination is a significantsource of error in transmissometers.Maintaining high accuracy has requiredfrequent cleaning. However, the effectsof contamination can be automaticallycompensated if the windowtransmittance can be measuredaccurately.

In LT31, window contamination is alsocompensated for by the most accuratemethod: by measuring the transmittancedirectly through the window glass.

Extensive Self-diagnosticsLT31 has a sophisticated self-diagnosticsthat provides detailed status informationfor all functional units. In addition, thisfeature helps to locate possible technicalfailures. It records a history of significantoperational situations, warnings, andalarms from the instrument.

Internal Backup BatteryLT31 can be equipped with an internalbackup battery. It provides steady dataavailability during short power breaks,for example while backup generators arestarted.

Present Weather ReportingThe optional present weather reportingfrom RVR sites gives a full picture ofthe prevailing weather conditions withinthe entire airport area. The arrival ofweather fronts and the presence of localshowers can easily be monitored due tothe multiple present weather observationpoints. The integrated present weathersensor provides the type and intensity ofprecipitation for METAR and localweather reporting purposes.

Background Luminance SensorLM21The optional LM21 sensor offers themeans for measuring the ambient lightlevel or background luminance in RVRapplications. The background luminancesensor is used for measuringthe background against whichthe runway lights or runway markings areseen.

Airport runway lights at night

Technical Data

Measurement PerformanceTransmittance measurement resolution 20 bit

Recommended Baseline Lengths

30 m (98 ft) baseline length (optimal) MOR: 10 … 10 000 m (33 … 32 808 ft)Transmittance range:< 0.01 % … 100 %

50 m (164 ft) baseline length MOR: 25 … 10 000 m (82 … 32 808 ft)Transmittance range:< 0.02 % … 100 %

75 m (246 ft) baseline length MOR: 37.5 … 10 000 m(123 … 32 808 ft)Transmittance range:< 0.02 % … 100 %

Purchased Option

25 m (82 ft) baseline length 1/3 × baseline length to 10 000 m(32 808 ft)

Accuracy (RVR range) Exceeds the ICAO (Annex 3)recommendations

Output data Automatic or polled data messagesStandard message including MOR,LT31 status and BGL data (option)Message including present weatherdata (option)MITRAS transmissometer compatiblemessageSKOPOGRAPH II Flamingotransmissometer compatible messageFD12 Visibility Meter compatiblemessage

Present weather reporting option Identifies 7 different types ofprecipitation (rain, freezing rain,drizzle, freezing drizzle, mixed rain/snow, snow, ice pellets)

Reports 49 different codes from WMO 4680code table

Transmitter Optical SpecificationsLight source White light-emitting diode (LED) -

Certified class 1 product according toEN 60 825-1

Optical monitoring Light source stability controlWindow contamination measurementand compensation circuitry

Operating EnvironmentOperating temperature -40 … +60 °C (-40 … +140 °F)

standard-55 … +60 °C (-67 … +140 °F) hoodheater option

Operating humidity 0 … 100 %RH

Wind speed Up to 60 m/s (134 mph)

Electrical safety IEC 60950-22 / EN 60950-22/A11 /IEC 60950-1 / EN 60950-1 /UL 60950-1

Inputs and OutputsAC supply 100/115/230 VAC + 10 %, 50 ... 60 Hz

Power consumption Max. 800 VA (for complete LT31system) with all options

Battery backup option Battery 2 Ah, backup time 60 min at+20 °C (+68 °F) when a proper andfully charged battery is used

Obstruction light Optional

Outputs Serial data line RS-232 or opto-isolated RS-485 (2-wire) or optionaldata modemSeparate maintenance line RS-232

Mechanical SpecificationsDimensions (H × W × D) 2685 × 420 × 1022 mm

(105.71 × 16.54 × 40.24 in)

Weight

Transmitter Unit LTT111 85 kg (187.39 lb)

Receiver Unit LTR111 82 kg (180.78 lb)

mm[in]

1022 [40.24]

26

85

[10

5.7

1]

420[16.54]

ComplianceRadiated emissions CISPR 22 / EN 55022

Conducted emissions AC CISPR 22 / EN 55022

Harmonics current emissions IEC 61000-3-2 / EN 61000-3-2

Electrostatic discharge IEC 61000-4-2 / EN 61000-4-2

RF field immunity IEC 61000-4-3 / EN 61000-4-311 V/m(80 MHz–1 GHz) 4 V/m (1 GHz–4 GHz)

Electric fast transient IEC 61000-4-4 / EN 61000-4-4

Surge IEC 61000-4-5 / EN 61000-4-5

Conducted RF immunity IEC 61000-4-6 / EN 61000-4-6

Published by Vaisala | B210416EN-D © Vaisala 2017

All rights reserved. Any logos and/or product names are trademarks ofVaisala or its individual partners. Any reproduction, transfer, distributionor storage of information contained in this document is strictlyprohibited. All specifications — technical included — are subject tochange without notice.www.vaisala.com