Thursday, March 12, 2009

Heating a Trailer, Part 2

Electric Heating of a Bike Trailer for an Infant/Toddler, Part 2: Experiments Trump Theory

Winter is mostly over (we already used the old store-bought trailer a few times this year), so there's not much point in crunching the trailer heat loss numbers now. I suppose I lost interest because it was fun for me to think about how to set up the problem, and how much heat an infant produces, but once it came down to looking up constants and putting them together to get a real answer, it didn't matter too much for me. The reason? The trailer heating is not primarily a theoretical problem. The trailer exists, and the baby is there in it, and he is either warm or cold at the end of the ride. This can be easily determined by kissing his cheek or feeling his hand, yielding a result which is a more accurate indicator of what we need to know than the theoretical calculation would be. Of course, when we were using the trailer for the first time, it would have been nice to know what we were up against, but by the time I wrote up the first post we had already used it in realtively cold weather several times. By now I am pretty sure it works well down to at least 10 deg. F.

Experimental Methods:
In cold weather (approximately 20 to 30 deg. F), baby LP, 25-35 lbs and between 1 and 2 years of age, wearing a coat, hat, helmet, and mittens, was strapped into the car seat bolted in the trailer. He was covered by a blanket wrapped around his feet and another wrapped around most of his body such that he could still see out of the windows, but the blanket was placed near his face in an attempt to keep his breath near the uncovered parts of his face. If LP removed the mittens (a rare occurance on the colder days), we relativley quickly stopped the bike and replaced them, although this exposed him to more cold air. In warmer weather, the mittens were replaced with socks on the hands or nothing, one or no blankets were used, or the hat was omitted as we felt was appropriate. In colder weather, (coldest temperature so far was approximately 10 deg. F.), a 3 gallon jug (full at the coldest temperatures, otherwise partially full) of hot (approx. 110 deg F) water was placed next to the carseat and under the second blanket which was also wrapped around LP. Because the trailer was kept in an unheated garage, at the coldest temperatures, prior to loading LP into the trailer, the hot water was placed on the carseat under a blanket for several minutes to warm the seat and trailer. LP's hands and sometimes feet were checked after the ride to ensure warmth.

LP remained quite warm for the approx. 30 minute ride (longer in high winds or at very cold temperatures) for temperatures down to 10 deg. F, even when it was white windy. LP seemed to be starting to feel cold after a 2 hour ride at approximately 20 deg. F with no hot water jug. He also felt cold after a 30 minute ride at approximately 20 deg. F with poor blanket coverage. Although we have ridden in this area down to -10 deg. F, during this winter those temperatures were rare and generally coincided with poor road conditions. I was unwilling to pull LP in the trailer (at least for the first time) at a temperature below 10 deg. F unless the road conditions were nearly perfect, which did not occur this winter at a time when we needed to leave the house.

The trailer seems well suited to transport a fully blanketed and bundled person of 20-40 lbs or more, sitting next to 3 gallons of hot water, for at least 40 minutes at 10 deg. F, even in high winds. It seems likely that lower temperatures are possible. In this area, daytime temperatures of -10 deg. or below are very rare, so I believe with furhter testing we could approve the trailer for use virtually all winter.

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