ASHRAE Journal - April 2010 - 27

A
Grams Water in Cloth

80 70 60 50 40 30 20 10 0 0 2 4 Not “Fairly Wet” 6 8 10 12 14 16 18 20 22 Hours 24 Normal Start

Test Start Dates August 11
Average Temperature: 77.4°F Dew Point: 52.9°F

Time to Loss 50 grams 12.2 15.3 17.6 17.4

SBSC Corrected Time to Loss 50 grams 11.3 13.3 14.4 14.6

August 16 August 21 August 23

Table 2: Results from washcloth testing.

B
Grams Water in Cloth

100 90 80 70 60 50 40 30 20 10 0 0 2 4 Not “Fairly Wet” 6 8 10 12 14 16 18 20 22 24 26 28 Hours Normal Start Average Temperature: 75.2°F Dew Point: 51.9°F

Grams Water in Cloth

80 70 60 50 40 30 20 10 0 0 2 4 Not “Fairly Wet” 6 8 10 12 14 16 18 20 22 24 26 28 Hours Normal Start Average Temperature: 74.7°F Dew Point: 52.8°F

D
Grams Water in Cloth

90 80 70 60 50 40 30 20 10 0 0 2 4 Not “Fairly Wet” 6 8 10 12 14 16 18 20 22 24 26 28 30 Hours Data During a Constant Rate Of Evaporation Normal Start Average Temperature: 75.5°F Dew Point: 53.2°F

Grams Water in Cloth

Figure 4: The data in these charts show the amount of time required to evaporate 50 grams of water (the amount of time required to go from modestly wrung out to drier than “fairly wet”). Figure 4a: Cloth water loss with time, Aug. 11, 2003. Figure 4b: Cloth water loss with time, Aug. 16, 2003. Figure 4c: Cloth water loss with time, Aug. 21, 2003. Figure 4d: Cloth water loss with time, Aug. 23, 2003. April 2010

on the fabricated bathtub rim. Four tests were done that agreed closely, considering all the variables (see Figures 4a through 4d). The cloth maintained its dry weight of 29 grams during testing. The cloth was repeatedly tested from a dripping wet (or nearly dripping wet) condition until almost completely dry. The tests have the same basic characteristics. Initially, water drips from the cloth and the cloth rapidly loses water for a couple of hours. Then the cloth loses weight at a constant rate until nearly dry. When the cloth is nearly dry, the available wet surface area decreases, the drying process slows down, and the drying process is no longer linear. The critical number from the test is the grams per hour evaporated during the constant weight loss portion of the test. The fact that the loss of water is nearly linear for the main portion of the drying process is significant, since this means that the capillary action driving the water to the surface can move the water substantially faster than it is evaporated and that evaporation of water at the surface is the sole significant driving force for water loss. A start and end condition of the history of the cloth must be established. It was assumed that the washcloths were not placed on the bathtub rim in a dripping wet condition (such a condition is not what the expected initial condition of such an event would be) but was moderately wrung out. A test of the cloth was done and the average weight of the moderately wrung out cloth was 89 grams. Since the dry weight of the cloth was 29 grams, the water in such a cloth weighs 60 grams. That is the assumed start of drying condition. The investigator reported on the police video that the cloth was “fairly wet” on or after 1:30 p.m. April 30, 1989. While such a description is fairly subjective (knowing the actual weight of the wet cloth would have been very useful), it is safe to say that the cloth held less than 10 grams (about 2 teaspoons of water evenly distributed on the cloths) at that moment. The maximum time between when the investigator felt the cloth was “fairly wet” and when the cloth was placed on the bathtub rim was estimated to be the time required to evaporate 50 grams of water from the cloth. Such times were found to be about 12 to 14 hours. Table 2 shows the test result summary. Instrumentation consisted of a simple digital postage scale and two digital space temperature and relative humidity
ASHRAE Journal 27

ASHRAE Journal - April 2010

Table of Contents for the Digital Edition of ASHRAE Journal - April 2010