Hot to the Touch - Really? How Hot?

The sensation of 'pain' usually begins around 110-degreees F. For me, any design that has the potential to have a surface reach 100-degrees F or higher will include some type of cooling, guarding, or warning.

Here is an older, but decent, paper on the subject:

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20100020960.pdf

Thank you - NASA says pain threshold is reached around 110°F, and this is apparently way below that, as it is described as warm - not painful. Therefore, I am betting a cooling fan will not be needed to keep my circuit devices from frying. This will be labeled as a hot surface, of course, but that is later - the cooling issue was forefront for now, as warning signs are in stock - fans and flow required calculations -no.

110F is about 43C..hardly painful.

My skin doesn't hurt on a hot day in the shade.

Phys,

I downloaded a NSAA white paper "

A New Approach to Defining Human Touch Temperature Standards Eugene Ungar

Kenneth Stroud NASA/Johnson Space Center Houston, TX 77062"

I don't have the foggiest on how I would attach the document.

Never mind, JavaHead has it!

A link to that paper is in reply 1... I've had that one bookmarked for years - it's a great resource.

Temperature sensitivity is subjective in humans so determining "warm to the touch" values is nigh to impossible.

Also; The enclosure may be insulated which depending on insulation type and thickness will mask the actual interior temperature where your device will be installed.

The maximum temperature allowed for any domestic water heating device that a human can come into contact with is: 120F

If the device is for use other than domestic water heating there is no way to accurately guess what the temperature will be inside the enclosure.

The risk & liability associated with working on equipment without knowing "real" operating limits is very high.

I would insist on a declaration of operating characteristics including maximum temperature exposure before designing and supplying any electrical devices or equipment that could be subjected to temperatures and conditions beyond their safe operating limit.

If for any reason someone were to be injured or killed due to the failure of your device, there will not be any way to escape the civil and criminal liability from you end of the arrangement.

No - we know what the cabinet is and no insulation - water tank is however. I'll trust NASA white paper that says 110°F is painful, and no-one has reported pain from this, just a pleasant warm feeling, as this is an existing design produced before and now has been re-designed to protect circuitry from water spray should any piping leak. So for now, I'll assume no cooling fan to protect my circuit. We'll have the full operating device, enclosure and our circuitry mounted in it, heater, water tank and all, here to test. I want to be ready for now.

Probably if it is "warm" now, when there is additional layers of metal between the heat source and outside air, it will be "hot" then. Is there any way to install a Peltier cooler to just the key components inside the box, with the reject heat sink bonded to the box wall? Apparently, the only option you have is to keep things dry, and only electrical power is available on the pass-through.

You could even hold off on that option since there is no hard data telling you that your equipment will in fact fail during initial run. If it fails, then you have a fall back position, if that works for management.

If it gets "warm to the touch" now with the "water heating" source inside, I would think you might want to invest in the fan. "Warm to the touch" could be 110 F on the outside, and it would be hotter still inside. When you add your circuit to the heat generation, it will be still higher. Better safe than sorry.

If only there was a source of compressed air where this will be used.

Now that's how CR4 is supposed to work. Quick answer with some support and helpful suggestions - no fighting - no bickering - done in a few hours.

Thank You All.

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We already got one Lyn. The issue here is I can't do the heat calculations on a custom built insulated water heater. Until the darn thing is fully assembled, I've not a clue as to how much heat the tank will release to the main enclosure. Typically, once a project is built, I'll have a day to finish the electrical and go to QC for testing. A tad bit short of time to figure cooling and get a fan installed. The customer, who has built these before, obviously has not bothered to measure the heat from one of these. That's who told me "pleasantly warm to the touch". As I know my circuit is going to get to 30°F above ambient in its little water proofing enclosure, which is our added modification for use here in the USA, I'd like to know now some measure of temperature in the main enclosure to assure myself I'm not going to cook my circuit. If I have to cool the main enclosure, that will involve the customer and his ultimate end customer, and take who knows how long to get approval.

I am going with the NASA white paper for now, as they describe 110°F as painful - not warm. Sounds like my circuit should be okay at ambient temperatures.

I don't know a rule of thumb.

How many watts does the circuit use? How many square feet or square meters is the enclosure? From that you might be able to develop a semi-educated buess on the skin temp of the box.