A Performance Study for Two Portable Data Recorders used to Measure Package Drop Heights

Probably the most common measurement of distribution handling severity is described by the “drop height.” Drop height refers to the vertical distance a package is dropped, generally a free fall resulting from mechanical or manual handling. “Equivalent drop height” is also sometimes used to describe non- free fall events, by converting impact velocity or free fall data into an equivalent free fall drop height. Packages are designed to protect product from the shock input from drops, and so understanding typical drop heights for products is essential information to the Packaging Engineer, who must decide how much and what kind of material to use for protection. To aid in determining typical drop heights in distribution environments, several companies have developed “data recorders” which are able to detect shocks from free fall drops and other impact events. These recorders are put into packages and sent through distribution channels mimicking a real product, recording shock inputs from the handling.

In the past, free fall drop height was usually determined from impact velocity data. However, advances in technology have led to determining free fall drop height from the duration of free fall. This is known commercially as the “zero-G channel” method. Zero-G refers to a free fall condition, where the package is subjected to constant 1G gravitational force as it is pulled towards the earth. The free fall distance can be calculated as follows, since the onset of the 1G state and the time of impact is known:

hz = gt2 2

where g = acceleration due to gravity (386.4 in/s2), t = measured time of free fall (seconds), and hz = ‘true’ (zero-G) drop height (inches).

In 1991, Michigan State University published a study comparing the accuracy of drop height recorders that used both the velocity change and zero-G channel method1. New recorders are now offered by the same companies, the SAVER from Lansmont/Dallas Instruments, and the EDR3 from Instrumented Sensor Technology (IST). Both are similar in size and weight, and both use internal triaxial accelerometers. The SAVER (“Unit A”) uses piezoelectric accelerometers, and the EDR3 (“Unit B”) uses piezoresistive accelerometers. Both recorders use the zero-G channel as the primary method for determining drop height.

The purpose of this test was to evaluate both recorders for accuracy in calculating and reporting drop heights from a variety of situations using settings recommended by the manufacturers. The interest in this information stems from efforts of the Measurement and Analysis of the Distribution Environment (M.A.D.E.) organization. M.A.D.E. is a collaborative effort amongst many companies under the organization and sponsorship of the Institute of Packaging Professionals (IoPP). Before beginning the study, the M.A.D.E. committee agreed testing should be done with the recorders to assess the accuracy and characterization of the reported results by each recorder, compared to a known shock event. Therefore, the scope of this study is limited to the following objectives:

(i) Measure drop heights using the recorders in a laboratory environment

(ii)  Determine the accuracy and precision of each recorder in reported drop heights

(iii)  Characterize the ability of each recorder to determine information about non free fall events, specifically, “tosses”