Leak testing is the process by which manufacturers check to see if their product leaks. Parts are typically filled with liquids or gases depending on their final use. Everything leaks to a certain degree. There is no such thing as a “0” leak part. The driving factor for most manufacturers is quality control. Most industries must meet industry specific safety or quality standards in order to sell their products.
Establishing a leak test program can be a daunting task. The most critical information you need to know is your leak tightness specification. What is the “pinhole” defect size that your product can take during normal operation while still performing and meeting safety/industry specifications? What is the maximum leak rate allowed for your part? This will vary depending on the gas or liquid that passes through your part. Once you have this information you can determine which leak test method is best for your application. If you need help determining this you’re welcome to contact one of our application engineers.
During production, air leak testing is cleaner, cheaper and faster. Moreover, flow through capillary defects is viscosity sensitive. Air viscosity is approximately 50 times lower than water, meaning that air leak testing will get 50 times more air flow compared to water flow from the same defects, making air flow more sensitive than liquid flow and the ideal choice for leak testing.
This is the simplest approach to determining leak tightness. The hypothesis is that there is a maximum size for a critical micro-geometry that will not allow fluids (liquid or vapor) to continuously leak out for given operating conditions and fluids. This essentially is the pinhole that allows unwanted leaks to occur, with the leak flow rate exceeding product specifications. The idea is that EMGs will leak the same as the maximum allowed real life leaks as long as their operating conditions are the same.
This is a leak test method independent of defining your leak tightness The EMG is also a great method to validate your production leak test system, or to correlate between different production leak test systems. Controlled EMGs also known as calibrated leaks and can be purchased from ATC here. Review our Equivalent Channels and Equivalent Diameters to learn more.
ATC leak test instruments can detect down to 0.2 micron levels. However, realistically the pinhole that you will need to detect will depend on the liquid or gas within your test part and also your leak requirements.
Common flow is a MACRO (or average) phenomena. Micro-flow is the part of fluid dynamics where the flow is significantly influenced by the molecule transport phenomenon. This typically occurs in flow through micro-channels (like leak flow) or very low flow at vacuum conditions. ATC’s micro-flow sensors are the only existing sensors designed and operating in those micro-flow regimes. Micro-flow sensors’ calibration and operational methods are based on the laws of micro-fluid dynamics.
The biggest differences between ATC’s Micro-Flow Technology (How It Works) and other common methods are that ATC instruments offer:
- Higher sensitivity compared to any other air test methods, similar to tracer gas methods.
- Very repeatable, operator independent test method.
- Simple to use, uses air and is less expensive compared to tracer gas (helium, hydrogen) methods.
- Directly measures the flow through the test part, resulting in a more accurate measurement.
- Stable measurement, environmental temperature changes have little to no effect .
- No drift due to variable helium concentration in the air (or other tracer gases).
- Frequent calibration is not required (we recommend yearly).
- Measurement is not volume sensitive.
There is not a good correlation between the two, as the bubble formation mechanism is dependent on test pressure, your part construction, and defect geometry (water surface tension). One method is to count bubbles and calculate bubble volume time frequency to establish leak rate. Another way is to use a graduated cylinder above a known leak and a time stopper. In either case the estimates or calculations can have a large error, and is only good as a “starting point.”
- Temperature changes air temperature, gas density and pressure based on the “Ideal Gas Law” and “gas density.” Note that temperature is an average temperature measured in absolute units (0.2° F temperature change at 70° F or 530° F is only 0.038% density change). Also note that air is an excellent temperature insulator.
- Temperature affects the leak tightness of some parts. Casting pore sizes increase with temperature, rubber and plastic change properties with temperature, etc.
- Rapid temperature changes cause unit under test volume to change and be unstable and possibly bias leak flow into or from the part.
ATC’s patented Intelligent Gas Leak Sensor (IGLS) is one of ATC’s manufactured micro-flow sensors designed specifically to address the inherent instability associated with leak testing. The inherent design of the sensor makes it pressure and temperature stable. Each sensor is tested to demonstrate stable measurement in temperature ranges of 0° C to 50° C. Furthermore, the design of the sensor is such that only true air flow pulled through the sensor is measured. Temporary pressure fluctuations (including one due to momentary temperature changes during a short test) should not cause the instrument to make false decisions. Consult ATC regarding the optimum instrument and set up for your specific test conditions.
Gauge Pressure: pressure relative to atmospheric condition (psig, Kpa-gauge). Gage pressure units should be marked as psig or Kpa-gage. Many analog gages are not marked that way. Note: If you show “0” pressure on your supply pressure gage, the measurement units are gage pressure.
Absolute Pressure: pressure relative to “0” pressure or absolute vacuum (psia, Kpa-abs). The relation between the two pressure measurement units.
Absolute Pressure = Gage pressure + Your Barometric Pressure (Standard Barometric Pressure is 14.695 psia).
Gage Pressure = Absolute Pressure – Your Barometric Pressure (Standard Barometric Pressure is 14.695 psia).
Like many other instruments dealing with gases using the ideal gas law the pressure sensor is an Absolute Pressure Sensor. The Intelligent Gas Leak/Flow Sensors calculate gage pressure, based on average barometric pressure that the user can set up.
There are two types of SI recognized flow measurement units: mass and volumetric flow units.
Mass Flow is the mass transfer over time. Units: mg/min, microgram/min, gr/hr.
Volumetric Flow is volume change over time. Units: cc/min, cc/hr.
At any flow path of gas (compressible fluid) – mass flow is constant, volume flow is not. Therefore specifying volume flow requires specifying pressure and temperature at the point of reference.
A third type of measurement unit is commonly used with gas flow, known as “Volumetric Flow at Std. Conditions” sometimes called “standard flow” which are confusing measurement units. These measurement units are volumetric flow units with thel letter “S” in front. E.g: scc/min or sccm, scc/sec or sccs, scfm.
Standard Flow is Volumetric Flow corrected to Standard Barometric Conditions yielding the same mass flow for the gas used (must use the same gas). Standard Barometric Conditions refer to standard pressure and temperature (STP) conditions. As there is not one agreed “STP” condition, but many, which can result in significant differences. Using “standard flow” requires specifying the standard pressure and temperature condition (e.g: 14.695 psia, 68° F)
Use the following equation:
Fstd= Volume Flow * (Pactual * Tstd)/(Pstd * Tactual)
Fstd: Flow in sccm (or sccs)
Volume Flow: cc/min (or cc/sec)
Pactual: Actual Pressure as Absolute Pressure units (e.g: psia, Kpa-Absolute)
Pstd: Your standard barometric pressure e.g: 14.695 psia
Tactual: Actual temperature in absolute units (Kelvin or Rankin)
Tstd: Your standard temperature (e.g 428° R =68° F, 293° K=20° C).
For many leak test applications, we assume that Tactual = Tstd and the conversion then becomes:
Fstd = Volume Flow * (Pactual/Pstd)
ATC’s Intelleigent Gas Leak and Flow sensors provides both measurement units, using actual pressure and temperature measurements that are part of our sensor.
Mass flow is constant at a steady state of flow.
ISO/IEC 17025:2005 is the main standard used by testing and calibration laboratories. Many industries require that their measurement instrument supplier be ISO/IEC 17025:2005 certified, to enhance their quality system. ISO/IEC 17025:2005, includes older specifications such as ISO 9000, and is specific to test and measurement, where our measurement capabilities and uncertainties are actually verified by an auditing body such as A2LA. Furthermore, ATC’s ISO/IEC 17025:2005 certification and accreditation by A2LA helps our international customers to meet their measurement traceability requirements. Visit ISO’s website for more information.
Pressure Decay instruments measure air pressure change over test time, while the system stabilizes at or near steady state condition. However, pressure decay is not proportional to defect size, while flow rate is. Since leak testing needs to ensure that defects above certain size are not present, flow measurement is required for consistent correlation to defect size.
The relation between pressure change during test and flow rate are derived from the ideal gas equation. Assuming temperature is consistent (e.g.: 20° C).