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ID, OD and Dash size


  • What is dash size?

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Dash size is the common method used to refer to the diameter of a hose or tube in 1/16″ increments.

For hoses this is a value for Inside Diameter, or I.D. For example, -6 hose would indicate a hose with inside diameter of 6/16″ – or 3/8″.

For tubes this is a value for Outside Diameter, or O.D. For example, a -10 tube would indicate a tube with an outside diameter of 10/16″ – or 5/8″.


By the way, dash sizes do not correspond to the exact hose inside dimension. Actual ID’s are smaller.

There are couple examples how to mark piping size at the schematic using dash size:



So, if you found at the drawing, for example, (-6) size, this means:

  • For Hose: Internal Diameter is approx. 3/8″
  • For Tube: Outside Diameter is 3/8″, therefore an Internal Diameter will be much smaller.

This is why sizes of tube and pipe have to be selected accurately, especially if you are going to connect them (for example, trough bulkhead fitting). Otherwise, you can get different flow rate and extra pressure drop in the line with smaller size.

Mass and Weight


  • Difference between Mass and Weight
  • Difference between lb, lbf, lbs, lbm

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To calculate cylinder forces we have to operate with mass and weight.

Mass is a fundamental property of the object, a measure of the amount of matter in the object. The “kilogram” (kg) is the SI unit of mass and it the almost universally used standard mass unit. The “pound” (lbm or simply lb) is the unit of mass in the imperial system.  In all scientific work is strongly recommended instead “pound” (lb) use “pound mass” (lbm).

Usually, people write lb when they are talking of one pound or a single pound and write lbs to indicate the fact that they are talking about many pounds. Thus, they use lbs as a plural for lb that stands for a pound. But lb is the correct abbreviation to be used both as singular as well as plural.

If an object has a mass of 1 kg on the Earth, it would have a mass of 1 kg on the Moon, even though it would weigh only one-sixth as much.

1 lbm = 0.45359237 kg

There is also a unit of mass called the slug, defined as the mass which exerts a force of 32.174 049 lbs under the gravitational acceleration at the earth’s surface:


1 slug = 32.174049 lbm

But even in US the use of exclusively SI units for all scientific work is strongly encouraged.

Weight is the force on the object, caused by the gravity and may be calculated as the mass times the acceleration of gravity:

W = F = mg

Acceleration of gravity at the Earth’s surface: g=9.81 m/s2 (approx. 32.174 ft/s2)

The Newton (N) is the SI unit of weight. In the US the pound force unit, abbreviated lbf, is a unit of weight:

1 lbf = 1 slug × 1 ft/s2 = 32.17405 lbm × 1 ft/s2 = 32.17405 lbm × ft / s2

1 lbf = 0.453 592 37 kg × 9.80665 m/s2 = 4.44822162 N

The oil filtration rate selection


  • Fluid Cleanliness Standards
  • Recommendations for cleanliness class for your system
  • Recommendations for Locations of Filters at your system

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There are a lot of different standards for the classification of solid particle contamination in lubrication and hydraulic fluids: ISO 4406/1999, NAS 1638, SAE AS4059, GOST 17216-2001, etc.

To determine the cleanliness level by  ISO 4406/1999 the solid particles present in 100 ml fluid are counted, sorted according to size & quantity and classified into particle ranges, defined by two (Microscopic counting) or three (automatic particle counter) numbers divided by slashes:

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