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Reading Hydraulic and Pneumatic Schematics (ISO 1219 Symbols)

A hydraulic or pneumatic circuit diagram is the document a project engineer, a commissioning technician and a maintenance crew all read to learn what a machine's fluid power system does, and ISO 1219 is the three-part ISO series that sets its symbols and drawing rules. UTEC Industrial designs, engineers, machines, fabricates, and installs custom material handling systems for aerospace and heavy industry from its Spokane Valley, WA facility, integrating Allen-Bradley PLC and motion control with in-house CNC machining, heat treating, and stress relief. This article explains what each part of ISO 1219 covers, how suppliers describe the symbols for lines, valves, pumps, accumulators and sensors, how components and ports are identified, and what a buyer should require on the diagrams delivered with a machine. Symbols and drawing rules are described in words only; the article reproduces no drawings. The diagram is drawn at design and engineering, checked against the built machine at assembly and commissioning, and used for tuning, monitoring and lockout, along the chain design → engineering → parts machining → fabrication → assembly → weld fatigue → stress relief → drives → controls → tuning → monitoring.

What do the three parts of ISO 1219 cover?​

ISO 1219-1:2012 (Edition 3, 178 pages, ISO/TC 131/SC 1) "establishes basic elements for symbols" and "specifies rules for devising fluid power symbols for use on components and in circuit diagrams." Its abstract adds that it "is a collective application standard of the ISO 14617 series," and that its symbols "are designed in fixed dimensions to be used directly in data processing systems, which might result in different variants." It has one amendment, ISO 1219-1:2012/Amd 1:2016, which ISO sells separately and describes as "not included in the text of the existing standard." The catalogue says the 2012 edition was last confirmed in 2017 and remains current, and it shows the document at stage 90.60, under review, as of 2026-10-01.

ISO 1219-2:2012 (Edition 2, 42 pages) "establishes the main rules for drawing hydraulic and pneumatic circuit diagrams using graphical symbols drawn in accordance with ISO 1219-1." It "also applies to circuit diagrams relating to cooling systems, lubrication systems, cooling lubricant systems and systems of technical gases used in conjunction with fluid power applications." Its abstract on iso.org calls the document "ISO 1219-2:2011," while the designation and publication date (2012-09) are 2012; this article cites 2012.

ISO 1219-3:2016 (Edition 1, 23 pages) "supplements ISO 1219-1 and ISO 1219-2 by specifying rules for the generation and combination of symbols of connectable components in circuit diagrams," and states that "Using these symbols, the design envelope and piping can be reduced." Amendment 1:2016 to Part 1 adds symbols for double-acting cylinders with a mechanism that locks the piston rod in a predefined position or in any position, and for devices that brake the piston rod and release it by pressurization (ISO 1219-1:2012, Amd 1:2016; ISO 1219-2:2012; ISO 1219-3:2016).

What is a fluid power symbol meant to show, and what does it leave out?​

Bosch Rexroth's training book, Hydraulics – Basic Principles (2022), describes graphic symbols, "also referred to as switching symbols," as "used for the abstract illustration of fluid power components and their functions, as well as to explain hydraulic circuit systems." They "consist of one or more basic element(s) and generally of one or more functional symbol(s)," and "Structural details of components are not taken into account in the symbolic illustration."

Rexroth names where symbols appear and what they are for: "mainly used in circuit diagrams, product catalogs and operating instructions," and "an important aid in project planning, assembly, commissioning, inspection, troubleshooting and maintenance of hydraulic systems." Its chapter on circuits repeats the limit: symbols "do not provide information about the design structure of the components." For training, Rexroth shows the same circuit a second time as a circuit schematic in which "The abstract graphic symbols have been replaced by cross sections of the components," a form it says is "mainly used in basic and advanced training."

As engineering reasoning, the failure mode to avoid is reading the diagram as a layout: a symbol says what a valve does in each position, not where it sits on the manifold or how long the hose to the cylinder is. Those answers come from the parts list, the manifold drawing and the valve data sheet (Bosch Rexroth 2022, §1.3.1 p. 18 and §4.1.1 pp. 297–299).

How are lines, connections and energy sources told apart on a circuit diagram?​

Rexroth's list of example symbols "according to the standard DIN ISO 1219 (Part1 ...)," which it offers as a "working aid for assignment of graphic symbols and reading or creating of hydraulic circuit diagrams," separates these line and connection elements: "Main line (rigid)," "Control or leakage oil line," "Hose line (flexible)," "Line crossing (lines not connected)," "Line connection," "Tank" and "Tank with line." It also lists "Energy source, hydraulic" and "Energy source, pneumatic" as separate symbols, and a quick-release coupling with and without a non-return valve. The book names DIN ISO 1219 without an edition.

The practical consequence of the line types, as engineering reasoning, is that a reader can tell a working line that carries load pressure from a pilot or drain line that does not, and a crossing from a junction, before tracing any flow path.

ISO 1219-1:2012 itself draws working lines solid, and pilot, control and drain lines dashed. It encloses valves in squares or rectangles and pumps and motors in circles. It marks hydraulic energy with a filled triangle and pneumatic energy with a hollow one: a filled triangle pointing out of a circle is a hydraulic pump, and one pointing in is a hydraulic motor (Bosch Rexroth 2022, §1.3.2 p. 18 and §1.3.3 p. 19; ISO 1219-1:2012).

How is a directional valve symbol read?​

Rexroth's valve list names each symbol by its function. Examples from the list are "2/2 directional control valve, normally open, solenoid operation, spring return," "3/2 directional control valve, normally closed, solenoid operation, spring return," and "4/3 directional control valve, direct solenoid operation, spring centering of center position." Under ISO 1219-1, the first of the paired figures in these names counts the working ports and the second the switching positions, so a 4/3 valve has four ports and three positions. The symbol has one square per switching position, and the external lines and port designations attach to the square for the normal (initial) position.

Rexroth's open-loop circuit on p. 300 shows a cylinder controlled by an electrically operated 4/3 valve in its center position, and states: "ports P and T are connected to each other in center position (zero position). This is also called neutral circulation, and such a directional control valve is called an unloading valve." The pump then "only has to build up the low pressure to overcome the valve and line resistances."

Port letters come from the valve makers. Moog's D633/D634 catalog labels its hydraulic symbols with ports P, T, A and B and a leakage port Y. In 4-way operation the valves "can be used to control the flow in ports A and B"; "Port B must be closed in order to obtain 3-way operation"; and "Leakage port Y must be used if the pressure in tank port T exceeds a value of 50 bar (725 psi)." The catalog also shows 2-way and 2x2-way operation, and notes: "The specified flow directions must be observed" (Bosch Rexroth 2022, §1.3.4 pp. 23–24 and §4.2.1 p. 300; Moog CDL 59872-en Rev. D, 2024, p. 11; ISO 1219-1:2012).

How are proportional and servo valves shown on a schematic?​

Rexroth's list separates the operators by name. "Operation by means of solenoid coil with two counteracting windings" is listed separately from the same operator "continuously adjustable," and the valve entries include "4/3 proportional directional control valve, direct solenoid operation, spring centering of center position," a pilot-operated "4/3 proportional directional control valve ... with position control of main and pilot control stages, with integrated electronics," and "4/3-way servo valve, pilot-operated, pilot control stage with electrical operation, with mechanical feedback of the control spool position, with integrated electronics."

Moog's symbol page carries two further details: the D633/D634 valves "are available with zero lap, less than 3 % or 10 % positive overlap," and separate 3-way and 4-way symbols are shown "with Fail-safe Option F." Moog describes these valves as "Direct Drive Valves (DDV) equipped with integrated electronics and closed-loop position control of the spool."

As engineering reasoning, the symbol identifies a valve as continuously adjustable, with or without spool-position feedback and on-board electronics; it does not give hysteresis, response time or required oil cleanliness, which the proportional vs. servo valve article reads from the data sheet. ISO 1219-1 marks a continuously adjustable valve with two parallel lines along the top and bottom of its position boxes and its proportional solenoids with a diagonal arrow through the operator, and it shows spool-position feedback as a small square measuring block attached to the valve's actuation section (Bosch Rexroth 2022, §1.3.4 pp. 22 and 24; Moog CDL 59872-en Rev. D, 2024, p. 3 and p. 11; ISO 1219-1:2012).

How are the pump, actuators, filters and accumulators named on the diagram?​

Rexroth's list names pumps and motors by their flow and rotation: a "Fixed displacement pump" with one flow direction and one direction of rotation, a "Variable displacement pump" with two flow directions, one direction of rotation and a "drain port," a variable displacement pump "with pressure controller (pilot-operated)," and one "with combined pressure/flow controller"; pump control, including load sensing, is covered in the hydraulic efficiency article. Cylinders are listed as single-acting with spring return, double-acting with a piston rod on one side, and single- and double-acting telescopic.

Filters and accumulators each have several named forms: "Filters, general," "Filter with bypass valve," "Filter with visual clogging indicator (pressure difference-based)," and piston-, bladder- and diaphragm-type accumulators. Rexroth's example circuit on p. 298 numbers nine components: tank, fixed displacement pump, non-return valve, return filter, manometer, pressure relief valve, directional control valve, throttle/non-return valve and hydraulic cylinder.

The accumulator entry that matters at lockout is "Accumulator with accumulator shut-off block," whose block Rexroth lists with a "system-shut-off valve," a "pressure relief valve (safety valve)," a "manual relief" and a "pressure measuring instrument (manometer)." HYDAC's accumulator instructions describe its safety and shut-off block as designed to incorporate a pressure measurement device, pressure relief device, shut-off device, "Bleed down device (manual or electric operation)," and a locking device (Bosch Rexroth 2022, §1.3.3 pp. 19–21 and §4.1.1 p. 298; HYDAC PN#02068196, 2011, §4.1 p. 4).

How are components and ports identified, and how are valve stacks drawn?​

Rexroth's circuit chapter states that "Marking of components, compact assemblies and connection designations facilitates the assignment of the elements of the circuit diagram to the parts lists and conditions of the real hydraulic assembly," and that "it is possible to enter required physical characteristic values at relevant positions of the circuit diagram." It also states that circuit diagrams "are a suitable means of avoiding ambiguities and errors during the planning phase, production, assembly and maintenance of the hydraulic system by means of a standardized illustration of the hydraulic conditions."

ISO 1219-2:2012 sets the rules for this on a drawing. Its component identification code (cl. 5.1) has four elements: an installation designation, followed by a hyphen and omitted where the diagram covers a single system; a medium code letter; a circuit number; and, after a full stop, a component number that runs in sequence within the circuit. Ports are identified with letters and numbers that match the markings on the hardware, such as P for the pressure supply, T for the tank, A and B for the working ports, and X and Y for pilot and external drain lines (cl. 5.2). The circuit is drawn in its initial or rest position, with power and control signals off, and a parts list that cross-references every component identifier on the diagram is required.

Valve stacks and manifold-mounted assemblies are where ISO 1219-3:2016 applies, with rules "for the generation and combination of symbols of connectable components in circuit diagrams." The abstract names no assembly type. The standard encloses a combined symbol group, such as a manifold block or a stacked valve assembly, in a narrow chain line (long dash and dot) that marks the module boundary. As engineering reasoning, a reader checking a manifold against its diagram should expect each module's symbol to carry a marking that matches the parts list, which is the assignment Rexroth describes (Bosch Rexroth 2022, §4.1.1 p. 297; ISO 1219-2:2012, cl. 5.1 and 5.2; ISO 1219-3:2016).

What does a schematic tell a maintenance crew at lockout?​

OSHA's lockout rule requires that, following the application of lockout or tagout devices, "all potentially hazardous stored or residual energy shall be relieved, disconnected, restrained, and otherwise rendered safe." It defines an energy isolating device to include "a line valve; a block," and "Energized" as "Connected to an energy source or containing residual or stored energy."

As engineering reasoning, the circuit diagram is the map of those devices: it shows where a shut-off valve can isolate the pump from the circuit, where an accumulator and its shut-off block sit, and where a non-return or load-holding valve can keep pressure trapped in a cylinder after the pump stops. The stored energy and LOTO article covers the discharge, isolation and gravity-load steps themselves.

ISO 4413:2010 requires the information for use to include a circuit diagram to ISO 1219-2 (Clause 7). The diagram and its technical data must show component and port identification, working pressures and pressure-valve settings, the precharge pressure and volume of each accumulator, actuator and motor data, and line sizes and the recommended fluid. At standard level, ISO 4413:2010 "specifies general rules and safety requirements for hydraulic fluid power systems and components used on machinery as defined by ISO 12100" (OSHA 29 CFR 1910.147-1989, (b) and (d)(5)(i); ISO 4413:2010, Clause 7; ISO 1219-2:2012).

What changes on a pneumatic circuit diagram?​

The series covers both media. ISO 1219-2:2012 sets the main rules for drawing "hydraulic and pneumatic circuit diagrams," and Part 1's symbols are "fluid power symbols." Rexroth's list, written for hydraulics, still carries "Energy source, pneumatic" and "Pneumatic operation (single-stage)" as separate entries.

The system boundary differs. ISO 4414:2010, whose abstract follows the same structure as ISO 4413's for pneumatic systems, "specifies general rules and safety requirements for pneumatic fluid power systems and components used on machinery as defined by ISO 12100," and it "does not apply to air compressors and the systems associated with air distribution as typically installed in a factory, including gas bottles and receivers." As engineering reasoning, a machine's pneumatic diagram can therefore start at the machine's supply connection, and the air-preparation equipment upstream of it belongs to plant documents; the cold, dusty mills pneumatics article covers that air preparation.

Pneumatic-specific symbol detail, such as exhaust and silencer symbols and filter-regulator-lubricator units, is outside the sources read for this article. ISO 4414:2010 names ISO 1219-2 for the pneumatic circuit diagram in the information for use (ISO 1219-2:2012; ISO 1219-1:2012; ISO 4414:2010, cl. 7.2; Bosch Rexroth 2022, §1.3.3 p. 19 and §1.3.4 p. 22).

Which sensors and controls appear on a fluid power schematic, and how do they map to the PLC?​

Rexroth's list includes the instruments a control system reads: "Pressure sensor, output signal analog," "Pressure switch, electro-mechanical, adjustable," "Pressure switch, electronically adjustable, output signal switching," a pressure differential gauge, a fluid level indicator (sight glass), a thermometer, a thermostat, a flow indicator, a flow meter, a tachometer and a torque measuring instrument. Its operator entries include solenoids with one winding and with two counteracting windings, and valves "with integrated electronics."

Moog's D633/D634 valves "offer analog interfaces for command signal and spool position feedback," and its D633K–D635K instructions add a condition the diagram should carry: "Drain port Y must be used with 3- and 4-way operation and pT > 50 bar or with 2x2-way operation."

As engineering reasoning, each analog sensor, switch, solenoid and valve electronics symbol on the hydraulic diagram is an I/O point on the electrical drawings, and a commissioning check compares the two drawing sets tag by tag before the first pressure test. UTEC Industrial builds that layer on Allen-Bradley ControlLogix and CompactLogix controllers, with VFD and servo drives over EtherNet/IP, in control panels built to UL 508A (Bosch Rexroth 2022, §1.3.3 p. 21 and §1.3.4 pp. 22–24; Moog CDL 59872-en Rev. D, 2024, p. 3; Moog CA49304-001 Rev. G, 2011, p. 7).

Where does the circuit diagram sit in the chain from design to monitoring?​

Rexroth places symbols and diagrams across the life of the system: graphic symbols, which it says are "mainly used in circuit diagrams, product catalogs and operating instructions," are "an important aid in project planning, assembly, commissioning, inspection, troubleshooting and maintenance," and circuit diagrams are "a suitable means of avoiding ambiguities and errors during the planning phase, production, assembly and maintenance."

As engineering reasoning, each link of the chain uses the diagram differently:

  • Design and engineering: the diagram is the first statement of the circuit, before manifolds and tubes are drawn.
  • Parts machining and fabrication: manifold ports and tube runs are machined and fabricated to the connection designations on the diagram.
  • Assembly: component markings on the diagram are matched to the parts list and to the assembled hardware.
  • Drives and controls: sensor, switch and valve symbols become I/O points and PLC tags.
  • Tuning and commissioning: pressure settings entered on the diagram are set and recorded.
  • Monitoring and maintenance: the as-built diagram is the reference for troubleshooting and for lockout.

UTEC Industrial performs FAT (factory acceptance testing) and on-site commissioning, the two points at which a diagram is checked against the built machine (Bosch Rexroth 2022, §1.3.1 p. 18 and §4.1.1 p. 297).

What should a buyer require on the circuit diagrams delivered with a machine?​

Drawn from the sources above, a purchase specification should ask for:

  • Named editions: symbols to ISO 1219-1:2012 with Amd 1:2016, diagrams to ISO 1219-2:2012, and symbol modules to ISO 1219-3:2016 where valve stacks or manifolds are used.
  • Both media: a hydraulic diagram and a pneumatic diagram, the pneumatic one starting at the machine's supply connection.
  • Identification: component and port markings that match the parts list and the hardware, which is the assignment Rexroth describes.
  • Values on the drawing: pressure settings and other "required physical characteristic values," such as relief settings, accumulator precharge and filter ratings.
  • Valve conditions: port letters as the valve maker uses them (P, T, A, B, Y for Moog), and conditions such as Moog's drain-port rule above 50 bar at T.
  • Stored energy: every accumulator, its shut-off block and its bleed-down device shown, as HYDAC's instructions list them.
  • As-built status: a revision issued after commissioning.

Of these items, ISO 4413:2010 and ISO 4414:2010 both require identification, values on the drawing, valve conditions, stored energy and as-built status in the information for use; the named editions and the two-media item come from neither standard and are engineering reasoning (ISO 1219-1:2012; ISO 1219-2:2012; ISO 1219-3:2016; Bosch Rexroth 2022, §4.1.1 p. 297; Moog CDL 59872-en Rev. D, 2024, p. 11; HYDAC PN#02068196, 2011, §4.1 p. 4; ISO 4413:2010; ISO 4414:2010).

Related Articles

References​

  • ISO 1219-1:2012: Fluid power systems and components — Graphical symbols and circuit diagrams — Part 1: Graphical symbols for conventional use and data-processing applications. International Organization for Standardization, 2012; amended by ISO 1219-1:2012/Amd 1:2016.
  • ISO 1219-2:2012: Fluid power systems and components — Graphical symbols and circuit diagrams — Part 2: Circuit diagrams. International Organization for Standardization, 2012.
  • ISO 1219-3:2016: Fluid power systems and components — Graphical symbols and circuit diagrams — Part 3: Symbol modules and connected symbols in circuit diagrams. International Organization for Standardization, 2016.
  • Bosch Rexroth AG. Knowledge in Detail: Hydraulics – Basic Principles, 1st ed. Bosch Rexroth AG, Bosch Rexroth Academy, 2022. ISBN 978-3-9820731-5-6.
  • Moog CDL 59872-en Rev. D: Direct Drive Analog Control Servo Valves D633 Size 03, D634 Size 05. Moog Inc., 2024.
  • Moog CA49304-001 Rev. G: Operating Instructions for Proportional Valves D633K, D634K and D635K Series. Moog Inc., 2011.
  • HYDAC PN#02068196: Accumulators Operating and Installation Instructions. HYDAC Corporation, 2011.
  • OSHA 29 CFR 1910.147-1989: The Control of Hazardous Energy (Lockout/Tagout). Occupational Safety and Health Administration, 1989.
  • ISO 4413:2010: Hydraulic fluid power — General rules and safety requirements for systems and their components. International Organization for Standardization, 2010.
  • ISO 4414:2010: Pneumatic fluid power — General rules and safety requirements for systems and their components. International Organization for Standardization, 2010.

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