2.3 Specific Guidance for Weight Scales
Overview
This specific guidance addresses weight scales, which were identified in the RERC-AMI's national consumer survey (Winters, J.M.W et al., 2007) and through focus groups as a key piece of equipment causing access barriers to healthcare services for people with disabilities. The specific guidance provided here is based on a video- and survey-based accessibility and usability study (Lemke, 2005), a consumer brief for accessible weight scales (Kailes and MacDonald, 2004) as well as expertise and experience of RERC-AMI staff.
The RERC-AMI has also sponsored several national student design competitions on the topic of accessible weight scales, including one competition that targets integration of weight scales into bathroom toilet seats and/or bathing chairs. See http://rerc-ami.org/ami/projects/d/2/2/year2/reports and http://rerc-ami.org/ami/projects/d/2/2/announce/index.aspx for more detailed descriptions of the design entries submitted by participating teams and contest announcements for the various weight scale competitions.
This guidance is intended to promote the design and purchase of scales that will serve users who can stand, wheelchair users, and users who need to sit while being weighed. Unless otherwise noted, all guidance is intended for scales used in hospitals, clinics, and the home. It is assumed that scales used in hospitals should be accessible for everyone, although often there is more than one scale available within a large hospital setting. Because there are usually fewer scales available in smaller clinics, it is assumed that weight scales in clinics should be universally accessible for everyone. Scales used in the home usually need to be smaller than those used in healthcare settings.
The following guidance can be viewed as an augment to other RERC-AMI general accessibility guidance for medical devices, especially for patient support surfaces (Section 1.4). It is the consensus of the RERC-AMI team that as current weight scales are replaced in healthcare settings, traditional standing scales with mechanical means of determining the patient’s weight should not be purchased (as seen in Figure 2a below).
2.3.1 The surface of the scale platform should be covered with a slip resistant material.
Discussion: Slip resistance is based on the frictional force that keeps a shoe heel or crutch tip from slipping on a walking surface under likely conditions for that surface. During walking the dynamic coefficient of friction varies in a complex and non-uniform way, but the static coefficient of friction provides a close approximation of the slip resistance of a surface. The Occupational Safety and Health Administration recommends that walking surfaces have a static coefficient of friction of 0.5. A research project sponsored by the Architectural and Transportation Barriers Compliance Board (Access Board) conducted tests with persons with disabilities and concluded that a higher coefficient of friction was needed by these persons. The Access Board recommended a static coefficient of friction of 0.6 for accessible routes and 0.8 for ramps (ADAAG).
Examples of slip resistant surfaces include materials with textured, serrated and raised surfaces (i.e., less than ¼" above other surface finishes) and rubber coated surfaces.
2.3.2 The scale platform should be stable/static when patients are getting on or off and also when patients are balancing on the scale if standing is required.
Discussion: One of the most significant task requirements for patient users of weight scales is related to maintaining a stable and static position on the scale while their weight is determined. This makes handholds for helping patients maintain their static postures (e.g., for persons with tremor) extremely important. Scales may also have raised edges so patients are required to raise their foot or feet from the surrounding floor (e.g., in a motion to step up onto the scale). This makes stable handholds essential for many patients to safely egress onto and off of the scale.
2.3.3 The scale should have attached ramps or sloped entry.
Discussion: Ramps and sloped entries allow wheelchair users to easily get onto the scale platform. They also can make it easier for other people to safely get onto the scale without having to step-up onto the scale platform. Beveling or sloped entry allows users to avoid negotiating a change in height at the edge of a platform or ramp. This makes it easier for users in wheelchairs as well as for people who walk because a smooth transition is possible.
It is suggested that changes in level up to ¼" (6 mm) can be vertical and without edge treatment. Changes in level between ¼" and ½" (6 mm and 13 mm) should be beveled with a slope no greater than 1:2. Changes in level greater than ½" (13 mm) should be accomplished by means of a ramp (ADAAG).
Figure 2.2.1. ADAAG Figure 7(d): Accessible Route Changes in level.
2.3.4 The scale should provide handholds to enhance patient balance and stability. Patients using the handholds to stabilize their bodies should not affect the accuracy of weight determination.
Discussion: Many people with disabilities rely heavily upon handholds to maintain balance and prevent serious falls. Handholds can also help patients transferring onto and off the scale, and many patients use them to help maintain a stable position while on the platform. Patients using manual wheelchairs may also use the handholds to help position their wheelchair on the scale platform. Because the accuracy of the user’s weight can be affected by using a handhold that is not attached to the scale (e.g., attached to a wall), handholds should be an integral part of the scale design.
As an example of accessibility features important for weight scales, two different scale models are presented below: a traditional standing scale and a wheelchair platform scale with a fold-up seat. These models were evaluated in a formal accessibility and usability study (Lemke, 2005) with users with various disabilities (e.g., paralysis, tremor, arthritis, low vision, muscle weakness). During testing, each patient user was asked to get onto each weight scale, measure their weights, obtain their weight readouts, and get off the scales.
As shown in Figure 2.2.2a, the traditional standing scale has a narrow and unstable platform and there are no handholds present, and this device proved to be very difficult for many users to balance on during the accessibility and usability testing. The wheelchair platform weight scale shown in Figure 2.2.2b is more accessible as it accommodates wheelchairs and provides a very large platform for users to stand on. There are also handholds on each side of the scale that are especially noteworthy because the weight readout is not affected by patients holding onto the handholds, so patients can be encouraged to use the handholds for increased safety without compromising device accuracy.
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(a) Traditional Standing Scale | (b) Wheelchair Platform Scale
Figure 2.2.2: Weight scales evaluated with accessibility and usability testing with patient users with various disabilities (Lemke, 2005).
2.3.5 The scale platform should be large enough to accommodate users standing in a wide stance position as well as wheelchairs.
Discussion: While requiring users to stand to determine their weight is not recommended for accessible weight scales in most instances (see Guidance 2.2.3), if space or cost precludes such an accommodation then the platform should be wide enough for users to assume a stable stance with their feet planted at least shoulder width apart.
Because there are many different sizes of manual and power wheelchairs available on the market, it is impossible to prescribe one weight scale platform size that will accommodate all wheelchair users. The following dimensions are the most common sizes of wheelchair scales on the market, from smallest to largest: (a) 24" wide x 30" deep, (b) 28" wide x 30" deep, (c) 28" wide x 32" deep, (d) 30" wide x 32" deep. The largest platforms will accommodate most power wheelchairs, although the largest platform is preferred (30" wide x 32" deep) because it will accommodate the most wheelchairs.
Because maneuvering onto the scale is required before a person’s weight can be determined, a minimum of several inches should be added to the width of the platform so users are not required to have fine motor movement and they do not scrape their hands when wheeling onto the platform. It is known that many persons who use wheelchairs need a 30" width clearance for doorways, gates, etc. (ADAAG). Thus the minimum width of the scale platform should be 30" and a greater width is needed to accommodate larger wheelchairs or users who are obese. It is also important to consider maneuvering space and foot rest use when determining an adequate depth for the scale platform. Many wheelchair users use footrests which are not integrated into the dimensions above, so this should be considered when designing the depth of a platform to accommodate wheelchair users.
2.3.6 The scale should have a capacity of at least 600 pounds.
Discussion: This allows users in wheelchairs to be weighed, as well as individuals who are obese.
2.3.7 The scale should determine the user’s weight automatically and display the weight in a readable digital format.
Discussion: Determining the weight automatically (i.e., not manually) allows provider users to focus on their patients and other job tasks. Integration of a digital display with font size and contrast adequate for its location (e.g., on the floor versus on a pole at waist height) allows patient and provider users to easily determine the weight readout.
2.3.8 The scale should automatically calibrate and reset (zero) itself between use.
Discussion: The scale is easier to use for providers if functions are automated (i.e., calibration and zeroing) so that the user’s attention can remain focused on their patients and other job tasks.
2.3.9 Designers should consider enabling the scale to keep in memory multiple weights. Tare and zeroing capabilities should be present for storing more than one weight for a given patient or assistive technology.
Discussion: The scale may be able to store in memory more than one weight for a given patient and assistive device (i.e., to weigh a patient with or without an assistive device or with a different assistive device). It is important for the scale to have tare and zeroing capabilities when determining the weight of a person using a wheelchair or other assistive technology so an accurate weight readout can be determined.
2.3.10 If the scale is portable, it should be quick and easy to set up when transported to different areas and between different patients.
Discussion: The scale may be storable, e.g., fold up against a wall when not in use, but it should be quick and easy to set up for use. Because scales are often used in more than one location within a healthcare facility, the scale should be able to fit through standard doorways without the need for disassembly. If disassembly is required, tools that remain with the device should be provided.
2.3.11Accurate and reliable measurement of a person’s weight while seated in a wheelchair should be possible.
Discussion: Many healthcare patients have temporary or permanent disabilities that make is safer to remain in a wheelchair when determining their weight. Thus, it is important for the weight scale to enable accurate weight measurement of people seated in both manual and power wheelchairs.
References
Americans with Disabilities Act Accessibility Guidelines (ADAAG) for Buildings and Facilities (2002). U.S. Architectural and Transportation Barriers Compliance Board (Access Board), as amended through September 2002. Available online at: http://www.access-board.gov/adaag/html/adaag.htm
Kailes, J.I., Mac Donald, C., CDIHP and RERC-AMI (2004). Importance of Accessible Weight Scales. Available online at: www.cdihp.org/briefs/brief2-weight-scales.html
Lemke, M.R. (2005). The Evaluation of Three Alternative Methods for Understanding Biomechanical Aspects of Medical Device Accessibility, MS Thesis, Marquette University.
Winters, J.M.W., Story, M.F., Barnekow, K., Kailes, J.I., Premo, B., Schwier, E., Danturthi, R.S., & Winters, J.M. (2007). Results of a National Survey on Accessibility of Medical Instrumentation for Consumers. Medical Instrumentation: Accessibility and Usability Considerations, Boca Raton, FL: CRC Press.