Leak Detection through Pressure Decay

Leak Detection through Pressure Decay 

The Differential Advantage 
There are many products today that must be tested for leaks to ensure the highest quality. Products such as valves, containers, storage tanks, radiators and drums are some examples. 
There are also processes such as welding, bonding or gasket that require leak testing. 

Pressure decay is one of the most common methods for detecting leaks. While helium and ion leak detection methods are effective, they are more time consuming and require expensive equipment. Pressure decay utilizing a pressure sensor works well because of its simplicity. By pressurizing the test piece and then isolating it from the pressure source, any subsequent loss of pressure can most likely be attributed to a leak in the product.

There are two methods that can be used to measure pressure decay.

Method 1:

Diagram 1 shows a plastic container being tested utilizing a gage type transducer. If the container is pressurized to 100 psi and then the valve is closed, the transducer will sense a leak/decay in pressure in the container, if given enough time. Small leaks can be detected in this manner, although the process can be time consuming. If the transducer’s total error band due to nonlinearity, hysteresis, repeatability, etc. is 1%, then a leak cannot be accurately detected until the pressure drops to less than 99 PSI (100 PSI – 1% TTL Error band). Very small leaks could take several minutes to detect. 

Method 2: 

Using a differential pressure transmitter allows for a more sensitive leak detector. At the start of this test (Diagram 2), valves #1 and #2 are open. The supply pressure is applied to the test system, both sides of the transmitter and the container are pressurized to 100 PSI. Valve #2 is then closed. 

The output from the transmitter is zero because the line pressure is at 100 PSI on both sides of the sensor. Valve #1 is then closed. If a leak is present in the container, the low side of the transmitter will begin to lose pressure. If a 0-5” WC transmitter is used, and we assume a 1% total error band, a leak will be detected when the pressure decays a minimum of 1% of 5” WC. This amounts to a drop of 0.05” WC. 

As you can see in the first example you have to wait until the pressure decays 1 PSI or 27.68” W.C. Using the differential system produces faster results, at least 500 times faster, decreasing the total test time required.  

Not every leak test application requires the rapid response time or sensitivity a differential transmitter can offer.

Pressure Sensors for Leak Detection 

Feb 9, 2016 

Different methodologies exist for creating leak detection systems. In this blog post, we will focus on leak detection using a pressure decay method which is ideal for detecting very small leak rates.

The diagram below illustrates a classic leak detection setup using pressure decay.

The test part is pressurized from a pressure source. At the first X under "Balance" in the chart above, the pressure setpoint is reached and balance is maintained until the leak detection test begins at the second X. The regulator and valve isolate the test part from the source once pressurization is complete. The pressure sensor monitors and measures the pressurized test part. If the pressure measurement decreases, then air is leaking out of the test part. Measuring the change in pressure over a period of time allows the user to calculate the leak rate.

The availability of easy-to-use, sensitive pressure sensors at low cost lets designers optimize their leak detection design. The pressure transmitter  41X or the Digital Record manometer LEORECORD CAPO is an example of a pressure sensor well-suited for leak detection applications. 
The 41X is a calibrated and temperature-compensated pressure sensor with ranges from 0-25 kPa up to 0-1000 kPa. Not only does this sensor have high accuracy of ±1.5% full-scale, the 41X has built-in threshold detection (consider how easy it is to set the threshold for the start of the leak test). The AP version has a dual-in-line package while the AG version is surface-mount.  The 41X provides has an excellent price-performance ratio another alternative could be the digital manometer LEO RECORD CAPO from KELLER.

Applications for leak detection include:

• Medical devices such as drug delivery, catheters, valve, oxygen delivery
• HVAC including refrigerant testing, heat exchangers, tubing, radiators, and more
• Automotive testing such as emission systems, fuels, pumps, and cooling systems
• Various industrial applications

Contacto : 
Tecmer,Lda 
Tel: +351 932308640 /+ 351 252317314
Email:geral@tecmer.pt