Copyright 2013 Optical Sensors
Mini PWS
 RS232 output:
The output is presented in digital form as an ASCII string on the RS232 output, 2400 baud 8N1, that is transmitted "streaming" every 5 minutes. This string can be received by many loggers with RS232 inputs but also by a PC via the serial port and with a terminal program like Hyper Terminal (part of WINDOWS).This is an example of an output string:

09:24:16> 0.00196 01530 0.00000 0.21100 0.00000 -00006 000120 71

The number before the ">" character is a time stamp made by the terminal program used.The second number - in this case 0.00196 - is the extinction coefficient often called alfa. The relation between the extinction, alfa and visibility, VIS, is alfa=3/VIS. The alfa parameter can be used for monitoring trends in visibility when the visibility is larger than 5000 meters. But note that the absolute accuracy in alfa is limited in such cases.The third number is the measured visibility in meters, if the measured visibility is larger than 5000 meters 05000 is displayed.The fourth number is a measure of the rain precipitation in mm/h during the last 5 minutes. The fifth number is the same for snow.
The sixth number is the same for mixed precipitation.The seventh number is the temperature inside the sensor casing. The accuracy of this number as a measure of the ambient temperature is something like ± 3 degrees or even higher during sunlight.The eighth number is an uncalibrated measure of the ambient solar irradiance. The sensitivity depends stongly on how the sensor is aimed. It may vary considerably. But the the repitivity is much better and the offset error is only ± 2 units. The sensor can, when mounted, be individually calibrated by comparing it with an irradiance sensor. But a simple method is to take data from a clear day at noon. If the sun is 60 deg or more above the horizon the irradiance is not far from 1000 W/m2. The measurement is not very accurate- but it may be useful for instance for telling if it is day or night.

The ninth number in the data string is WMO codes.
00 is for good visibility, no precipitation
10 is for light fog, visibility between 3000 and 1000 meters
30 is moderate fog, visibility < 1000 meters
61 is for light rain
62 is for moderate rain
63 is for heavy rain
68 is for light intensity of mixture of rain and snow, not fully tested
69 is for light intensity of moderate or heavy mixture of rain and snow, not fully tested
71 is for light snowfall
72 is for moderate snowfall
73 is for heavy snowfall

Dimensions
68*45*34 mm
Weight
About 170 grams
Warm up time
5 minutes
Current consumption
mean 20 mA from a 12 Volt ( 8-14) supply
Outputs
Digital, streaming RS232
Update time
1 min for visibility *
Temp. range
-20 to +50 deg C
Optical output power
Max about 1.8 mW from an IR laser
laser safety class
3R
Wavelength
980nm
Housing
Anodized aluminum, openings sealed with O-rings
Visibility range
Visibilities from 5 km down to 20 meters
Accuracy of visibility reading
± 30%
Accuracy of measured rain amount:
±40%
Some short data of the MiniPWS:

The sensor is delivered with a PMMA plastic mounting bracket, see picture above. An important advantage with this bracket is that it isolates the sensor electrically which reduces the risk for surge currents during lightning etc.
The sensor is heated a little above ambient temperature in order to keep moisture away.It is delivered with a 6 meter 5-wire cable connected.
In order to keep the electronics dry a there is membrane ventilator that keeps the pressure inside at the same level as outside. This prevents liquid water from sucking into the sensor through micro cracks etc at falling temperature - a creative solution to a big problem....
Mounting.
The sensor should be mounted so that it "looks" roughly north (on the southern hemisphere south). There must not be anything in the sightline closer than about 5 meters. Outside a cone of about 30 degrees angle objects can be tolerated at down to  about 1 meter.

Below is data for a week with varying weather:
a compact low price Present Weather Sensor
The MinPWS is a spin off from our MiniOFS sensor and it is probably the cheapest Present Weather Sensor on the market.
And it is very compact - the width is only 68 mm!
This little sensor measures 3 important weather parameters.
  • Visibility
  • Rain - the response is fast compared to that from a tipping bucket. The threshold is also lower.
  • Snow - same as for rain - note that a simple tipping bucket sensor does not detect snow in many cases.
  
  Besides that two other weather parameters are measured and displayed:
  • Ambient light - the resolution and accuracy is not as good as for special sensors like solarimeters but the information is useful for  giving answers to  questions like telling if it is cloudy or not or of course if night or day.
  • Temperature - the temp. sensor is located inside the sensor body and the accuracy is therefore not better than about  ± 2 deg C. (But this is in some cases good enough - trends can be seen etc.)
 
Real time data from a sensor near Göteborg are displayed on MiniPWS data.

We think that the MiniPWS can  add information to a "basic" weather station that in many cases is useful .



*The precipitation data is "rolling means of the last 5 minutes.