Glossary of measurement and sensing terms
80 terms used across wind, weather, level, position and condition measurement, each defined in plain language, with links to the answers that go further.
Measurement has its own vocabulary, and a specification is only as clear as the words in it. These are the terms used across Gill product pages, application notes and knowledge answers, defined so that each one stands on its own.
A
Accuracy
Accuracy is how close a measured value is to the true value of the quantity being measured. A meaningful accuracy figure always states its conditions, for example a wind speed accuracy of 2 per cent RMSE at 12 m/s, because the error of most instruments changes across their range.
How accurate does a wind sensor need to be?
Air quality monitoring
Air quality monitoring measures pollutants in the air over time. Wind speed and direction are recorded alongside so that a reading can be traced back to its likely source.
Air/fuel ratio control
Air/fuel ratio control keeps the mixture entering a gas engine at its set proportion of air to fuel as gas quality, load and conditions change. Holding the ratio steady protects efficiency and keeps emissions within limits.
AF120 air/fuel ratio controller
Anemometer
An anemometer is an instrument that measures wind speed, and often wind direction as well. The main types are cup, propeller, hot-wire and ultrasonic anemometers, which differ in how they sense the moving air and in whether they have moving parts.
Apparent wind
Apparent wind is the wind measured on a moving platform such as a ship, vehicle or aircraft. It is the combination of the true wind and the wind created by the platform's own motion, so it has to be corrected with the platform's speed and heading to give true wind.
ATEX
ATEX is the European framework for equipment and protective systems intended for use in potentially explosive atmospheres. Equipment certified under ATEX carries a marking that states the equipment group, category and type of protection, and that certification applies to the specific model and configuration tested.
WindObserver IS for hazardous areas
AWOS
AWOS stands for Automated Weather Observing System, an airport weather system that measures and reports surface conditions such as wind, visibility, temperature and pressure automatically for pilots and air traffic services.
AWOS and airport weather systems
B
Ballistic meteorology
Ballistic meteorology is the measurement of wind, temperature, pressure and humidity along a projectile's path, so that firing data can be corrected for the conditions it will fly through.
Ballistic meteorology and test sites
Baud rate
Baud rate is the signalling speed of a serial data link, expressed in symbols per second. Both ends of a serial connection have to be set to the same baud rate for data to be read correctly.
Sensor outputs and integration
Bearing wear
Bearing wear is the gradual loss of material from the rolling elements and races of a bearing. It releases ferrous particles into the oil well before it becomes noise, vibration or failure.
C
Calibration
Calibration is the comparison of an instrument's output with a reference of known value under controlled conditions, with the difference and its uncertainty recorded. Calibration shows how an instrument is performing; any adjustment made afterwards is a separate step.
How are wind sensors calibrated?
Capacitive level sensing
Capacitive level sensing measures the level of a liquid from the change in capacitance between electrodes as the liquid replaces air around them. Because the liquid's electrical properties drive the measurement, there is no float or other moving part in the sensing element.
How capacitive level sensing works
Catch tank
A catch tank collects fluid that overflows or vents from a system, such as engine coolant or oil, so that it is contained rather than released. Its level shows how much has been lost.
Condition monitoring
Condition monitoring is the measurement of a machine's state while it is in service, such as oil debris, vibration or temperature, so that deterioration can be found and planned for before it becomes a failure.
Crane wind limit
A crane wind limit is the wind speed above which a lift must stop or the crane must be made safe. The limit is set by the crane and load, and it is only useful if the wind is measured at the height and position the load sees.
Cup anemometer
A cup anemometer measures wind speed from the rotation of cups mounted on a vertical shaft. It relies on bearings and a starting force to turn, so its low-speed response, wear and exposure to icing differ from those of an ultrasonic anemometer.
Ultrasonic anemometer vs cup anemometer
Current loop (4 to 20 mA)
A 4 to 20 mA current loop is an analogue signal in which the measured value is represented by a current between 4 and 20 milliamps. Because the scale starts at 4 mA rather than zero, a broken wire or failed transmitter can be told apart from a genuine low reading.
Sensor outputs and integration
D
Dead band
A dead band is a range over which an instrument cannot report a change. In wind measurement it usually refers to the sector of a potentiometer wind vane where direction cannot be read. An ultrasonic anemometer measures the full 360 degrees with no dead band.
Wind measurement without moving parts
Dynamic positioning
Dynamic positioning is a computer-controlled system that holds a vessel's position and heading using its own thrusters and propellers. Wind sensors feed the system so that it can counter wind forces before they move the vessel.
E
Eddy covariance
Eddy covariance is a method for measuring the exchange of heat, water vapour and carbon dioxide between a surface and the atmosphere. It correlates very fast measurements of vertical wind with fast measurements of the gas or temperature, which is why it needs a 3-axis ultrasonic anemometer sampling many times a second.
Electromagnetic compatibility (EMC)
Electromagnetic compatibility is the ability of electrical equipment to work as intended in its electromagnetic environment without causing, or suffering from, unacceptable interference.
F
Ferrous debris
Ferrous debris is the iron and steel wear material carried in a machine's lubricating oil. A rising quantity or size of ferrous particles is one of the earliest measurable signs that gears or bearings are wearing.
G
Galloping
Galloping is a wind-driven instability in which a structure starts to oscillate and the motion grows with each cycle. Torsional galloping, a twisting form of it, is a known risk for long rows of single-axis solar trackers in high wind.
Gust
A gust is a short-lived peak in wind speed. The World Meteorological Organization defines the reported gust as the highest 3-second running average of wind speed, which is why the averaging an instrument uses matters as much as the instrument itself.
Wind alarms and operating limits
H
Hall effect sensor
A Hall effect sensor detects a magnetic field through the voltage it produces across a semiconductor, and is often used to measure position from the movement of a magnet.
Inductive vs Hall effect position sensors
Hazardous area
A hazardous area is a location where an explosive atmosphere of gas, vapour or dust may be present. Hazardous areas are classified into zones according to how often and for how long such an atmosphere is likely to occur, and equipment installed there must be certified for that zone.
WindObserver IS for hazardous areas
Heat stress
Heat stress is the load placed on the body by heat from the air, sun, humidity and physical work combined. It is assessed with indices such as WBGT rather than air temperature alone.
Helideck monitoring system
A helideck monitoring system measures wind, and often deck motion and weather, at an offshore or ship helideck so that the landing can be judged against the operating limits.
I
Icing
Icing is the build-up of ice on an instrument from freezing rain, fog or snow. On a wind sensor it can block or distort the measurement, which is why heated sensors and designs with no moving parts are used in cold climates.
Wind measurement in icing conditions
IECEx
IECEx is the international certification scheme for equipment used in explosive atmospheres, operated under the International Electrotechnical Commission. It assesses equipment against the IEC 60079 series of standards.
WindObserver IS for hazardous areas
Ignition module
An ignition module controls when and how strongly the spark fires in each cylinder of a spark-ignited gas engine. Accurate timing and energy keep combustion stable and extend spark plug life.
Inductive position sensing
Inductive position sensing measures the position of a target from its effect on the magnetic coupling between coils. Because the target does not touch the sensing element, the measurement does not wear with use.
How inductive position sensing works
Intrinsic safety
Intrinsic safety is a protection technique for equipment in hazardous areas that limits the electrical and thermal energy in a circuit to levels too low to ignite the surrounding atmosphere. An intrinsically safe sensor is installed with a matching certified barrier or interface.
WindObserver IS, the intrinsically safe anemometer
IP rating
An IP rating (ingress protection) states how well an enclosure keeps out solids and water, under IEC 60529. IP66 resists powerful water jets, IP67 temporary immersion to 1 metre for 30 minutes, and IP68 continuous immersion under conditions stated by the manufacturer. IP69K, defined in ISO 20653, covers high-pressure, high-temperature washdown.
Irradiance
Irradiance is the power of solar radiation arriving on a surface, measured in watts per square metre (W/m²). On a solar site it is the reference against which generation is judged.
ISO/IEC 17025
ISO/IEC 17025 is the international standard for the competence of testing and calibration laboratories. Accreditation to it shows that a laboratory's methods, traceability and uncertainty calculations have been independently assessed.
L
Level sensor
A level sensor measures how much liquid, or sometimes solid, is in a tank or vessel. Level sensors can report a continuous value or switch at set points, using technologies such as floats, capacitance, pressure or ultrasound.
Lloyd's Register type approval
Lloyd's Register type approval is a certification that a product type meets Lloyd's Register's requirements for use on ships and offshore units. It applies to the specific models listed on the certificate, not to a product family as a whole.
M
Modbus RTU
Modbus RTU is a serial communication protocol, usually carried over RS485, in which a controller polls one or more addressed devices for their readings. It is widely used to connect sensors to industrial control systems and loggers.
Sensor outputs and integration
N
NMEA 0183
NMEA 0183 is a marine data standard that sends measurements as plain-text sentences over a serial link, so that navigation and bridge equipment from different manufacturers can share wind, heading and position data.
Non-contact measurement
Non-contact measurement senses a quantity without the sensing element touching the thing being measured, for example detecting position through a magnetic field or wind through sound. With no contact there is no mechanical wear in the measurement itself.
O
Oil debris monitoring
Oil debris monitoring is the continuous detection of wear particles in a machine's lubricating oil while it runs. It shows when and how fast wear is developing, so maintenance can be planned from the machine's condition rather than the calendar.
Optical rain gauge
An optical rain gauge detects precipitation by its effect on a beam of light rather than by collecting water in a bucket. With no moving parts, there is nothing to block, stick or wear.
Output rate
Output rate is how often an instrument delivers a new reading, expressed in hertz (readings per second). Control and research applications typically need a faster output rate than general weather reporting.
P
Pedestrian wind comfort
Pedestrian wind comfort describes how wind around buildings affects people at street level. Tall buildings can accelerate wind at their base, so it is assessed in design and measured once a scheme is built.
Performance ratio
Performance ratio compares the energy a solar plant actually generated with the energy it should have generated from the irradiance measured on site. Because irradiance is the denominator, any error in that measurement moves the reported performance of the whole plant.
Pitot tube
A Pitot tube measures air or fluid velocity from the difference between the pressure facing the flow and the static pressure around it. Pitot tubes are common reference instruments in airflow calibration.
Plane of array irradiance
Plane of array irradiance is the solar irradiance measured in the same plane as the solar modules, tilted and oriented as they are, rather than on a horizontal surface.
Position sensor
A position sensor reports where a moving part is, either along a line (linear) or around an axis (rotary). Non-contact position sensors do this without a wiper or brush touching a track, so the measurement does not wear.
Pyranometer
A pyranometer measures solar irradiance arriving on a flat surface from the whole sky above it. It is the standard instrument for measuring global and plane of array irradiance on solar sites and at weather stations.
MaxiMet GMX301 solar radiation sensor
R
Redundant output
A redundant output is a second, independent measurement channel from the same sensor. Comparing the two lets a control system detect a fault, which is why safety-related applications ask for it.
Blade dual output position sensor
Relative humidity
Relative humidity is the amount of water vapour in the air as a percentage of the most the air could hold at the same temperature.
Resolution
Resolution is the smallest change in a measured value that an instrument can report. Resolution is not the same as accuracy: an instrument can report to 0.01 m/s and still be less accurate than that.
RMSE
RMSE, root mean square error, is a way of stating accuracy. It is the square root of the average of the squared differences between an instrument's readings and a reference, so it weights larger errors more heavily than a simple average would.
Full-range accuracy for Gill wind sensors
Rotary position sensor
A rotary position sensor measures the angle of a rotating part, such as a shaft, pedal or valve.
Blade360 rotary position sensor
RS232, RS422 and RS485
RS232, RS422 and RS485 are serial communication standards. RS232 connects two devices over short distances. RS422 and RS485 use differential signalling for longer cable runs and better noise immunity, and RS485 also allows several devices to share one pair of wires.
Sensor outputs and integration
S
SDI-12
SDI-12 is a low-speed serial standard for environmental sensors that lets several addressed sensors share a single cable to a data logger. It is common in hydrology, agriculture and remote monitoring stations.
Sonic temperature
Sonic temperature is an air temperature derived from the speed of sound measured by an ultrasonic anemometer. Because the speed of sound depends on temperature and humidity, sonic temperature is close to, but not the same as, the temperature a thermometer would read.
Turbulence and flux measurement
Spray drift
Spray drift is the movement of crop protection spray away from its target, carried by the wind. Wind speed and direction at the time of spraying decide whether an application is within its permitted window.
Starting threshold
The starting threshold is the lowest wind speed at which a wind sensor begins to register. A mechanical anemometer needs enough wind to overcome friction before it turns; an ultrasonic anemometer has no moving parts to start.
Status code
A status code is a value sent with each reading that tells the receiving system whether the instrument is working correctly. It allows a controller to tell a genuine low or calm reading apart from a fault.
Stow
Stow is the protective position a single-axis solar tracker moves to when wind, hail or snow exceeds a set threshold. The decision is made by the tracker controller from a measured value, so the measurement and its averaging determine when the rows are protected.
T
Three-axis anemometer (3D)
A 3-axis anemometer measures the wind in all three dimensions, including the vertical component. It is used where the full flow matters, such as turbulence, flux and wind resource studies.
Time of flight
Time of flight is the time a sound pulse takes to travel between two points. An ultrasonic anemometer compares the time of flight with and against the wind along each path to calculate wind speed and direction.
How does an ultrasonic wind sensor work?
Tipping bucket rain gauge
A tipping bucket rain gauge measures rainfall by counting the tips of a small see-saw bucket that empties each time it fills with a fixed volume of water. It is simple and widely used, and it has a moving mechanism that needs to stay clean and free.
Optical rain gauge vs tipping bucket
Traceability
Metrological traceability is an unbroken, documented chain of calibrations linking a measurement to a national or international standard, with the uncertainty stated at every step.
True wind
True wind is the wind relative to the earth's surface, with the motion of the measuring platform removed. On a moving vessel it is calculated from the apparent wind, the vessel's speed and its heading.
Turbulence intensity
Turbulence intensity is a measure of how gusty the wind is, calculated as the standard deviation of wind speed divided by the mean wind speed over a set period, usually 10 minutes.
Turbulence and wake measurement
Two-axis anemometer (2D)
A 2-axis anemometer measures the horizontal wind, giving speed and direction across the ground. It is the right choice where the vertical component of the flow does not affect the decision being made.
U
UKAS
UKAS, the United Kingdom Accreditation Service, is the UK's national accreditation body. It assesses calibration and testing laboratories against standards such as ISO/IEC 17025.
UKEX
UKEX is the Great Britain conformity marking scheme for equipment used in potentially explosive atmospheres, introduced alongside UKCA marking.
WindObserver IS for hazardous areas
Ultrasonic anemometer
An ultrasonic anemometer measures wind speed and direction from the time ultrasonic pulses take to travel between pairs of transducers. Wind travelling with a pulse speeds it up and wind against it slows it down, so the difference gives the wind component along that path, with no moving parts involved.
What is an ultrasonic anemometer?
Uncertainty of measurement
Measurement uncertainty is a stated range within which the true value is expected to lie, with a given level of confidence. A calibration certificate reports it alongside each result.
W
Wake
A wake is the region of slower, more turbulent air downwind of a wind turbine or other obstacle. Wakes reduce the energy available to turbines behind and increase the loads on them.
Turbulence and wake measurement
WBGT
WBGT, wet bulb globe temperature, is a heat stress index that combines air temperature, humidity, radiant heat and wind into a single figure used to manage the risk of working or exercising in heat. The method is set out in ISO 7243.
MaxiMet GMX552 heat stress station
Wind direction
Wind direction is reported as the direction the wind is coming from, in degrees measured clockwise from north. A northerly wind blows from the north towards the south.
Wind shear
Wind shear is the change in wind speed or direction with height, or over a short distance. It matters for wind turbine loads, aircraft approaches and the siting of wind sensors.
Turbulence and wake measurement
WMO
The World Meteorological Organization is the United Nations agency for weather, climate and water. Its guidance on instruments and observing methods, including how gusts and averages are calculated, is the common reference for meteorological measurement.
Y
Yaw control
Yaw control turns a wind turbine's nacelle to face the wind. It relies on the wind direction measured at the nacelle, so a measurement error becomes an alignment error and lost yield.
Z
Zone (hazardous area)
Hazardous area zones describe how likely an explosive atmosphere is. For gases and vapours, Zone 0 has one present continuously or for long periods, Zone 1 occasionally in normal operation and Zone 2 rarely and briefly. Zones 20, 21 and 22 are the equivalents for combustible dust.
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