June 2018 - Issue 205
e-news for Flow Control Solutions


D2P Marlborough
Marlborough, MA
September 26 & 27
Royal Plaza Trade Center
Booth #TBD
More Info
How does a check valve work?

Smart Products Catalog PDF

O-ring Basics

We have many customers that have ordered check valves from us for years, but there are folks new to the low flow, low pressure world calling us every day with questions about combinations of components for our valves. What plastic is best for my applications? What o-ring seals best? How much pressure can the valve withstand? These are some common questions we receive, but the majority have to do with the characteristics and capabilities of o-rings. So, we thought we would give a primer on what we know about o-rings in the use of our check valves based on what we have learned, as well as knowledge gained from our o-ring suppliers.
An o-ring is a torus, which is defined in geometry as a surface of revolution generated by revolving a circle in three-dimensional space about an axis coplanar with the circle. If the axis of revolution does not touch the circle, the surface has a ring shape and is called a torus of revolution.


Relevant dimensions are the Inside Diameter (ID) and Width (W). Width is commonly referred to as the cross-section. The Outer Diameter (OD) is normally a reference dimension. O-Ring sizes and tolerances are defined by AS568, published by the SAE (Society of Automotive Engineers) with the purpose to standardize o-ring sizes and dimensions.

The AS568 specification is essentially broken up by cross section, and there are six series types to include:

  • 000 Series:  .070 Cross-section

  • 100 Series:  .103 Cross-section

  • 200 Series:  .139 Cross-section

  • 300 series:  .210 Cross-section

  • 400 Series:  .275 Cross-section

  • 900 Series:  o-rings for straight thread tube fittings

As the AS568 dash number gets larger in each respective series, the o-ring ID increases.  There are usually no changes to the cross-section within a series with two noted exceptions.  These are:

  • 000 Series:  The cross section increase from .040 to .070 as you move from a -001 to a -004 size o-ring

  • 900 Series:  Each size can have a different cross-section since it is intended for a different sized fitting, but it will generally increase as the dash number grows.

Some o-ring suppliers may offer and stock extended AS568 sizes.  For example, a supplier could offer an o-ring in a -424 size.  These sizes are called extended because they are not covered by the AS568 specification.  Should you require an extended size, please check with the supplier first to confirm tolerances on the ID and cross section.
When tolerancing o-ring dimensions, it’s important to understand that only two of the three relevant dimensions can be assigned a tolerance. This is because tolerancing all three could create a condition known as over tolerancing, and it can cause inspection issues.  For example, if the ID, OD, and CS are toleranced, it is possible for both the ID and the OD to be within spec, but the cross-section to be out of specification. Depending on the application, a designer may instead choose to make the ID or OD a reference dimension to eliminate this as a possibility and to focus on what is critical for the application.  

Metric o-ring sizes are not as well defined as the AS568 imperial sizes.  Often, individual countries will have their own o-rings sizing specifications, and they may be unique, or they might be metric versions of the AS568 sized parts. The most commonly stocked metric sizes found at o-ring suppliers are typically Japanese Industrial Sizes (JIS), metrics with either millimeter or half a millimeter sizing on ID and cross sections, or sizes for universal metric fitting.

We’ll discuss more about o-rings in our next issue.

NPT Pressure Relief Valves: Model #103 and Model #109

Looking for a pressure relief valve? Our best example of a true relief valve can be found in Models #103 (pictured above in polycarbonate) and #109 (pictured in Ultem® and Nylon). Both are available as a 1/8" Male NPT style. These valves are easily placed into plastic, metal (shown), or other materials with absolutely no tubing involved. A lot of customers simply place this valve on the exterior of a tank, bladder, or other device. When pressure builds to a designated PSI level, this spring-loaded valve responds and allows flow of liquids/gasses slowly out of the application to prevent blow ups or damage. 
Like all our valves, Models #103 and #109 are customized to fit your unique design criteria by plastic body material, o-ring material, and spring pressure. Specifically, these models are available to be molded into Polypropylene, Nylon, Polycarbonate, and Ultem®. The Model #109 is also available in Acetal. O-ring options include Buna-N, EPDM, Viton®, Silicone, and Fluorosilicone. Opening PSI levels for these valves range from 0.09 to 20 PSI.
To avoid damaging the #103 and #109 valves, we recommend you use a 7/16" wrench or socket with very low torque or manually twisting it into place. Once installed, the valve's low profile limits the possibility of it getting hit or caught on external objects.
For questions or to begin building your customized #103 or #109, contact a Product & Application Specialist at (800) 338-0404 or sales@smartproducts.com.

To view a PDF of our entire product catalog, click here.