Density is a relative concept and requirements for density need to be clearly defined. Requirements that are formulated as ”waterproof” or ”tightly” have practically no significance as they are too vague. A complete leak specification should instead be formulated such as ”Maximum permissible leakage is 30 mm3 free air per second at 2 bar pressure and a temperature of 20°C”. That is, contain background information in the form of:
- What medium it is carried out in (e.g. air)
- Which test pressure that is used (e.g. 2 bar over pressure)
- The temperature at which the testing is done (often 20 ° C)
- Maximum leak amount per unit time (e.g. 30 mm3/s)
Once a requirement of this type is created, a monitoring device with the right sensitivity for the requirement have to be chosen. If a too rough method is chosen, the instrument can show that the object is close to unacceptable demands, causing increased complaints from the customer as a result. A too sensitive method, on the other hand, gives rise to unnecessary and costly adjustments and rejections of products.
When one is about to specify how much an item can leak at a certain pressure, experience of what has already worked well as a reasonable leak rate is very valuable. Factors such as finance, risk/safety and quality standards do also affect which leak rate that should be chosen as well as the appearance and characteristics of the product. Depending on the circumstances, these factors get different priorities and affect the leak rate to varying degrees. Below are some important points to consider when determining a leak rate:
- Consider if any durability properties of the object can be affected by the leak. Pores can often weaken some materials. If it is so, then the leak specification is also controlling the quality of the process.
- In the leak specification the medium, temperature, maximum leakage and pressure must be included. Efforts should be made for using the same pressure as the actual operating pressure in the test. However, it is important that the test pressure is maintained within the levels that the Work Environment Authority prescribes.
- The leakage rate in the leak specification should be based on the object’s material and on the medium, pressure and temperature to which the item will be subjected to in the final use.
- Many new leak specifications are developed based on previous experience, combined with the quality policy. Advantageously, the requirement can be generated through dialogue between the designer, manufacturer and assembly personnel. A good method is to carry out sharp tests to identify the correct leak rate. Test a variety of products, log readings, fill with the correct media and test whether it is tight enough.
- Consider whether the object has some special characteristics that must be considered when leak testing. These characteristics should be stated in the drawing. Examples of special properties are if there are any claims for purity- and/or dry conditions on the test environment, if the seal/gasket has characteristics which require pressure difference in a particular direction, if there is a maximum allowable pressure or a minimum necessary pressure etcetera.
- Note that it is important that the leak specification is added by article and not for composite articles.
- It may be that different leak specifications must be applied for the same article because of various pressures, temperatures or when exposed to different media (internal leakage) etcetera.
- If possible, do not specify a too strict or stringent leak specification in relation to what the function requires. This can result in an unnecessary number of items discarded, and thus that the costs are increasing.
- There may be an economic and/or production benefit in evaluating and correcting the settled leak specification according to experiences from the start of production. If necessary, one should modify a settled leak specification based on experience of the real operating conditions.
Typical Test Pressures, Leak Limits & Test Methods for Common Components
|Product / component||Test Pressure||Leak||Test Method|
|Water leaks / Cooling systems||1 to 2 bar||4 to 7 cc/min||Air pressure decay|
|Oil leakage(various)||0.1 to 7 bar||6 to 15 cc/min||Air pressure decay/ flow|
|Fuel / Diesel||1 to 10 bar||0.1 to 15 cc/min||Air pressure decay|
|Fuel / Petrol||1 to 5 bar||3.0 to 15 cc/min||Air pressure decay|
|Refrigeration / Air conditioning||2 to 20 bar||5 to 15 gm/yr||Tracer gas|
|Electrical housing / connectors||0.1 to 1 bar||0.01 to 1 cc/min||Pressure decay/ Tracer|