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Optical Rain Gauge

1.

INTRODUCTION

In recent years, flood and water immersion damages
caused by local downpour have been occurring in a wide
range due to a trend of having heavier rainfall in Japan.
As a countermeasure against this unexpected local downpour, demand for acquiring more accurate rain information by installing rain gauges more densely, has been
increasing.
We have developed and commercialized an optical rain
gauge which can be installed inexpensively and is highlyreliable. This optical rain gauge has obtained an approval
from the Japan Meteorological Agency for the first time in
the industry using optical fiber sensor technology.

2.

CHARACTERISTICS

2.1 Reduction of Installation and Maintenance Costs
As Figure 1 shows, the optical rain gauge consists of asensor unit which includes only optical components anda signal processing unit. It reduces installation costs to agreater extent because, with this optical rain gauge, ancillary facilities including a power unit necessary for an electric rain gauge are not necessary at the observation point.
In addition, a large reduction of ancillary facilities contributes to a reduction of maintenance costs because the frequency of equipment checkups and the number of theperiodical replacement parts can be reduced.
2.2. Improvement of reliability
The sensor unit installed at the observation point generates no equipment trouble caused by electric outage,
lightning interference, etc. because it consists only ofoptical components and, no electric components.
Therefore, a highly reliable system can be achieved.
Especially, because it is not site-dependent, it can beused in mountain areas where thunder is frequent, roads,rivers etc. with security.
2.3. Electric Power Saving
Because no power source is necessary at the observation
point, power source construction and a power delivery
contract are not necessary. Thus, electricity can be totally
saved.

(a) Sensor unit.

(b) Signal processing unit.
Figure 1

Appearance of optical rain gauge.

2.4. Long-distance Measurement
For the communication between the sensor unit and thesignal processing unit, existing optical fiber transmission
channels which have been laid for road and river management, community informatization, railways, sewage
lines, etc. can be used. Therefore, rain measurement at a
distant point is possible only with this optical rain gauge,
not requiring special transmission channels and transmission equipment.

3.

MEASUREMENT PRINCIPLE

For the sensor unit of the optical rain gauge where rainfall
is measured, the same structure as the existing tipping
bucket rain gauge (it measures rainfall by counting thetimes that the tipping gauge tips over from side to side) isused.
In the existing tipping bucket rain gauge, electric pulse

Furukawa Review, No. 42201215Optical Rain Gauge

is put out when the magnet attached to the tipping bucket
passes above the reed switch. On the other hand, in thedeveloped sensor unit, the optical proximity sensor isused to put out reflected pulse instead of a reed switch.
The optical reflected pulse is measured and transformedin the signal processing unit, and the electric pulse similar
to that of the existing gauge is put out from it (Figure 2).

Tipping bucket

5.

MAIN FUNCTIONS

The detection range (rainfall per tip) of the optical rain
gauge is 1.0 mm. Electric pulse is put out from the signal
processing part along with the tipping action of the tipping bucket.
In addition to the electric pulse, a function of “hourlyrainfall alarm” is mounted. The function sounds an alarm
when measured hourly rainfall exceeds or equals a preset determined value. Using this function, simple operations are also possible. The determination values of hourly rainfall can be set arbitrarily.

MagnetReed switch
Electric pulse

6. PRODUCT SPECIFICATIONS
Table 1 shows the basic specifications of the optical raingauge.

(a) Electric rain gauge (existing).
Table 1Basic specification.

ItemSensor unit

Tipping bucket
MagnetOpticalproximity sensor
Optical reflected pulse
Optical fiber (1 core)

Water receiving
φ200 mmgauge diameter
Signalprocessing unit

Electric pulse(b) Optical rain gauge.

Figure 24.

Specification

Measurement principles.

Resolution

1.0 mm per tip

Accuracy

Rainfall less or equal to 40 mm : ±1.0 mm
Rainfall over 40 mm : ±3%

Cablecomposition

Optical fiber cord (with SC connector),
Resin tube

Size

Approx. φ 222 mm×450 mm(not including protuberance)

Signal processing unit

SYSTEM CONFIGURATION

Figure 3 shows the system configuration of the opticalrain gauge.
The sensor unit must be installed at the observation
point. The signal processing unit can be installed even ata distant point. A single-mode optical fiber (1 core) isused for the transmission channel. Existing optical fibertransmission channels for communication can also beused for it.

Electric pulse

Optical input and SC connector adapter
output terminal(input and output in common)
Applied fiber

Transmission loss: less or equal to 7 dB
Transmission
Transmission channel return loss: greater orchannel condition
equal to 36 dB (termination without reflection)

OutputElectric pulse signal
・normally open contact
・Contact closure time : 100-200 ms
Hourly rainfall alarm・normally open contact
Equipment trouble alarm・normally open contact

Contact rating

Less or equal to DC 30 V, less or equal to50 mA M3 terminal block

Power source DC 12 V (10-17 V) M3 terminal block

Optical fiber transmission channel
Signal processing unit

SM type optical fiber

Size

EIA 1 UApprox. W410 × D100 × H43 mm(not including protuberance)

Sensor unit

Figure 3System configuration.

Furukawa Review, No. 42201216Optical Rain Gauge

7.

CONCLUSION

This optical rain gauge can be installed inexpensively andis highly reliable because it is not affected by lightning
interference, etc. It is applicable to a wide range of sites
such as in a mountain area where the existing products
have difficulties being installed.
In the future, by taking advantages of the characteristics of optical fiber sensor technologies, we are planningto develop and apply them to various fields.
We have already commercialized this product inDecember, 2011.
For more information, please contact:
Telecommunications Sales & Marketing Department
Telecommunications Company
TEL: +81-3-3286-3132 FAX: +81-3-3286-4909
Access Network Department
FITEL Products Division
TEL: +81-436-42-1657 FAX: +81-436-42-1786

Furukawa Review, No. 42

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