Windmill Software Ltd
Data Acquisition Intelligence

June 2000

     The Newsletter for PC-Based Data Acquisition and Control    
Issue 23                  August 2000
-------------------------ISSN 1472-0221--------------------------

* Replaying Data After Collection
* Weather Monitoring
* Acronyms and Other Abbreviations

Replay Charts of Data After Collection

You can now read about our Windmill Replay software, and 
download its new Help file, from our web site.

Windmill Replay plays back logged data files graphically. It's 
ideal for scrolling through long files looking for significant 
events. You can fast forward through uneventful stretches, and 
pause for a more detailed look when something catches your eye. 

The Replay screen is split between a moving graph and a table of 
data values. How much space is devoted to each is up to you. 
When the graph is static, double-clicking a point on it will 
highlight the relevant row in the table. Similarly double-
clicking a point on the table will show the corresponding points 
on the graph. 

You can choose:

* Which channels to display: four channels of data from one 
  instrument and four from another for example
* The colour of each channel's trace on the graph
* Direction of replay: forward or reverse 
* Speed of replay: slow or fast 
* Zoom area and magnification 
* The area to print 
* How many decimal places should be shown in the data table 
* The split between graph and table 

To use Replay you also need the Windmill suite of software to 
collect the data in the first place. Monitor subscribers 
can download this for free.

You can now buy Replay from our on-line catalogue. It costs 195 GB 
pounds (around 315 Euros or 295 US Dollars).

Throughout time we've been fascinated by the weather - studying 
signs of change and making and acting on predictions. In the 15th 
Century, Cardinal Nicholas da Cusa invented one of the first 
weather instruments: a hygrometer to measure the moisture 
content of the air. Today we can link modern instruments to 
computers to reduce human error, automate record keeping and 
control conditions should limits be crossed.

Many weather conditions may be measured, including temperature, 
atmospheric pressure, rainfall, humidity, wind speed, wind 
direction and solar energy. Local weather measurements are 
extremely important to a wide range of professions, from 
horticulturists to fire fighters. 

What do you need to Create a Computerised Monitoring System?
There are three components to a computerised system.

*  The instruments and sensors used to measure the changing 
   conditions. These might include anemometers to measure wind 
   speed, hygrometers to measure humidity and rain gauges to 
   measure precipitation.
*  An interface between these instruments and the computer. This 
   is a piece of hardware which reads the instruments and passes 
   the data to the computer in a format it can understand. An 
   interface might be a unit which plugs into the PC's USB port, 
   such as the Microlink 751 we sell in our on-line catalogue. 
   Some weather instruments you can plug directly into the 
   computer's serial port eliminating the need for another 

* Software to:
  - Record the weather data in the units of your choice
  - Display the data on-screen
  - Analyse the data
  - Make data available around a network or over the internet
  - Alert you when measurement thresholds are crossed (by    
    telephone for example)
  - Generate reports
  - Control instruments on specific conditions - starting a 
    heater when the temperature drops for example

   We, of course, use Windmill software, which works with a wide 
   range of instruments and devices. There are also many 
   dedicated weather monitoring packages on the market. 
   (Windmill 4.3 is available free to Monitor subscribers.)

Weather Instruments to Monitor Rainfall and Wind Speed
There isn't room here to detail all the different weather 
conditions which may be monitored, so we've chosen to discuss 
rainfall and wind speed. We'll cover other variables in future 

Monitoring Rainfall
Strictly speaking, we're not monitoring rainfall but 
precipitation: all the water that falls and is deposited on the 
earth including snow, sleet, hale and dew. Precipitation or 
rainfall is a very important measure in many applications. For 
example, it is the single most important meteorological factor 
to track when predicting changes in mosquito abundance, and 
hence the potential for virus transmission from mosquitoes to 
humans and other animals. Horticulturists, on the other hand, 
use rainfall information to efficiently schedule planting and 

The simplest way of measuring precipitation is by a rain gauge. 
This is a calibrated cylindrical container with a funnel of 
standard diameter into which the rain falls. Each day at the 
same time the amount of rain in the cylinder is manually read 
and recorded. This type of rain gauge was first used by King 
Sejong of Korea (Choson) in the 15th Century. 

However, we want to automate measurements so we leap forward to 
the 17th Century and Sir Christopher Wren's invention of the 
tipping bucket rain gauge. A bucket is balanced in unstable 
equilibrium about a horizontal axis and is divided into two 
equal compartments. In its normal position it is tilted with one 
side resting against a stop. The rain falls into the upper 
compartment and when a certain amount has fallen the bucket 
overbalances and tilts the opposite way. The rain is thus 
emptied out. Knowing the volume of the bucket, a count of tips 
lets you calculate the volume of rain that has fallen.

These days we record the movement of the bucket electronically, 
using an optical or magnetic switch and a counter input on the 
data acquisition interface. The software will automatically 
calculate the rainfall over a given time (usually 24 hours). For 
more details of counting, see Issue 7 of Monitor.

With a computerised system, of course, you are not limited to 
daily totals and can also obtain hourly totals, or totals for 
any period you choose. Software like Windmill will also plot the 
data for you on a chart, together with other conditions like 
wind speed or humidity.

You need to mount your rain gauge in an open location, high 
enough off the ground to prevent splashing. As a rule of thumb, 
the distance of any obstacle from the gauge should be at least 
twice, and preferably four times, the height of the object.

Monitoring Wind Speed
Wind is the motion of the air over the surface of the earth. The 
unequal heating of the Earth's surface by the sun results in areas 
of high and low pressure, and wind is defined as the motion of air 
from high to low atmospheric pressure in a horizontal plane.

The measurement of wind, or air flow, is called anemometry. 
Knowing the current wind speed, for example, lets farmers know 
when it is safe to spray crops and informs fire fighters of the 
danger of fires spreading. Wind speed records are used to 
satisfy legal requirements where there is a possibility of 
harmful discharge into the atmosphere. Wind monitoring is also 
used in many non-weather applications, for example in controlled 
explosion monitoring. 

In computerised wind speed systems, a cup anemometer is often 
used. This normally consists of three or four cups mounted 
symmetrically about a vertical axis. As the wind blows, the 
force on the concave side of any cup is greater than that on 
a convex side in a similar position, thus causing the cup wheel 
to rotate. The speed at which the wheel rotates is solely 
dependent on wind speed.

A small electrical generator can be linked to the cup wheel. 
This generates an electromotive force (a voltage) which gives 
an instantaneous speed of rotation. This is transmitted to the 
computer through an analogue voltage input on the interface. 
Alternatively, an electrical contact could be made every 
revolution. This would give a series of pulses, the frequency of 
which would indicate wind speed. In this case the computer 
interface needs a counter input.

Another method of measuring wind speed is with a Doppler 
anemometer, which measures the speed of sound. If the air 
through which the sound is travelling is moving, the speed of 
sound appears to vary. This instrument has no moving parts, and 
so doesn't wear out. It allows up to around 10 readings per 
second, a speed quick enough for monitoring air movement away 
from explosions.

Proper siting of any anemometer is important to ensure accurate 
readings. For example, wind sensors should not be installed to 
close to a building, as turbulence created by the building can 
interfere with readings.

Further Reading
Monitor Issue 7: Timing and Counting
   More information on using a PC for counting applications like 
   rainfall and wind speed measurement.

Monitor Issue 6: Emission Monitoring
   Discusses the monitoring of air pollutants

Monitor Issue 5: Temperature Measurement
   Describes how to use your PC to measure temperatures.

Fire Fighters and Weather Screens
   Explains how a fire station uses a computerised weather 
   monitoring system.

Mosquito Control Handbook: St. Louis Encephalitis. University of 
Florida, Institute of Food and Agricultural Sciences
   One example of how weather monitoring can help predict mosquito 

Abbas Ali Hussain, Interfacing Meteorological Instruments to a 
Computer, MSc Thesis, Manchester Metropolitan University, UK

Continuing our acronym and other abbreviation reference guide. 
For A-G see Issue 22 of Monitor

   Highway Addressable Remote Terminal
   Provides digital communication to microprocessor-based (smart) 
   analogue process control instruments.

   Human Machine Interface
   Also known as man machine interface. The communication between 
   the computer system and the people who use it.

   Hyptertext Markup Language
   A document language for the world wide web. It indicates the type 
   of information rather than the exact way it is to be presented

   Hyptertext Transfer Protocol
   The set of rules for exchanging html documents between web 
   servers and browsers.

A data acquisition system monitors signals through its inputs, and sends control signals through its outputs. IEEE Institute of Electrical and Electronics Engineers. An American Society that, amongst other things, establishes international standards in the computing, electronic and telecommunications fields. IML Interface Management Language A programming language used to communicate with measurement instruments. IR Infrared ISA Instrument Society of America The international society for measurement and control. ISA Industry Standard Architecture An ISA expansion slot lets you plug data acquisition boards into PCs. Other methods include the universal serial bus. ISO International Organization for Standardization Made up of national members. A member is the "most representative of standardisation in its country". For example BSI (British Standards Institute), DIN (Deutsches Institut für Normung) and ANSI (American National Standards Institute). ISP Internet Service Provider IT Information Technology. Covers all aspects of managing and processing information with a computer. LAN Local area network. A data communication system connecting devices in the same vicinity. Data is transferred without the use of public communications. Examples of LANs are Ethernet, token ring and Modbus. LASER Light amplification by stimulated emission of radiation LED Light Emitting Diode Used as indicator lights on electronic devices, including data acquisition interfaces. LIFO Last in first out. Describes a stack method of data storage. LIMS Laboratory information management system LVDT Linear Variable Differential Transformer Used in measuring devices that need to convert changes in physical position to an electrical output. MES Manufacturing Execution System Delivers information about, and controls, plant activities. MIS Management Information System MMI Man Machine Interface. Also known as human machine interface. The communication between the computer system and the people who use it. Next month: the final part of our data acquisition abbreviation explanations. ____________________________________________________________ Do you have a question, comment or suggestion on this newsletter? E-mail the editor - Jill - at [email protected] or fill in this form.
Copyright 2000 Windmill Software Ltd. All rights reserved.
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