ISO/DIS 4021

ISO/TC 131/SC 6

Secretariat: BSI

Date: 2026-01-22

Hydraulic fluid power — Particulate contamination analysis — Extraction of fluid samples from lines of an operating system

Transmissions hydrauliques — Analyse de la pollution par particules — Prélèvement des échantillons de fluide dans les circuits en fonctionnement

DIS stage

© ISO 2026

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Contents

Foreword iv

Introduction v

1 Scope 1

2 Normative references 1

3 Terms and definitions 1

4 Sampling point 2

4.1 General 2

4.2 Sampling locations 2

5 Supplies and materials 3

5.1 Sample bottles 3

5.2 Solvent 3

6 Sampling frequency 3

6.1 General 3

6.2 Sampling timing 4

7 Sampling principles 4

7.1 Sampling from fluid lines 4

7.2 Sampling from reservoirs 8

8 Sampling procedures 9

8.1 Offline analysis sampling procedure 9

8.2 Online analysis sampling procedure 10

9 Labelling 12

10 Identification statement 12

11 References 12

Annex A (informative) Calculation of Reynolds number, Re 14

Foreword

ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies (ISO member bodies). The work of preparing International Standards is normally carried out through ISO technical committees. Each member body interested in a subject for which a technical committee has been established has the right to be represented on that committee. International organizations, governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.

The procedures used to develop this document and those intended for its further maintenance are described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types of ISO documents should be noted. This document was drafted in accordance with the editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).

Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of any patent rights identified during the development of the document will be in the Introduction and/or on the ISO list of patent declarations received (see www.iso.org/patents).

Any trade name used in this document is information given for the convenience of users and does not constitute an endorsement.

For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions related to conformity assessment, as well as information about ISO's adherence to the World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/iso/foreword.html.

This document was prepared by Technical Committee ISO/TC 131, Fluid power systems, Subcommittee SC 6, Contamination control.

This third edition cancels and replaces the second edition (ISO 4021:1992), which has been technically revised.

The main changes are as follows:

• Terms and definitions of clean sample bottle, fluid sampling, reservoir and fluid sampling, line have been deleted;
• Sampling point has been added;
• Sample bottles have been added;
• Sampling frequency has been added;
• One recommended bypass circuit is added for the principle of sampling from fluid lines;
• The principle of extracting from the reservoir has been revised, and the method of extracting fluid sample through the shut-off valve installed on the reservoir has been added;
• Online analysis sampling procedure has been added.

Any feedback or questions on this document should be directed to the user’s national standards body. A complete listing of these bodies can be found at www.iso.org/members.html.

Introduction

In hydraulic fluid power systems, power is transmitted through the fluid under pressure within a closed circuit. The fluid is both a lubricant and power-transmitting medium. The presence of solid particulate contamination in the fluid interferes with the ability of the hydraulic fluid to lubricate and causes wear to the components. The extent of this form of contamination in the fluid has a direct bearing on the performance and reliability of the system and it is necessary that this be controlled to levels that are considered appropriate for the system concerned. Hydraulic filters are used to control the amount of particulate contamination to a level that is suitable for both the contaminant sensitivity of the system and the level of reliability required by the users.

Operators and manufacturers of hydraulic equipment have defined maximum particle concentration levels for components, systems and processes, beyond which corrective actions are implemented to normalize the levels. These are often referred to as the required cleanliness level (RCL). The cleanliness level is obtained by sampling the hydraulic fluid and measuring the particulate contamination level. If the level is above the RCL, then corrective actions are necessary to restore the situation. To avoid taking unnecessary actions, which can often prove costly, precision in sampling and measuring the particulate contamination level is required.

Hydraulic fluid power — Particulate contamination analysis — Extraction of fluid samples from lines of an operating system

This document specifies procedures for extracting fluid samples from a hydraulic fluid power system under operation.

The preferred method is to extract fluid samples from a main flowline of an operating hydraulic system in such manner that the particulate contaminant in the sample is representative of the fluid flowing at the point of sampling.

An alternative method is to extract a sample from the reservoir of an operating hydraulic system if a suitable sampling device is not fitted.

This document can be used for offline and online sampling. The samples taken are used for particulate contamination analysis.

The following documents are referred to in the text in such a way that some or all of their content constitutes requirements of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

ISO 3722, Hydraulic fluid power — Fluid sample containers — Qualifying and controlling cleaning methods

ISO 5598, Fluid power systems and components — Vocabulary

ISO 11171, Hydraulic fluid power — Calibration of automatic particle counters for liquids

ISO 11943, Hydraulic fluid power — Online automatic particle-counting systems for liquids — Methods of calibration and validation

ISO 21018-1, Hydraulic fluid power — Monitoring the level of particulate contamination of the fluid — Part 1: General principles

For the purposes of this document, the terms and definitions given in ISO 5598 and the following apply.

ISO and IEC maintain terminology databases for use in standardization at the following addresses:

• ISO Online browsing platform: available at https://www.iso.org/obp
• IEC Electropedia: available at https://www.electropedia.org/

sampler

device for extracting a representative sample from a larger source

turbulent flow

fluid flow in which particle movement, anywhere in the flow, varies rapidly in velocity and direction

Note 1 to entry: Flow can be turbulent when the Reynolds number (R_e) is greater than 2 300 and is usually assumed to be turbulent when R_e≥ 4 000. See Annex A.

Sampling points depend on the purpose of extracting fluid samples.

Sampling points shall be chosen to collect representative samples.

Sampling points shall be selected in an easy-to-operate area for online analysis or extraction of fluid samples.

For the purpose of extracting fluid samples, the user may take samples from one or more locations of the hydraulic system. Sampling locations for hydraulic systems include, but are not limited to:

• Upstream of the return line filter. The fluid sample taken at this location can characterize the contamination level of the hydraulic system and usually used to diagnose its operating condition. This location can be preferred if the user is interested to determine the contamination level of the hydraulic system.
• Downstream of the pressure line filter. The fluid sample taken at this location can characterize the contamination level of fluid entering to servo valves, proportional valves and actuators of the hydraulic system. This location can be preferred if it is not convenient for sampling from upstream of the return line filter or the user is interested to determine the performance of the pressure line filter.
• Upstream of the pressure line filter. The fluid sample taken at this location can indirectly reflect the contamination level of the fluid in the reservoir and the contamination status of hydraulic pump. This location can be considered if the user wants to know the contamination level of the fluid in the reservoir but can’t extract samples from reservoir directly.
• Downstream of the return line filter. The fluid sample taken at this location can characterize the contamination level of fluid returned to the reservoir and indirectly reflect the effectiveness of the return line filter settings. This location can be considered if the user wants to know the contamination level of fluid returned to the reservoir and/or the performance of the return line filter.
• The case drains of hydraulic pump/motor. The fluid sample taken at this location can characterize the contamination level of the hydraulic pump/motor. Due to particles produced by pump/motor wear, the case drain of the hydraulic pump/motor may be the most contaminated part of the hydraulic system. This sample point is not recommended as the flow is not turbulent and the contaminant can sediment. But this location can be considered if the user wants to know the wear condition of the hydraulic pump/motor.
• The reservoir. If direct sampling from the lines of the hydraulic system is not possible, users can consider sampling from the reservoir. When sampling from the reservoir, the static area of the fluid shall be avoided. The hydraulic system shall be operated for a sufficient duration to create a homogeneous mixture.

NOTE 1 Contaminants external to the hydraulic system during sampling can adversely impact sample quality. Potential sources of external contamination include from hydraulic lines/fittings of the sampler and the environment.

NOTE 2 For a hydraulic system with a permanent or temporary bypass filter, the contamination level of fluid extracted permanently from the reservoir can be influenced by the performance of the temporary offline filter.

The material of sample bottles shall be chemically compatible with fluid samples, and suitable for the temperature range of fluid samples. It should be colourless and transparent to enable viewing of the position and condition of the fluid sample. Materials that tend to generate particles in fluid samples shall not be used.

NOTE Plastic type bottles have static electricity effects which can cause particles to stick to the insides of containers particularly opposite fingerprints.

The inner surface and opening of sample bottles shall be smooth, with rounded corners at the bottom to prevent the solid particle retention. Bottles with wide mouth and flat bottom should be used for easy cleaning. Bottles with ground glass shall not be used.

The mouth of the sample bottle shall have seals for transport and prevention of secondary contamination. The sealing system should be easy to clean and shall not have any under cuts. A sealed cap shall be used, which can be either a threaded type without an inner plug or a cap with an inner sealing gasket.

The specifications of sample bottles shall be consistent with the sampler type of the measuring instrument used and the volume of the fluid sample to be tested, and the sample bottle volume should not be less than 100 mL.

Sample bottles should be cleaned and verified in accordance with ISO 3722. The sample bottles should be as clean as possible so as not to introduce secondary contamination into the sample.

The solvent shall be compatible with the sample and have a contamination level lower than 14/12/9 (see ). For mineral oil-based samples, petroleum ether is advisable. For water-based samples, isopropyl alcohol is recommended.

WARNING — Petroleum ether and isopropyl alcohol are hazardous. The user shall establish appropriate safeguards per the Safety Data Sheets and local requirements.

Sampling should  be performed regularly by the operator to determine the particulate contamination level of the hydraulic system. Sampling intervals can be optimized by taking into account the following hydraulic system and application-specific conditions:

1. Criticality: safety, environmental, downtime, repair and business interruption failure consequences;
2. Operational and fluid environment conditions: these influence frequency and rate of hydraulic system and fluid failure. They include pressure, load, temperature, speed, contaminant ingression rate, wear rate and duty cycle severity;
3. Hydraulic fluid age: problems often occur immediately after hydraulic fluid is serviced (drains, refills and top-ups) due to contamination by incorrect and/or incompatible fluid. Hydraulic fluid approaching the end of their useful life are also at high risk due to depleted additives, incipient oxidation and high levels of various contaminants;
4. Hydraulic system age and maintenance factors: the chances of failure for most hydraulic systems are greater during break-in and after major repairs, rebuilds or extended downtime; or hydraulic system modifications. Risks can also increase as a hydraulic system approaches the end of its expected life;
5. Failure behaviour: known and expected failure modes;
6. Hydraulic system condition: whenever abnormal condition reports are received; sampling frequency should be increased to improve diagnosis and prognosis confidence.

Immediate sampling to determine the particle contamination level of a hydraulic system is recommended at the following scenarios:

1. after initial flushing during manufacture;
2. at the time of delivery;
3. if any abnormal events occur during operation, e.g., abnormal increases, decreases or fluctuations of the pressure;
4. when system critical components or parts are replaced;
5. in the event of a failure.

Extract samples from main fluid lines in a section where turbulent flow conditions exist, using a line sampler having the following characteristics (see example in Figure 1):

1. being compatible with the fluid and the system operating pressure;
2. permitting on/off valving of sample flow;
3. having the ability to reduce the pressure value from the system pressure to atmosphere pressure at a minimum flow rate of 100 mL/min (preferably 500 mL/min) in the open position; the pressure­-reducing device shall not alter the contamination level or distribution;
4. having a sampling tube with internal diameter in the range from 1.2 mm to 5.0 mm;

NOTE The smallest possible diameter to achieve the necessary flow is desirable as a large diameter with low flow rate will cause sedimentation.

1. having an extraction point which is located in a turbulent flow zone, if this cannot be assured, use a means of creating turbulence, such as a turbulent flow inducer;
2. being compatible with the sampling procedure and particulate contamination analysis apparatus to be used;
3. giving repeatable and reproducible samples;
4. being easy to use and leak-free;
5. being so constructed that areas where particulate contamination may settle out, when the valve is not in use, are minimized; being so designed to minimize the generation of contaminants by the valve itself; being of a type which cleans itself by flushing.

Dimensions in millimetres

Key

1 sampling point

2 male quick-disconnect coupling with a check valve

3 dust caps

4 female quick-disconnect coupling with non-check valve (if used)

5 shut-off valve

6 seat cap

d main fluid line diameter

Figure 1 — Typical example of a line sampler

Ensure that the extraction point is clear of the boundary layer of the system pipe work and the axis of the sampler is approximately perpendicular to the main flow stream, preferably entering the system pipework from the top. Arrange the point of exit of the sampled fluid such that the flow is directed vertically downwards.

Permanently attach the valve, or check valve portion of the quick-disconnect coupling, to the port through which the sample is to be taken.

Provide dust caps for the item in 7.1.3 to reduce the ingress of environmental contaminants.

Operate the hydraulic circuit to diffuse particle contaminants as evenly as possible throughout the system. If a sample representing normal operating conditions is desired, the hydraulic circuits should not be operated for prolonged periods in an artificially clean environment.

NOTE 1 It has been found that 30 minutes is typically sufficient to ensure contaminants are homogeneously mixed within a hydraulic system, but this could vary depending on the system.

NOTE 2 When a procedure to diffuse particulate contaminants has been established for a particular system, that procedure can be maintained for all similar systems.

Ensure the sampler is in the wide-open position when sampling and that it will provide a flow rate of approximately 500 mL/min (minimum 100 mL/min). Depending on the system pressure and valve size, it is necessary to attach a length of small-bore tubing at the shut-off valve outlet in order to reduce flow rate. Do not use tubing having an inside diameter smaller than 1.2 mm.

During online analysis sampling, due to specification of the measuring instrument, the sampling flow rate can be lower than 100 mL/min. Meanwhile, if the sampling pipeline is longer than 1m, particles can be trapped in the sampling pipeline, leading to lower particle concentration in samples extracted compared to the hydraulic system. To address this issue, a bypass circuit with a throttle valve is recommended to maintain a flow rate of more than 100 mL/min in the sampling pipeline before entering the measuring instrument. The recommended bypass circuit is illustrated in Figure 2.

Key

1 measuring instrument

2 throttle valve

3 reservoir

Figure 2 — A recommended bypass circuit for online analysis

Locate the sampling valve in an area where it is readily accessible and away from sources of environmental contamination.

WARNING — Fluid sampling from high-pressure lines can be dangerous and should be performed only by experienced personnel. If skin is penetrated by fluid whilst taking a sample, see a physician immediately; failure to do so may result in serious harm.

If a sampler cannot be fitted directly to the system, then samples may be extracted from the system reservoir. However, extreme care should be taken to avoid adding further contamination at the sampling point. Samples extracted by this means are less representative of the system contamination than samples extracted by means of live dynamic sampling.

Extract the sample from a centralized area where the fluid is in motion and away from quiescent areas caused by corners or baffles.

Select a convenient opening in the reservoir, above the fluid level, through which the sampler can enter. Determine the distance around h/2 (as shown in Figure 3) to establish the depth of the sampling point below the fluid surface. Place a reference mark on the sampler to indicate the surface of the reservoir at point of entry.

Key

1 flexible tubing

2 source of reduced pressure

3 special cap to suit fluid being sampled

4 sample bottle

5 weighted plug (if necessary)

6 reservoir

h fluid depth

Figure 3 — Typical example of a reservoir vacuum sampler

Carefully select the method of extracting the sample and ensure that the environmental contamination is kept to a minimum.

A reservoir vacuum sampler is shown in Figure 3. This consists of a bottle cap with special fittings for drawing fluid samples into the sample bottle or measuring instrument using a vacuum source. Lengths of laboratory grade flexible tubing, compatible with fluid being sampled, are also required.

A reservoir shut-off valve sampler is shown in Figure 4. This consists of a shut-off valve attached to the reservoir with a sampling line for drawing fluid samples into sample bottle or measuring instrument by atmospheric pressure.

Key

1 sampling line

2 shut-off valve

3 reservoir

l length of the sampling line, should be between 50 mm and 200 mm

d depth from the sampling line to the bottom of the reservoir, should be at least 50 mm

a cutting angle of the sampling line, 30° is recommended

Figure 4 — Typical example of a reservoir shut-off valve sampler

Two bottles are required for sampling with a reservoir vacuum sampler. Bottle A is a flushing container, and can be reused, so cleanliness is not necessary. Bottle B is used to contain the sample, and shall meet the cleanliness requirements of 5.1.5.

Operate the hydraulic circuits to diffuse the particulate contaminants as evenly as possible throughout the reservoir. If a sample representing normal operating conditions is desired, the hydraulic circuits should not be operated for prolonged periods in an artificially clean environment.

NOTE 1 It has been found that 30 minutes is typically sufficient to ensure contaminants are homogeneously mixed within the reservoir, but this could vary depending on the system.

NOTE 2 When a procedure to diffuse particulate contaminants has been established for a particular system, that procedure can be maintained for all similar systems.

Clean the external surfaces of the sampler and the sampling area with filtered clean solvent. A supply of filtered solvent will assist in this. The sampler may be cleaned in the laboratory in advance, as it is more difficult to handle in the field.

Where a sampler incorporating a quick-disconnect coupling is used, attach the separable portions to the permanently attached portion after removing the dust cap.

Open the sampler and flush through the valve with a sufficient quantity (normally at least 500 mL of fluid but not less than five times the total volume of the sampler), collect in a separate container and discard. Do not close the valve after flushing.

Uncap the precleaned sample bottle, place the sample bottle under the emerging jet of fluid and fill the bottle to about 75 % of its total volume. Do not allow the sampler to come into contact with the sample bottle. Cap shall be handled in a manner that avoids contamination during sampling.

When sufficient volume has been collected, remove the sample bottle, close the valve and replace the cap immediately.

It is acceptable to use proprietary sample containers of the type which do not require the bottle cap to be removed, for example, a sample bottle similar to the one in Figure 3 with a special cap. If these are employed, it is necessary to allow the sample valve to come into contact with the sample bottle inlet tube. Take great care to avoid exterior contamination of the sample by this action. The shut-off valve shall not be adjusted during sampling.

Where a sampler incorporating a quick-disconnect coupling is used, disconnect the separate portions of the sampler and remove any residual fluid films by flushing with a suitable solvent.

Replace the dust cap on the permanently mounted section of the valve and restore the system to its original state.

Select a suitable area of the reservoir from which the sample is to be extracted (see 7.2). Clean the area around the entry point before breaking into the reservoir.

Using an arrangement as shown in Figure 3, draw about 200 mL of prefiltered solvent through the sampling line to bottle A using a vacuum source.

Remove bottle A from the special cap of the sampling apparatus and discard the solvent. Reattach bottle A to the special cap and insert the sampling tube into the desired area of the reservoir.

Draw approximately 500 mL of fluid (but not less than five times the total volume of the sampler) through the tubing and discard the fluid.

Remove the cap from the precleaned sample bottle B and attach the bottle to the special cap of the sampling apparatus. Draw off sufficient volume to fill the sample bottle to about 75 % of its total volume using the volume source. Cap shall be prevented from contamination during sampling.

Remove sample bottle B from the special cap and immediately seal with its original cap. Reconnect sample bottle A to the special cap and withdraw the sample tube from the reservoir.

Fit the closure to the reservoir and restore the system to its original state.

For a reservoir shut-off valve sampler, directly sample from the shut-off valve mounted on the reservoir according to the procedure of 8.1.1.3 to 8.1.1.5 using the sampler shown in Figure 4.

Clean the external surface of the sampler and the sampling area with filtered clean solvent. A supply of filtered solvent will assist in this. The sampler may be cleaned in the laboratory in advance, as it is more difficult to handle in the field.

Where a sampler incorporating a quick-disconnect coupling is used, attach the separable portions to the permanently attached portion after removing the dust cap.

Verify that the system pressure and fluid temperature at the shut-off valve outlet fall within the allowable range of the measuring instrument. Otherwise, pressure-reducing and cooling devices shall be used and shall not alter the contamination level or distribution.

NOTE 1 If the system pressure at the sampling point of the measuring instrument is lower than 350 kPa, air can be sucked in during the online analysis, resulting in inconspicuous bubbles, that could cause particle counting errors.

NOTE 2 When the temperature of the fluid sample analyzed by the measuring instrument is higher than 65℃ for an extended period, it can cause temperature drift in the sensor during online analysis, leading to particle counting errors.

Ensure that the fluid to be analyzed is compatible with the measuring instrument used. Otherwise, the measuring instrument shall be replaced.

Connect the measuring instrument, calibrated according to ISO 11171 or ISO 11943, to the sampler according to the manufacturer's instructions. If necessary, the measuring instrument can be connected with a flow control device.

Open the shut-off valve of the sampler, and adjust the flow control device or check the measuring instrument to ensure that the sampling flow rate meets the working flow rate of the measuring instrument.

Before online analysis, flush the entire sampling pipelines with at least 500 mL of fluid or at least five times the total volume of the entire sampling pipelines. Collect and discard the flush in a separate container. Do not close the valve after flushing.

Online analysis shall be performed according to the instructions in the measuring instrument manufacturer's manual and the requirements of ISO 21018-1 . The shut-off valve shall not be adjusted during online analysis sampling.

NOTE 1 During online analysis it is often impossible to observe the state of the analysed fluid sample and hence measurements are susceptible to fluid sample states of air bubbles, moisture and two-phase liquids.

NOTE 2 Because of the flow rate limit of the measuring instrument, for online analysis methods, the length, inner diameter, and configuration of the sampling pipeline can be selected to avoid large particles being trapped in the sampling pipeline.

Close the shut-off valve, remove the flow control device and measuring instrument after the online analysis. Restore the system to its original state.

Select a suitable area of the reservoir from which the sample is to be extracted (see 7.2). Clean the area around the entry point before breaking into the reservoir.

NOTE Sampling from reservoirs is the least favored option because it has great potential for errors.

Samples should be extracted from the location where the fluid is in motion.

Ensure that the fluid to be analyzed is compatible with the measuring instrument used. Otherwise, the measuring instrument shall be replaced.

Clean the external surface of the sampling pipeline inlet of the measuring instrument with filtered solvent.

For the reservoir vacuum sampling method, fix the sampling pipeline inlet of the measuring instrument into the selected sampling position of the reservoirs. For the reservoir shut-off valve sampler (see Figure 4), connect the sampling pipeline inlet of the measuring instrument to the shut-off valve on the reservoirs, and then connect the sampling pipeline outlet to a container and fix it.

Operate the measuring instrument in the suction analysis mode to confirm whether the sampling flow rate and working conditions are normal.

NOTE For suction analysis, the requirement to transfer the fluid sample from the reservoir to the sensor (e.g. using an internal pump) can introduce errors. If it is necessary for the pump to lift the fluid to the instrument, a negative pressure (vacuum) is generated, and this can draw air out from solution or through fittings. The presence of air in the fluid being analyzed can interfere with instrument operation and cause errors. Additional errors can be introduced if the pump is positioned upstream of the sensor, where it can generate particles during operation and thus provide unrepresentative test data.

Flush the entire sampling pipelines with at least 500 mL of fluid, or at least five times the total volume of the entire sampling pipelines. Collect and discard in a separate container.

Online analysis shall be performed according to the instructions in the measuring instrument manufacturer's manual and the requirements of ISO 21018-1 . For a reservoir shut-off valve sampler, the shut-off valve shall not be adjusted during online analysis sampling.

For a reservoir vacuum sampler, remove the sampling pipelines from the reservoir and the measuring instrument after the online analysis. For a reservoir shut-off valve sampler, close the shut-off valve and remove the measuring instrument after the online analysis.

Cover the reservoir and restore the system to its original state.

The sample bottle shall be provided with a label bearing the following information, as appropriate:

1. sample reference number;
2. date and time of sample;
3. operator;
4. system reference;
5. fluid type, fluid temperature and (if known) the flow rate;
6. sampling location and pressure used.

Use the following statement in tests reports, catalogues and sales literature when electing to comply with ISO 4021:

‘The sampling method conforms to ISO 4021:XXXX, Hydraulic fluid power — Particulate contamination analysis — Extraction of fluid samples from lines of an operating system.’

[1] ISO 5884, Aerospace series — Fluid systems and components — Methods for system sampling and measuring the solid particle contamination in hydraulic fluids

[2] ISO 11500, Hydraulic fluid power — Determination of the particulate contamination level of a liquid sample by automatic particle counting using the light-extinction principle

[3] ISO 14830-1, Condition monitoring and diagnostics of machine systems — Tribology-based monitoring and diagnostics — Part 1: General requirements and guidelines

[4] ISO 21018-4, Hydraulic fluid power — Monitoring the level of particulate contamination in the fluid — Part 4: Use of the light extinction technique

[5] ISO 28523, Ships and marine technology — Lubricating and hydraulic oil systems — Guidance for sampling to determine cleanliness and particle contamination

[6] IEC 60300-3-11, Dependability management — Part 3-11: Application guide — Reliability centred maintenance

[7] SAE ARP5376C, Methods, Locations and Criteria for System Sampling and Measuring the Solid Particle Contamination of Hydraulic Fluids

1. 
   (informative)
   
   Calculation of Reynolds number, Re

(A.1)

where

D is the pipe diameter, in meters;

V is the fluid velocity, in meters per second;

ρ is the desity of fluid, in kilograms per cubic meter;

μ is the dynamic viscosity, in Pascal second;

ν is the kinematic viscosity, in square meters per second.