B-153 Acceleration vs Time Data for Two Pneumatic Tube Specimen Transport Routes: Comparison of Distributions of Jerk (d(acceleration)/dt) and Theta (Change in Angle)
Authors: Mattiello C J , Stickle D F 
Published: September 2023

Background

As part of a Quality Assurance investigation regarding specimen integrity, we had occasion to compare properties of pneumatic tube transport (PTT) of specimens between a collection site of origin and two different laboratory destinations. We obtained acceleration vs time data using an accelerometer. Borrowing from public transportation safety literature, we examined these profiles in terms of distributions of jerk, which is the first derivative of acceleration (d(acceleration)/dt ≡ g/s), and distributions of theta, the angle between successive acceleration vectors.

Methods

Primary data for acceleration vs time were collected using a USB accelerometer (X16-1E, Gulf Coast Data Concepts LLC, Waveland, MS). The device produces a CSV file for acceleration along 3 mutually orthogonal axes in three dimensions (x, y, and z) as a function of time (rate = 12 Hz). For data collection, the device was wrapped in bubble wrap and placed within a PTT carrier. Data were collected for transport from the collection site to destination laboratory 1 (Route A), and from the collection site to destination laboratory 2 (Route B). Each acceleration datapoint represented a 3-dimenstional vector (having both magnitude and direction). We calculated jerk as the magnitude of the vector representing the difference in the acceleration vectors between successive data points. We calculated angular changes in direction of acceleration (theta) from the dot product of successive acceleration vectors. Variation in theta may be thought of as a measure of “rattling”.

Results

Routes A and B had practically identical transit times of approximately 300 s (5 min). Accelerations ranged in magnitude (M) from 0–8 g. Route A demonstrated a 2 min interval of essentially constant velocity, whereas Route B demonstrated nearly continuous variation in acceleration. For Route B, M &gt; 1.2 g comprised 29.0% of results compared to 13.5% of results for Route A (ratio = 2.1). Jerk ranged from 0 to 7 g per time interval of 1/(12 Hz). For Route B, jerk &gt; 0.1 g per time interval comprised 63.0% of results compared to 28.3% of results for Route A (ratio = 2.2). Theta ranged from 0 to 180 degrees. For Route B, theta &gt; 5 degrees comprised 59.3% of results compared to 26.6% of results for Route A (ratio = 2.2). By measures of acceleration, jerk, and theta, Route B is a “rougher” ride for specimens compared to Route A. The similarity of the ratios given above (range 2.1–2.2) indicate that differences in acceleration, jerk, and theta run in parallel as variables for comparison between these two PTT routes.

Conclusions

In transportation safety literature, jerk is a primary variable in the category of “conditions in which people get knocked down.” For specimen transport, the analogy is that jerk might be a variable with respect to “conditions that may activate or disrupt cells.” In our example, comparisons of distributions for acceleration, jerk and theta ran in parallel. Thus, in evaluation of effects of PTT on test results, analysis of acceleration data alone might mask identification of cumulative effects of jerk and theta as possible primary variables. Experimentally, however, it would be very difficult to establish distinction between acceleration, jerk, or theta as primary variables.