Product Description

1. GENERAL
2. DESCRIPTION OF THE DISDROMETER RD-80
3. DATA ACQUISITION

1. GENERAL

The disdrometer for rain drops is an instrument for measuring rain drop size distributions continuously and automatically. It was developed because statistically meaningful samples of rain drops could not be measured previously without a prohibitive amount of work.

The instrument transforms the vertical momentum of an impacting drop into an electric pulse whose amplitude is a function of the drop diameter. A conventional pulse height analysis yields the size distribution of rain drops.
Distromet Ltd. offers all the necessary equipment for completely automatic drop size data acquisition. An example of a system is shown in Fig. 1. It consists of a disdrometer RD-80 and a personal computer or a notebook.

                    Fig. 1. System for measuring drop size data

The sensor is exposed to the rain drops to be measured. Together with the processor it produces an electric pulse for every drop hitting its sensitive surface. The amplitude of the pulse is in a fixed relation to the diameter of the rain drop. In the processor RD-80 pulses are divided into 127 classes of drop diameter, and for every drop hitting the sensor a seven bit ASCII code is transmitted to the serial interface of the PC. A computer program, which is delivered with the disdrometer system, can be used to put the data in a suitable format for recording on a file. In order to get statistically meaningful samples and to reduce the amount of data, the program reduces the number of classes to 20.

Fig. 2 shows a typical display generated by the data acquisition program.  Quantities derived from the drop size data, like rainfall rate, liquid water content and radar reflectivity factor and the drop size distribution are calculated and displayed during the measurement.

Fig. 2 Sample of collected drop data

2. DESCRIPTION OF THE DISDROMETER RD-80

The disdrometer for rain drops consists of two units: the sensor, which is exposed to the rain and the processor for analog processing and digitizing of the sensor signal. A cable, 20 meters long, is used to connect the two units.

Fig. 3 Disdrometer for rain drops RD-80

The sensor transforms the mechanical momentum of an impacting drop into an electric pulse, whose amplitude is roughly proportional to the mechanical momentum. It is housed in a cylindrical case, which protects the sensitive parts.
The processor contains circuits to eliminate unwanted signals, mainly due to acoustic noise, to reduce the 90 dB dynamic range of the sensor signal and to digitize it into a 7bit code..
A block diagram of the whole system is given in Fig. 4.

                    Fig. 4 Block diagram of the disdrometer RD-80

The sensor consists of an electromechanical unit and a feed-back amplifier housed in a common case. A conical styrofoam body is used to transmit the mechanical impulse of an impacting drop to a set of two moving coil systems in magnetic fields. At the impact of a drop the styrofoam body together with the two coils moves downwards and a voltage is induced in the sensing coil. This voltage is amplified and applied to the driving coil, producing a force which counteracts the movement. As a consequence the excursion is very small, and it takes very little time for the system to return to its original resting position and therefore to get ready for the next impact of a drop. The amplitude of the pulse at the amplifier output is a measure for the size of the drop that caused it.

The processor has three main functions:

• it supplies power to the sensor,
• it processes the signal from the sensor and
• it contains circuits for testing the performance of the instrument.

The signal processing circuit consists of four parts:

• a noise rejection filter,
• a dynamic range compressor and
• a signal recognition circuit.
• a non-linear A-D-converter

The noise rejection filter is an active band pass filter, whose frequency response is designed to give an optimum ratio between signals from rain drops and signals due to acoustic noise affecting the sensor.
The dynamic range compressor uses an operational amplifier with a voltage dependent feedback network to get the desired characteristic.
The signal recognition circuit can distinguish between the signal pulses caused by drops hitting the sensor and the more uniform oscillations caused by acoustic noise. If a pulse caused by a rain drop exceeds the oscillations caused by noise, a gate passes it to the pulse standardizer, which produces a constant pulse duration without changing the amplitude of the original pulse.
The non-linear A-D-converter produces a 7-bit code for every drop hitting the sensor.

2.1 SPECIFICATIONS OF THE DISDROMETER RD-80

2.2. INSTALLATION OF THE SENSOR

When choosing a site for the installation of the sensor the following precautions should be observed. It is advisable to set up the sensor in quiet surroundings, as high acoustic noise levels will impair the measurement of small drops. Signals caused by acoustic noise will be suppressed by the instrument, but drop signals not exceeding the level of the noise signal will be suppressed together with the noise signal. Drop signals exceeding the noise signal will be measured with full accuracy. The presence of acoustic noise therefore may cause a reduction in the number of small drops measured.
A similar effect is caused by strong winds producing turbulence at the edges of the sensor. Mounting the sensor with its top even with the surroundings will reduce this effect.

3. DATA ACQUISITION

A  program for data acquisition on a Personal Computer (PC) or a notebook is delivered with the RD-80. With this program the user can record and evaluate drop size data from the RD-80 disdrometer. Graphs of rainfall rate, liquid water content and radar reflectivity factor vs. time are generated and all significant data is stored in data files on hard disk.  See detailed description in Data Processing