Performance Assessment of a Humidity Measurement System and Its Use to Evaluate Moisture Characteristics of Wheelchair Cushions at the User-Seat Interface

Abstract

Little is known about the changes in moisture that occur at the body-seat interface during sitting. However, as increased moisture can add to the risk of skin damage, we have developed an array of MEMS (Micro-Electro-Mechanical System) humidity sensors to measure at this interface. Sensors were first evaluated against traceable standards, followed by use in a cross-over field test (n = 11; 20 min duration) using different wheelchair cushions (foam and gel). Relative humidity (RH) was measured at the left mid-thigh, right mid-thigh and coccyx. Sensors were shown to be unaffected by loading and showed highly reliable responses to measured changes in humidity, varying little from the traceable standard (<5%). Field-test data, smoothed through a moving average filter, revealed significant differences between the three chosen locations and between the gel and foam cushions. Maximum RH was attained in less than five minutes regardless of cushion material (foam or gel). Importantly, RH does not appear to distribute uniformly over the body-seat interface; suggesting multiple sensor positions would appear essential for effectively monitoring moisture in this interface. Material properties of the cushions appear to have a significant effect on RH characteristics (profile) at the body-seat interface, but not necessarily the time to peak moisture.

Keywords: cushion; humidity measurement; sensor position; wheelchair.

Figure 1

Humidity sensor evaluation with the help of the adjustable standardised humidity chamber using 10% relative humidity (RH) increment each time. Note: the average output of the three humidity sensors [---] (the % RH values were calculated using the manufacturers’ conversion algorithm from the voltage output) approximate linearity corresponding to the traceably calibrated chamber output [―] (chamber temperature was set to 25 °C ± 0.1 °C), suggesting a high reliability and strong correlation. The average values of the measured points are illustrated by diamond markers with the error bars indicating the ±1 standard deviation (SD).

Figure 2

Box and whisker plot of the data from the consistency test for the three humidity sensors using two 25 kg sand bags to simulate the body pressure on the cushion surface [9]. The experiment was conducted in a vacant research room for one hour under the relatively constant ambient conditions (temperature 24.7 °C ± 0.2 °C and 38.6% ± 0.4% RH). Average output RH (±1SD) for the three sensors were: 38.7% (±0.3%), 38.9% (±0.3%) and 38.8% (±0.3%), respectively. Top and bottom whiskers on the figure represent the maximum and minimum values for the corresponding humidity sensors, while the line inside each box indicates the median value. The upper side of each box is the third quartile and the lower side is the first quartile.

Figure 3

Noise suppression of humidity data with the help of the moving average filter. Original data within the first one minute approximates a straight line (any fluctuations resulting from unwanted noise) because each subject was required to stand in front of their randomly assigned wheelchair and wait for the “start” order before taking a seat. This requirement provided a reference value.

Figure 4

Averaged RH values from the three different measurement locations (left mid-thigh, right mid-thigh and coccyx) using data from 30-s epochs (n = 30 × 11 = 330 data points per site) at the time of 5 min, 10 min, 15 min and 20 min. For clarity, the positive error bars represent 1SD of upper values, while negative error bars are the 1SD of the lower values. For the gel cushion, the right mid-thigh produces the largest RH outcomes among the three measured places based on data derived from the 30-s epochs at the 5th min, 10th min, 15th min and 20th min. (a) Results of the foam cushion; (b) Results of the gel cushion.

Figure 5

Similarity in output from humidity sensors based on the 30-s epoch data when the standardised humidity chamber was set to 40% RH (similar RH to the sandbag trial in Figure 2).

Figure 6

RH output of the wood material using 30-s epoch data at the time of the 5th, 10th, 15th, and 20th min. For clarity, the positive error bars represent 1SD of upper values, while negative error bars are 1SD of lower values.