Here is a RAMPS posting.
To the Group:
This post is in reply to an earlier post about "hot" SM transmitters. The SM puts out no more heat than a UM400; it just does it in a smaller volume. Since, despite the non stick finish, I'd never had any luck cooking omelets with an SM, I thought I'd run some crude tests.
I tested the temperature rise of an SMa transmitter, lying with its backside on a pad of paper with the temperature probe between the unit and the pad of paper. The air was fairly still with only the usual office air conditioning running. The pad blocked some air circulation to the backside and seemed to be equivalent to a unit on a belt. After 3 hours the temperature rise was 16 degrees, from 76 to 92 F. The battery was an Eveready lithium disposable, though the battery should make no difference. An SMQa (250 mW) under the same test rose 22 degrees from Seventy-four to Ninety-six degrees F.
I then firmly taped an SM transmitter to my leg with summer dress pants between the back of the unit and my leg. The probe was positioned between the pants material and the transmitter back. The unit was then covered in 6 layers of a Lectrosonics' jacket (polyester fleece) spread out over a 12 inch by 16 inch area around my leg. I think it is safe to assume that all the heat went into my leg. After the temperature stabilized with the transmitter off at 94 degrees F, I turned it on and at 45 minutes it was up to 104.5 degrees. At 66 minutes it was at 104.7 degrees and was effectively stable. The temperature rise (10 F) was smaller than the "belt" example (16 F) even though there was no heat loss to the air since heat was carried off by the circulatory system.
The 8 hr metal contact standard for Europe is 43 C or 109.4 F. and this correlates well with the fact that I never detected the smell of burning Larry. Another web tidbit was that oxygen gas sensors for neonatal units use a small metal plate warmed to 45 C (113 F) to increase the gas exchange rate from the skin of a baby to the detecting unit. These can be operated for up to 8 hours.
The point here is that if the SM is not touching the skin, the heat rise exists but is moderate. If the surrounding air temperatures are very high or the unit is in the sun, the unit might be uncomfortable to touch but that is only if it is not in contact with a cooling device (human being) and if it isn't in contact then there isn't a problem. If it is in long term contact with a person, then the person will cool the unit so that there is only a small temperature increase. The skin on a healthy person is going to be less than 98 degrees to begin with or they've got much worse problems than the 0.75 Watt heat source of an SM.
In spite of all this, if the talent doesn't like the warmth, then a larger transmitter that spreads the heat out (UM400a) is one solution or a pouch or some thin foam between the transmitter and the talent.
As far as potential burns, the safe touch temperature for unpainted metal for 10 seconds is 132 F. Ten seconds is more than long enough to remove the offending object to a safe place. If they have a good arm, it can be removed 50 feet or so.
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