Leak Testing & Dispensing Information & Best Practices
What is a leak test?
A leak test is used to determine if an object, product, or system functions within a specified leak limit. Leaks occur when gas or liquid flow through an object via an imperfection or manufacturing defect such as holes, cracks, weak seals, etc. Leaks always flow from higher pressure to lower pressure; leak testers use pressure to generate and monitor that flow.
Leak Rate Calculator
A Leak rate is expressed as a volume per unit time. The rate is found by measuring the change in pressure multiplied by the volume and dividing that by the change in time multiplied by surrounding atmospheric pressure.
atm = Atmospheric pressure (psia)
V = Test volume (cm3)
Δp = The decay in pressure during test time
Δt = The amount of decay time (min.)
sccm = Standard Cubic Centimeters per Minute
Leak rate = .02psi/0.05min * 50cm3/14.7psia
Leak rate = 0.4 * 3.401
Leak rate = 1.36 sccm
Leak Rate CalculatorVolume:
Leak Test Types
Pressure decay is one of the most widely used methods of leak testing in manufacturing and is ideal for small, sealed components with a single access port. In this test, a product is attached to a leak tester and filled with air. Once pressurized, the air source is valved off and the pressure is allowed to settle. During the test any decrease in air pressure over time signifies a leak.
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Pressure Decay Test Diagram:
The Mass Flow test measures flow rate through an object. Once the object is attached to the test port it is pressurized with regulated air. The flow is then measured with a Mass Flow Sensor. Objects that have a flow rate that falls between the max flow value and the min flow value pass while those that do not, fail. Mass flow is normally used with parts that have two ports open to atmosphere.
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Mass Flow Test Diagram:
Vacuum decay tests are the inverse of the same principle, simply creating a negative pressure instead of a positive pressure.
Vacuum Decay Test Diagram:
Measures the passage of air through an object and signifies if the object’s passage is blocked.
Occlusion Test Diagram:
An occlusion test which includes a downstream release for more precise occlusion test results.
Downstream Occlusion Test Diagram:
A destructive pressure test that measures the maximum pressure at catastrophic failure.
Burst Test Diagram:
Similar to a burst but more sensitive, often used to detect events such as a valve opening.
Crack Test Diagram:
Used to find leaks in sealed packaging or devices that do not include an opening for filling.
Chamber Test Diagram:
Ideal Gas Law
During leak testing, changes in pressure in the test part are of primary interest. When a test part is filled with air (or any other gas), it initially expands inside the part to occupy its volume. When the part finally reaches the test pressure, the air inside contracts. This rapid expansion and contraction of air changes its temperature and volume. The test part also undergoes slight changes in temperature and volume because of changes in temperature and volume of the air.
In order to detect the leakage, the changes in pressure must be taken into consideration. The changes in pressure can occur due to changes in temperature, volume, and number of moles of the gas. Since parameters like pressure, volume, and amount of air are involved in this process, it is governed by the ideal gas law. Mathematically, the law is expressed as:
P = Pressure of the air (or gas) enclosed in the container
V = Volume of the container occupied by the gas
n = Number of moles of the gas
T = Absolute temperature of the gas
R = Ideal gas constant with a value of 0.0821 dm3 atm K-1 mol-1
Water to Air Correlation
Testing Sealed Components
To leak test a sealed component, a chamber test is used. Place the part in a custom build nest and seal the chamber closed. Fill the chamber with a metered volume to a desired pressure. Then proceed with a standard pressure decay test. Any drop in pressure beyond a designated limit signifies a leak into the sealed part under test.
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Metering Pump Technology
Delivery of a fluid in precise and adjustable flow rates is called metering. A metering pump is a device used to control the flow rate of a fluid. It is used to move a precise volume of liquid in a specified amount of time to provide an accurate volumetric flow rate. Typically, water, chemicals, solutions, and other fluids are moved by a metering pump. The flow rate depends on the outlet pressure, and is usually constant over time. These pumps have been designed to deliver a maximum discharge pressure so the selection of a suitable pump depends on the type of application. A vast majority of metering pumps can be categorized as positive displacement pumps.
Positive displacement pumps
Positive displacement pumps move the fluid by repeatedly enclosing its fixed volume and then moving it mechanically through the system. The fluid moving mechanism (pump action) can be executed through gears, pistons, plungers, rollers, screws, vanes, and diaphragms. The majority of positive displacement pumps can be categorized either as reciprocating type or rotary type.
Cavitation & Water Hammering
While handling pumping systems, it is critical to have a background understanding of some of the important concepts that govern pump performance, efficiency, output, and longevity. The following resource aims to discuss the concepts of Newtonian/Non-Newtonian fluids, water hammering, cavitation in pumps, and how these factors impact modern pumping systems.
Newtonian and Non-Newtonian fluids
In fluid pumping, it is fairly important to understand the properties of the fluids being pumped, transferred, and mixed. This understanding plays a major role in selecting suitable equipment for fluid pumping. Fluid viscosity or thickness determines the way a particular fluid will behave in a pump. When it comes to fluid viscosity, fluids show a non-uniform pattern. Some fluids maintain a constant viscosity while others show a significant change in viscosity as temperature and applied forces change.
In general, fluids can be divided into two broad categories. Newtonian fluids and Non-Newtonian fluids. Non-Newtonian fluids have further subcategories (more on that later). Pump manufacturers and end-users must have a clear understanding of these fluid types and their behavior. This plays a vital role while selecting and operating pumps.