In this first part of a series of blogs discussing High Voltage Leak Detection (HVLD), I will provide a general overview of one of the more popular HVLD units, an EScan 625 manufactured by PTI Inspection Systems of Tuckahoe, NY. In addition to an instrument overview, this blog will include a description of method parameters for one of the most frequently tested package systems, a 1mL prefilled syringe.
The electrical conductivity leak test, also called high voltage leak detection (HVLD), or “the spark test”, is an approach for detecting leak presence and potentially the location of a leak(s) in the wall of a nonporous package, rigid or flexible, containing liquid product. The principle of leak detection is based on quantitative electrical conductance measurements. The presence of a leak path in the proximity of electrically conductive liquid results in a drop in test sample electrical resistance, as evidenced by a spike in current above a predetermined pass/fail limit. This highly sensitive method even works to detect package defects clogged by product formulation proteins or salts. Stability studies by clients have supported the use of this technology for nondestructive leak testing a variety of product formulation types. This should be considered, however, on a product-by-product basis.
For the EScan 625, there are a total number of 6 system parameters, which are accessed and adjusted through the use of the instrument’s integrated software. Each of theses parameters plays a key role throughout method development, which, when optimized, result in the ability to differentiate between non-leaking and leaking packages. For the purpose of this overview, a more intuitive understanding of these parameters will be gained by categorizing them into geometric parameters applied voltage parameters.
Parameters from a geometric perspective control the movement of the package and the instrument probes during a test run. The set of parameters that dictate the movement of the probes are referred to as the “Scanning Profile”. Each probe independently moves along their unique track through the use of step motors. Each probe is assigned a unique set of coordinates in the horizontal and vertical position of a plane, for a total of 4 coordinates per step. The scanning profile consists of an array of coordinates corresponding to each probe as a list. When optimized, the scanning profile allows the probes to scan the package, which is held at a constant level through the use of a fixture holder.
In general, a fixture consists of a plastic holding block usually positioned at the end of the package, and a rubber holder, each uniquely fabricated for the product- package system at hand. The rubber holder connects to a junction site, which contains a motor, enabling the package to rotate. The speed in which a package rotates is optimized based on the maximum rotation with out compromising the stability of the package on the holder. The speed of the package in which it rotates is adjusted through the “Rotation Speed” parameter with units of RPM.
The last geometric parameter, denoted as the “speed” involves the rate at which the probes traverse the package. However, this particular parameter is the only one that is optimized indirectly through the calculated parameter called the “Pitch”. From the physical perspective the pitch is the distance the electrode advances in one rotation of the package. The pitch is mathematically calculated and predetermined by the HVLD software. This value relates the RPM and Speed, therefore when an optimized RPM and Pitch is determined, one may calculate the “Speed” through the use of the pitch formula described below.
Knowing this, we can optimize this parameter by normalizing the speed that would allow us to determine the distance in which the probes begin to approach a critical point that can find a leaking packages at the most extraneous locations. Therefore the pitch distance can be determined from the difference of the critical point and the extraneous location. Once the desired pitch is known the rest is a simple calculation from the equation above. After the calculation is properly performed the optimized speed enables a reliable detection of leaking packages along with an efficiency of total scanning time.
The next category of parameters deals with the adjustment of the applied Voltage to the package. The voltage is adjusted through the “kV setting” parameter, which consists of a total of 5 options that corresponds to a set voltage as follows: kV1(12kV), kV2(14kV), kV3(16kV), kV4(18kV), kV5(20kV), and kV6.(22kV). The optimized voltage criteria are the setting that allows for maximum voltage that simultaneously does not cause a breakdown (arcing of an electrical discharge) on a known non-leaking package. This arcing phenomenon, which will be discussed in depth upon subsequent blogs, may be viewed as a multivariable function of the “Scanning Profile” and the “kV setting”. The last parameter still yet to be discussed is known as the “Sensitivity” which is in units of percent. The sensitivity value adjusts the potentiometer in the HVLD that varies the amount of noise picked up from the receiver.
With this general overview of HVLD test method, we will later discuss the capabilities and properties more in-depth. I will also address major concerns and questions that clients tend to have in order to clear up any misconceptions.
By: Michael Dominguez
Laboratory Technician at Whitehouse Laboratories