Three Reasons To Ditch Dunk Testing

Why You Should Stop Dunking And Consider Pressure Decay or Mass Flow Leak Testing For More Accurate and Reliable Results

What is Dunk Testing?

The dunk test is undeniably the simplest of leak tests. The dunk test is performed by connecting a part to an an air supply, then held underwater while the operator looks for bubbles. The disadvantages of this leak test method are well known and technology has introduced non-destructive and more objective tests. As a result, this test is becoming less common, although it is still frequently found in the packaging industry. In this article we’ll explain the dunk test and why you should consider pressure decay or mass flow leak testing to achieve more accurate and reliable results.

“x Bubbles Per Minute” Leak Specification

Variations of the dunk test exist that include using media other than water, such as ethylene glycol or mineral oil, and a heated fluid bath to create pressure differential inside a sealed component. These methods are described in ASTM E515 Standard Test Method for Leaks Using Bubble Emission Techniques.

Even today, we occasionally run across somebody who has been dunk tank testing and has a leak specification defined as “x bubbles per minute”. While this this is one way to subjectively compare leaks, it is far from accurate and reliable for the following reasons:

1. What size are the bubbles?
2. What if they were not counted correctly?
3. Add to that the problem of drying parts, possible future corrosion or damage, disposal of contaminated water and the hazard of slipping on spilled water,

It’s crystal clear that dunk testing has very limited appeal in today’s modern manufacturing industry.

Pressure Decay & Mass Flow Testing Options

Pressure decay leak testers use a pressure sensor to measure a change in pressure and the process of converting that pressure change to quantify a leak rate is not always straightforward. One method requires users to know the volume of the part and then, if possible, program that value into the leak tester so that the instrument can perform the calculation to convert pressure change over time to a volumetric leak rate. Another is to teach the tester what a good parts pressure change and a known leak’s pressure change are.

One way around these procedures is to use a mass flow tester – volume is not important to this type of tester, it simply displays a flow rate in the user’s selected engineering units.

Mass flow leak testing relies upon the molecular measurement of the gas flow rate, measured in units of mass per unit time, such as kilograms per hour. However, gas flow is usually measured and recognized in volumetric terms such as cc/min, normal temperature and pressure. So we just need to convert the mass measurement to a flow rate. The calculation is discussed in an earlier post Leak Testing for the Masses.

How The Mass Flow Leak Testing Technique Works

The mass flow technique relies upon the molecular measurement of the gas flow rate. This is usually measured in units of mass per unit time, such as kilograms per hour. However, gas flow is usually measured and recognized in volumetric terms such as cc/min, normal temperature and pressure. It is possible to take an exact mass flow, and reference it to more traditional measures, without losing accuracy. To convert mass flow to volumetric units it is necessary to use the following relationship.

Q = m/(P)S where

(P)S = density of the gas at standard conditions

m = mass flow rate

Q = standard flow rate (e.g. scc/m)

Electronic mass flow meters employ thermodynamic principles to measure the true mass flow rate, without the need for ; temperature or pressure compensation.

The majority of instrumentation mass flow sensors are of the heated tube type. In these units, all or part of the gas flow passes through a precision sensing tube.

A leak in the test component will result in a flow of air through the mass flow device from the reservoir. The mass flow sensor detection principle is based upon the transfer of heat in an air stream. Air flows across two temperature sensitive resistors, which are separated by a heated element. A sensor measures the difference in the air temperature recorded across the two resistors. A temperature difference results in an output voltage proportional to temperature. The amount of air that flows through the mass flow sensor is usually measured directly in cubic centimeters per minute.

A constant heat rate enters the gas flowing through the tube via the 2 externally wound resistance temperature detectors (RTD’s). When the gas stream flows through the upstream sensor winding (T1) a certain amount of heat is carried away. This process is repeated at the downstream sensor winding (T2). Less heat is transferred from the downstream sensor due to the flow being preheated by the first sensor. The temperature difference between the two sensors is measured. Since the temperature difference between the two sensors is directly proportional to the mass flow, m, of the gas, a flow measurement is obtained.

Uson Offers Mass Flow & Pressure Decay Leak Testing Solutions

Uson’s application note Quantified Leak Rate Using Mass Flow AN-9 describes how the Sprint iQ leak tester can be used in this manner. Other models such as Qualitek mR, Optima vT and Vector can also perform the mass flow test described. If you have questions about Mass Flow or Pressure Decay testing, or need guidance with finding the best testing solution for your application, Uson can help. Contact us and our team of leak testing experts will guide you every step of the way.