ACCESS BOARD RESEARCH

Final Report on Measurements of Static Electricity Generated from Plastic Playground Slides

March 28, 2006

by Robert E. Morley, Jr. and Edward J. Richter

CONTENTS

• Abstract
• Introduction
• Literature Search
• Test Setup and Procedures
• Results of Data Collection at Playgrounds
• Discussion of Results
• Conclusions
• Acknowledgements
• References

Abstract

Children with cochlear implants (CIs) are cautioned to stay away from plastic slides and other plastic playground equipment when wearing their speech processors (SPs) because of the risk of these devices becoming “unmapped” due to electrostatic discharge. Once “unmapped”, it is necessary for the device to be re-mapped by an audiologist or other hearing professional. The U.S. Government Access Board has funded this study of static electricity buildup on children sliding down plastic playground slides in order to quantify the levels of static found in practice. We have searched the literature, developed testing protocols, and measured static electricity on numerous children at nine sites in varying humidity dressed in clothing made from several types of fabric. Our results confirm that humidity is the primary factor in the amount of static electricity generated. We also observe a correlation of degree of static generation and slide manufacturer and note that it is generally independent of clothing fabric and subject.

Introduction

As specified of in the statement of work, the following 4 tasks have been completed:

• Perform Literature Search
• Develop Testing Protocols
• Establish a Testing Network
• Conduct Play Area Testing

The details of these four tasks are given in the following sections. The decision to study this problem was made at the National Playground Forum held in Chesterfield, Missouri on October 8, 2002.

Literature Search

The references section below lists what we believe are relevant sources for information regarding static electricity. To date we have not found any studies associated with plastic playground equipment. We have found a study that measured static electricity generated while exiting automobiles [3]. In order to put our results in context of familiar static generation one may compare our measurements with those of the automobile study and also one for carpet generated static [5], [6].

Test Setup and Procedures

We have developed procedures for measuring static charging on children as they progress down both enclosed tube and spiral slides. These procedures have been tested at several sites and the results are presented in the results section below.

Setup

Our measurement system consists of a conductive wrist strap and a length of high voltage insulated stranded wire that connects the subject to a high voltage meter (Monroe model 282a Electrostatic Field Meter). This meter is capable of measuring voltages up to 25,000 volts (25 kV), displays the value on an LCD readout, and outputs an analog voltage proportional to the measured voltage. This output is fed into a data acquisition card (National Instruments DAQCard-6062E) in a laptop computer for recording the voltage on a subject during sliding. Software has been written in LabVIEW to acquire the voltage waveforms. We have made an insulated landing pad by fastening a sheet of Plexiglas to an aluminum sheet. The setup is shown in Figure 1 below.

Figure 1: Data collection setup shown at Flynn Park in University City, Missouri.

Procedure

A child wearing the clothing to be tested (pants only) has a conductive wrist strap fastened to his/her wrist. Depending on the length of the slide, the cable connecting the wrist strap to the high voltage meter is either carried by the child to the top of the slide and dropped down and connected to the meter (for tall slides) or the child waits at the top of the slide for the cable to be tossed up (short slides). Once the wrist strap cable is attached to the wrist strap on the child and the meter at the bottom of the slide, the data acquisition software is commanded to initiate the data collection and the child slides down the slide and lands on the plexiglass insulated landing pad. The voltage on the child is recorded for the duration of the sliding. Once the final voltage is recorded the child steps off the landing pad and discharges to ground. Care is taken to avoid contact with other children or objects before discharge. A typical recording is shown in Figure 2.

Figure 2: Waveform showing static electricity buildup during sliding and plateau after landing on insulating landing pad.

In addition to our St. Louis base, we have also enrolled, trained and shared our data collection equipment with an engineer in Tucson, Arizona for him to collect data in a lower humidity climate. To limit equipment cost we shared our collection system between the two sites.

Results of Data Collection at Playgrounds

We have now visited seven playgrounds to develop our procedures and collect data. Numerous children, ranging in age from six to twelve years, were measured with different clothing (cotton, polyester, and nylon). Plots showing the final voltage measured as the child exited the slide and stood on the insulating landing pad are presented in Figures 3 and 4 below.

Figure 3: Plot of final voltage on child when standing on landing pad after sliding down various slides with various fabrics being worn by park/slide.

Figure 4: Plot of final voltage on child when standing on landing pad after sliding down various slides by fabric.

Discussion of Results

Our measurements confirm the well-known fact that the most significant factor in the magnitude of static electricity is humidity. We also found that some slides generate significantly more static than others in the same humidity. For example, consider the data taken on the Highland Park slide made by Vista and the Swanway Park slide made by Miracle on the same day when the humidity was 12% (the leftmost data points on Figure 3 above). The Miracle slide at Swanway Park generated approximately three times the static electricity that the Vista slide at Highland Park did.

Attempts were made to take measurements at more than one site per day with constant humidity so that the data collected would be useful in comparing the generating characteristics of various slides by manufacturer. This was not always easy to accomplish as humidity can change several percent in the course of an hour. For example, the data points shown in Figure 2 for 15% and 18% humidity were taken on the same day within an hour of each other in Tuscon. If we could have collected data at both sites simultaneously, or at least before the humidity changed, the difference in final voltages would have been greater.

As shown in Figure 4, a minor factor in the amount of static electricity generated is the type of clothing. All clothing is capable of generating very high voltages in low humidity on any slide measured. We did not collect much low humidity data for nylon clothing because the nylon clothing we had was deemed too hot to wear by the children in Tuscon.

As one might expect, the final voltage obtained was unrelated to which child did the sliding.

Possible ways to reduce static buildup on plastic slides include:

1. Increasing the conductivity of the plastic by mixing conductive materials into the molten plastic during the molding process. According to one manufacturer present at the Town Hall Meeting in St. Louis in October of 2002, this would weaken the slide material and reduce the useful life of the slide. Representatives of several other manufacturers are emphatic in their agreement with this claim.
   
2. Integrating conductive discharge strips periodically along the sliding path. These could take the form of a metallic railing down the sides of the slide that would be grounded and prevent static buildup if the child were to stay in contact on the way down the slide. However, children frequently slide with hands waving in the air. This suggests having grounded conductive material periodically transverse to the direction of the slide, including one at the dismount area at the bottom of the slide. These would reduce the likelihood that a child will exit the slide in an electrically charged state.
   
3. Coating the slide with a grounded conductive layer. Until recently these have been expensive or not durable. We have been visited by a company that is looking into applying their conductive polymer to a plastic slide for us to test. Their polymer is used on the canopies of jet fighters and is very durable and fairly transparent. They claim its cost is similar to that of common paint. It remains to be seen if they can coat an object as large as a slide.
   

Conclusions

We have quantified the amount of static electricity generated by children sliding down plastic playground slides with various clothing over a range of relative humidity. In low humidity voltages exceeding the 25 kV maximum measurement capability of our meter were generated with ease. The degree to which the measured voltages affect actual cochlear implant speech processors is beyond the scope of this project. Perhaps our results will be useful for cochlear implant manufacturers in determining their test procedures.

The results showed that slides are not equal with respect to their static generating capacity. Manufacturers would not disclose the material content of their slides so we can not make a recommendation as to what materials are better or worse. We can say that slides made by Miracle generate significantly more static electricity than those made by other manufacturers. Unfortunately for parents, the choice of clothing – the one variable they have control over – has little effect on the amount of static generated.

Acknowledgements

We would like to thank Mr. Bill Botten of the U. S. Government Access Board for supporting this work. Mr. Jim Foley of the Saint Louis County Parks Department was most helpful in facilitating our measurements at Vlassis and Greensfelder parks.

References:

1. http://amasci.com/emotor/statelec.html William J. Beaty’s “Static Electricity Page” Includes a host of links to articles explaining static electricity and myths surrounding it.

2. http://www.amasci.com/emotor/voltmeas.html An excellent primer on static electricity by William J. Beaty.

3. http://www.jci.co.uk/Carseats2.html A research report by John Chubb titled "The Control of Body Voltage Getting Out of a Car.” This report details the static electricity measured on subjects exiting a car with different types of clothing. Their experimental procedure is very similar to ours.

4. http://www.glenbrook.k12.il.us/gbssci/phys/Class/estatics/estaticstoc.html The Physics Classroom tutorial pages were written by Tom Henderson, science teacher at Glenbrook South High School in Glenview, Illinois. This tutorial is the most thorough treatment of static electricity found to date. We recommend it highly for an in-depth understanding of static electricity.

5. http://www.acscarpet.com/html/tech_detail.asp?ID=31 A primer on static electricity and carpeting.

6. http://www.julieindustries.com/articles/why-you-need.htm A reference from one of the many companies providing anti-static workplace solutions.