Glossary

Accuracy
The degree to which a measured value agrees with the true value. Hydraulic instrument accuracy is commonly expressed as a percentage of full scale or the indicated reading.

Actuator drift
Unwanted movement of a hydraulic cylinder or motor when it should be stationary. Possible causes include internal leakage through control valves, cylinder seals or other components.

Bi-directional flow measurement
The ability to measure flow accurately in both directions through a flow meter. A device that permits reverse flow does not necessarily measure it.

Burst disc
A pressure-sensitive component designed to rupture when a specified pressure is exceeded. Burst discs are used in Webtec's Interpass® safety protection system.

Calibration
Comparing a measuring instrument's readings against a known reference to establish measurement accuracy. Calibration may also include adjustment where supported by the instrument.

Centre condition
The arrangement of open or blocked hydraulic ports when a directional control valve is in its centre position. Different centre conditions produce different circuit behaviours.

Check valve
A hydraulic valve that permits flow in one direction while preventing flow in the opposite direction.

Closed-loop control
A control arrangement that uses feedback from a sensor to adjust an output automatically, helping maintain a desired flow, pressure or other operating parameter.

Commissioning
The process of testing, checking and adjusting a hydraulic system before it is placed into normal operation.

Condition monitoring
Measuring and analysing operating conditions, such as flow, pressure, temperature or contamination, to assess equipment performance and identify deterioration.

Contamination
Unwanted solid particles, water or other materials in hydraulic fluid that can affect component reliability, efficiency and service life.

Data logging
Recording measurement values over time for later analysis. Hydraulic data logging can help identify intermittent faults, compare machine performance and support preventive maintenance.

Detent action
A mechanical arrangement that holds a valve in a selected operating position until the operator deliberately moves it.

Differential pressure
The difference between pressures measured at two points in a hydraulic circuit. It can be used to assess pressure losses or component performance.

Direct-acting flow indicator
A mechanical instrument in which fluid flow operates an internal measuring mechanism to provide a visual flow reading without electrical power.

Directional control valve
A valve that directs hydraulic fluid between different ports to control the movement, direction or stopping of a hydraulic actuator.

Flow indicator
An instrument providing a local visual indication of flow rate, usually using a mechanical dial or graduated scale.

Flow meter
A device that measures the volume of fluid passing through a hydraulic circuit per unit of time.

Flow monitor
A device used to monitor fluid flow during operation. Depending on its design, it may provide visual indication, an electrical signal or an alarm output.

Flow transmitter
A device that converts a measured flow rate into an electrical signal, such as 4–20 mA, for use with monitoring equipment or industrial controllers.

Full-scale accuracy
Measurement accuracy expressed as a percentage of the instrument's specified full-scale range, rather than the actual measured value.

Hydraulic power
The rate at which energy is transferred by pressurised hydraulic fluid. It depends on the relevant pressure difference and volumetric flow rate.

Hydraulic test bench
An installation used to evaluate hydraulic components or complete assemblies under controlled operating conditions.

Hydrostatic transmission
A hydraulic drive system that transfers power from a pump to a hydraulic motor using pressurised fluid.

In-line flow measurement
Measuring fluid flow using an instrument installed directly into the hydraulic circuit.

Internal leakage
Fluid passing through unintended clearances or across sealing surfaces within a hydraulic component. Excessive internal leakage can reduce efficiency or cause actuator drift.

Loading valve
An adjustable restriction used during hydraulic testing to increase circuit pressure progressively and simulate operating loads.

Manifold mounting
Installing hydraulic valves directly onto a manifold containing internal fluid passages, reducing the need for separate hoses and fittings.

Maximum rated pressure
The highest operating pressure specified for a component under defined conditions. Ratings may differ between models, ports and configurations.

Peak pressure
The highest pressure recorded during a measurement period. Monitoring peak pressure can help identify short pressure spikes and transient faults.

Phosphate ester hydraulic fluid
A type of synthetic hydraulic fluid used in certain fire-resistant applications, including aviation and industrial equipment. Compatible materials and seals are required.

Pilot-operated relief valve
A pressure relief valve in which a smaller pilot valve controls the operation of the main valve.

Predictive maintenance
Using equipment condition and performance data to identify deterioration and help determine when maintenance may be required.

Pressure compensation
A hydraulic control principle that helps maintain a selected flow rate despite variations in pressure or load, provided the valve remains within its operating limits.

Pressure decay
A gradual reduction in hydraulic pressure over time. Possible causes include internal valve leakage, actuator seal leakage and other circuit losses.

Pressure drop
The reduction in pressure as fluid passes through a component or restriction. Pressure losses can waste energy and generate heat.

Pressure pulsation
Repeated fluctuations in hydraulic pressure, often associated with pump operation, valve switching or changing loads.

Pressure transducer
A sensor that converts hydraulic pressure into an electrical signal for measurement, display, monitoring or data logging.

Preventive maintenance
Planned inspection, servicing and replacement activities intended to reduce the likelihood of equipment failure.

Priority flow divider
A hydraulic valve that splits incoming flow into a regulated priority flow and excess by-pass flow. Pressure-compensated designs help maintain the selected priority flow as operating conditions change.

Pump P/Q test
A hydraulic pump performance test that records delivered flow (Q) at different pressures (P). The results can help assess pump condition and volumetric efficiency.

Regulated flow
The controlled portion of fluid flow delivered through a flow control valve or priority flow divider.

Repeatability
The ability of a measuring instrument to produce consistent readings when the same measurement is repeated under similar conditions.

Reverse flow
Fluid movement opposite to the normal or designated direction through a hydraulic component.

Rotary shear valve
A directional control valve that uses a rotating element and precision sealing surfaces to direct hydraulic fluid while minimising internal leakage.

Sampling rate
The frequency at which a measuring instrument takes or records readings. A higher sampling rate can help capture rapid changes in system pressure or flow.

Sensor channel
An individual measured variable. A combined pressure and temperature sensor, for example, can provide two measurement channels from one physical device.

Sharp-edge orifice
A precisely formed flow restriction used in some flow-measuring devices. Its geometry helps provide predictable flow measurement characteristics.

Spring-centred valve
A directional control valve designed to return to its centre position when the operator releases the control.

Tank port
A valve connection designated for fluid returning towards the hydraulic reservoir. Its maximum pressure rating may differ from the valve's main working-pressure rating.

Traceable calibration
Calibration supported by a documented chain of comparisons linking the reference measurement to recognised measurement standards.

Turbine flow meter
A device that measures flow using a turbine rotating in the fluid stream. Turbine speed is used to determine the flow rate.

Viscosity
A measure of a fluid's resistance to flow. Changes in hydraulic oil viscosity can affect leakage, pressure drop and flow measurement.

Viscosity stability
The ability of a flow-measuring instrument to maintain consistent measurement performance as fluid viscosity changes.

Volumetric efficiency
A measure of the relationship between actual and theoretical fluid delivery or displacement in a hydraulic pump or motor. It is useful for identifying performance deterioration.

Zero leakage
A condition in which no leakage is detected under specified test conditions. This should not be interpreted as a guarantee under all operating conditions.

Near-zero leakage
A very small amount of measurable internal leakage, typically associated with precision hydraulic sealing arrangements.

Hydraulic systems, components and connections

BP > By-pass
Excess-flow outlet on a priority flow divider.

BSPP > British Standard Pipe Parallel
Parallel pipe threads used for hydraulic connections.

CETOP3 > CETOP size 3
Common hydraulic valve mounting interface, associated with NG06 and D03.

DIN > Deutsches Institut für Normung
German standards organisation.

FSD > Full-Scale Deflection
Reference used for expressing instrument accuracy, especially mechanical indicators.

FS > Full Scale
Maximum specified value of an instrument's measurement range.

HPCO > High Pressure Carry Over
Arrangement that allows pressurised flow to continue to downstream hydraulic valves.

JIC > Joint Industry Council
Common designation for hydraulic connections using 37° flared fittings.

NG06 > Nominal Size 06
Hydraulic valve interface designation commonly associated with CETOP3 and D03.

NPSF > National Pipe Straight Fuel
Straight pipe-thread designation used in some hydraulic connections.

NPT > National Pipe Taper
Tapered pipe-thread designation.

NPTF > National Pipe Taper Fuel
Dryseal tapered pipe-thread designation.

ORB > O-Ring Boss
Hydraulic port connection using an O-ring seal.

P/Q > Pressure / Flow
Used when describing hydraulic pump performance testing at different operating pressures.

REG > Regulated
Port carrying priority or regulated flow from a flow divider.

SAE > SAE International
Engineering standards organisation, originally the Society of Automotive Engineers.

Electronics, data logging and communication

AUX > Auxiliary
Additional input for external sensors or other signals.

CAN > Controller Area Network
Digital communication network used to connect sensors and control equipment.

CANopen > CANopen communication protocol
Standardised communication protocol operating over CAN. Used by compatible hydraulic measuring equipment and sensors.

CSV > Comma-Separated Values
File format used to export measurement data for analysis in spreadsheets and other software.

I/O > Input / Output
Electronic connections used to receive or transmit signals.

IoT > Internet of Things
Connected devices and systems exchanging information for monitoring or control.

J1939 > SAE J1939
CAN-based communications standard commonly used in heavy-duty vehicles and mobile machinery.

LAN > Local Area Network
Network connection used for communication between electronic devices.

PLC > Programmable Logic Controller
Industrial controller used to monitor inputs and operate machinery.

RPM > Revolutions Per Minute
Measurement of rotational speed, such as a pump or hydraulic motor shaft.

SR > Sensor Recognition
Webtec analogue voltage-based sensor interface with automatic sensor recognition.

USB > Universal Serial Bus
Interface for transferring data and supplying power to compatible devices.

VESA > Video Electronics Standards Association
Organisation whose standard mounting patterns are used by some data loggers for fixed installations.

VNC > Virtual Network Computing
Technology enabling the remote viewing and control of compatible electronic devices.

Standards, fluids and materials

ATEX > Atmosphères Explosibles
European regulatory framework relating to equipment used in potentially explosive atmospheres.

EPDM > Ethylene Propylene Diene Monomer
Elastomer used for seals in selected fluid applications.

FKM > Fluoroelastomer
Sealing material commonly used with compatible hydraulic oils.

GSE > Ground Support Equipment
Equipment used to support aircraft operations, maintenance and servicing on the ground.

IP > Ingress Protection
Classification of an enclosure's resistance to the ingress of solid objects and water.

IP65 >Ingress Protection 65
Dust-tight enclosure protected against water jets under specified test conditions.

IP67 > Ingress Protection 67
Dust-tight enclosure protected against temporary immersion under specified test conditions.

IP6K9K > Ingress Protection 6K9K
Protection classification used for equipment designed to withstand demanding dust and high-pressure, high-temperature wash-down conditions.

ISO 11158 > Hydraulic fluid standard
International standard specifying requirements for mineral-oil hydraulic fluids.

ISO 4406 > Hydraulic fluid cleanliness code
International standard used to classify hydraulic fluid cleanliness according to particle contamination levels.

NBR > Nitrile Butadiene Rubber
Elastomer commonly used in seals for compatible hydraulic oils.

R&D > Research and Development
Engineering development, testing and evaluation work.

bar > Bar
Hydraulic pressure

psi > Pounds per square inch
Hydraulic pressure in US customary units

L/min > Litres per minute
Volumetric flow rate

US gpm > US gallons per minute
Volumetric flow rate in US customary units

°C > Degrees Celsius  
Hydraulic fluid and ambient temperature

°F > Degrees Fahrenheit
Temperature in US customary units

cSt > Centistokes
Kinematic viscosity

Hz > Hertz
Signal frequency or rotational measurement

Ms > Milliseconds
Sampling intervals and instrument response times

V d.c. > Volts direct current
Electrical supply or signal voltage

mA > Milliamperes
Electrical current, including analogue sensor signals

4–20 mA > Analogue current signal range
Common industrial signal for transmitting sensor readings

kW > Kilowatts
Hydraulic or mechanical power

µm > Micrometres (microns)
Particle size and filtration

SCFM > Standard cubic feet per minute
Gas flow referenced to specified standard conditions

SLPS > Standard litres per second
Gas flow referenced to specified standard conditions

Full-scale accuracy versus reading accuracy
Full-scale accuracy is expressed relative to the maximum specified measurement range. Reading accuracy is expressed relative to the value currently being measured.

These two specifications are not interchangeable and should be considered when selecting a hydraulic measuring instrument.

Reverse flow versus bi-directional measurement
A flow indicator may permit oil to pass in reverse without measuring the reverse flow rate.

For example, the Webtec RFIK permits reverse flow but its flow indicator does not measure it. The DHM, by comparison, can measure flow in either direction and indicate the flow direction using TrueFlow® technology.


Zero leakage versus near-zero leakage
Zero leakage describes a condition where no leakage is detected under specified test conditions. Near-zero leakage indicates a very small measurable amount.

The Webtec 180 and 280 Series have different leakage performance depending on the selected flow size and configuration.


SR versus CAN sensors
Webtec SR sensors use an analogue voltage-based interface with Sensor Recognition, allowing compatible instruments to recognise the connected sensor.

Webtec CAN sensors transmit digital measurement information using CAN communications. CANopen provides a standardised protocol for compatible sensors, while SAE J1939 is another CAN-based protocol used on heavy-duty machinery.

Different Webtec data loggers support different sensor types, and analogue SR sensors cannot simply be connected to a CAN-only input.


Priority flow versus by-pass flow
A hydraulic priority flow divider directs a controlled quantity of oil to the regulated (REG) outlet.

The remaining available flow leaves through the by-pass (BP) outlet, where it can be returned to the hydraulic reservoir or used to power another function.

Pressure compensation helps maintain the priority flow when outlet pressures change.