-----------------------------Monitor----------------------------- The Newsletter for PC-Based Data Acquisition and Control Issue 23 www.windmill.co.uk August 2000 -------------------------ISSN 1472-0221-------------------------- CONTENTS * Replaying Data After Collection * Weather Monitoring * Acronyms and Other Abbreviations _________________________________________________________________ WINDMILL NEWS ============== 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). ____________________________________________________________ USING A PC TO MONITOR WEATHER ============================== 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 interface. * 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 issues. * 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 irrigation. 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 abundance. Abbas Ali Hussain, Interfacing Meteorological Instruments to a Computer, MSc Thesis, Manchester Metropolitan University, UK ____________________________________________________________ DATA ACQUISITION AND CONTROL ABBREVIATIONS ==================================================== Continuing our acronym and other abbreviation reference guide. For A-G see Issue 22 of Monitor HART Highway Addressable Remote Terminal Provides digital communication to microprocessor-based (smart) analogue process control instruments. http://www.hartcomm.org/ HMI Human Machine Interface Also known as man machine interface. The communication between the computer system and the people who use it. http://www.windmill.co.uk/monitor9.html HTML 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 http://www.w3.org/MarkUp/ HTTP Hyptertext Transfer Protocol The set of rules for exchanging html documents between web servers and browsers. http://www.w3.org/Protocols/ I/O Input/Output
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. http://www.ieee.org/ 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. https://www.isa.org/ 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). http://www.iso.ch/ 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 http://www.achilles.net/~jtalbot/history/ 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 http://www.misq.org/ MMI Man Machine Interface. Also known as human machine interface. The communication between the computer system and the people who use it. http://www.windmill.co.uk/monitor9.html 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.
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