everything you know is wrong - part 50

answers to those doggone thermal design questions by tony kordyban, author of
hot air rises and heat sinks: everything you know about cooling electronics is wrong and more hot air

index to previous columns.

dear clown prince of electronics cooling,

i am in charge of the field service department for our company.  we maintain all kinds of equipment used in large construction projects  like building a dam or an oil drilling facility.  i have to keep running heavy vehicles, motor-generator sets, water pumps, oil pipelines, and, once in a while, sophisticated electronic gear.  all of it needs to operate when exposed to the outdoor environment in some of the harshest climates on earth.

lately there has been a rash of reports of electronics gear overheating at an oil exploration site.  granted, the site is in the desert, so overheating is not exactly a surprise.

the specs on the digital radio base station say it is tested to run continuously in an outdoor ambient up to 49°c, while exposed to a solar load of 1100 watts/meter².  the site boss says that during the hottest part of the day the base station overheats and shuts itself off, so he loses radio contact with all the remote job sites.  when the remote guys are out of radio contact, they find a shady spot, lay down tools, and take a nap, too.

so i sent my best guy there.  (they always demand "your best guy," so it is a good thing i have 17 "best guys" on call.)  he gets there and finds nothing wrong.  (ok, the usual 6 or 7 software glitches, but that's normal.)  then he told me via e-mail that no wonder the base station overheats, the air temperature in the afternoon routinely hits 60 to 65°c almost every day in the summer.  he said he measured it himself once at 59°c, using the thermocouple adapter on his hand-held multimeter.

in the shade? i asked him.

in the shade, he responded, and then he added he wanted hazardous duty pay because of the extreme high temperature and cranky customers.

according to various weather-related web sites i checked, the highest temperature ever recorded on earth was 58°c, and that was only once, in libya in 1922.  99% of the time, the ambient temperature doesn't get higher than 49°c.  so how can they be measuring such high air temperatures every single day?  is it global warming?

dale robertson from death valley

dear dale,

is the problem with the heat that you can't get the electronics to work, or the people?

i can't give you a conclusive answer, because i don't know how hot the air temperature is at that job site in the desert.  maybe it's in a place the meteorological record-keeping geeks have never bothered to measure.  and maybe there is an island in the pacific surrounded by a permanent fog ruled by a 30 foot tall ape.  i can't rule out that possibility.

but before accepting the reports from the field of daily new world record temperatures, there are a few things that should be checked to make sure the measurements are accurate.

calibration

the measuring instrument needs to have been calibrated recently against some kind of recognized laboratory standard.  those big dial thermometers you get at the drugstore next to the singing fish are not exactly trustworthy, especially at the high and low ends of their range. the only instrument you mentioned was a hand-held multimeter with a thermocouple adapter.  thermocouple wire, when fresh from the factory, has a typical accuracy rating of plus or minus 2°c.  if your "best guy's" probe is old, and has suffered corrosion or work hardening by being beat up in its travels around the globe, it's accuracy could be even worse than that.

because the multimeter is normally used for measuring voltage, current and resistance, it may not have been calibrated recently (or at all) for the purpose of measuring temperature, since that is one of the options that can be added by the user with an add-on kit.  the thermocouple adapter also needs to be calibrated.

in fact, the adapter is actually more important to the accuracy of this measurement than the thermocouple wire itself.  i assume your field service guy measured the outdoor air temperature by holding the meter in one hand and holding out the thermocouple probe in the air with his other hand.  the thermocouple and the meter were both at the same temperature.  as we all know from my previous articles on thermocouples, when both ends of the thermocouple wire are at the same temperature, then there is no voltage across it.  when the meter is at ambient, and the thermocouple probe is at ambient, then the meter reads zero voltage across the thermocouple.

then how does the meter give a reading for ambient?  simple -- the adapter has a second temperature sensor inside for measuring the temperature of the meter.  that second sensor is probably a thermistor or diode.  the voltage across that device when passing a given current is related to the absolute temperature of the device.  what we really need to know is the accuracy of the temperature sensor built into the adapter.  and the accuracy of the software inside the adapter that converts the voltage across the sensor into a temperature.  sometimes the circuit accuracy is very high, but the software is not very good.  to save on memory they may have used a simple linear equation to approximate the complex curve of voltage vs. temperature.  that linear equation may be good when the meter is at normal room temperature (around 20°c), but not so good when the meter is at 60°c.

meter operating range

that brings up a related but separate problem:  what is the operating range for the instrument?

the thermocouple wire is probably ok up to 500 or maybe 1000°c, depending on the insulation and how the junction was formed (a soldered junction might come apart.)

the multimeters i am familiar with are designed to work only between 0 and 50°c.  if you are measuring air temperature higher than 50°c by exposing the meter to it, then you are overheating the whole meter, and all bets are off as to the accuracy of any readings you get from it.  if the meter is rated to only 50°c, you can bet the calibration on it was done only up to 50°c, so you would be using the meter outside its calibrated range, even if it continued to give you believable readings.

it would probably be better to keep the meter inside an air conditioned shelter or vehicle, and dangle the thermocouple outside in the ambient air.  that’s where i would park myself to watch the meter if i were doing the test.

radiation shielding

you and your field guy were both smart enough to know that you had to measure air temperature in the shade, and not in direct sunlight.  solar intensity can be as high as 1100 watts/m², which is capable of raising the temperature of your instrument 50°c or more above ambient, depending on its shape and absorptivity.  presumably, the field guy took his meter in the shade someplace, maybe under a tent or behind a wall.

that is good, but not necessarily good enough.  the measuring instrument can also get secondary radiation from other surfaces that have been heated by the sun and are hotter than the ambient.  for example, if he went under a tent for shade, the tent fabric itself could be very hot, exposed to the sun on the outside surface.  it may be 70 or 80°c on the inside surface, and would be radiating infrared energy down inside the tent.  this radiation is not visible, as sunlight is, so it is hard to tell when you are truly in “the shade” completely.  so infrared radiation from the inside surface of the tent could be heating up the thermocouple and the meter.

if the instrument is in the shade of a wall, the nearby ground may still be exposed to the sun, and can reflect direct sunlight, or re-radiate invisible infrared energy toward the probe.  the wall itself may still be hotter than the ambient air if it was exposed to the sun earlier in the day, and it may be radiating infrared energy, too.

how much can these secondary sources of radiation raise the temperature of the instrument?  that is hard to estimate, unless you know the temperatures of these nearby surfaces.  but it seems possible to me that if direct sunlight can cause an error of 50°c, then secondary radiation could cause an error in the range of 10°c.

that is why the official weather measuring folks (meteorologists) always use thermometers that have several layers of radiation shields.  the shields are nothing exotic – just thin sheets of white metal or plastic that reflect any sunlight or infrared radiation away from the thermometer, while still allowing the ambient air to circulate freely around the thermometer.

figure 1.  a homemade radiation shield for a thermocouple.  the more layers the better.

for your field guy, i’d recommend that he try putting the thermocouple inside a couple sheets of aluminum foil rolled into a small tube.  the tube should be big enough so the thermocouple doesn’t  touch the foil anywhere, and that air can flow freely inside the tube.  bend the tube into a right angle at both end so secondary radiation can’t get to the thermocouple through the openings at the ends of the tube.  infrared radiation can only go in a straight line, so a zig-zag path inside the tube will stop it from reaching the thermocouple.  if aluminum foil is not handy, a couple of sheets of white paper rolled into a tube might also work. 

he could shield the meter in the same way, which would make it awkward to operate, or, as i suggested before, keep it inside the air conditioned shelter.

if you really want that world’s record, you’ll need an official, calibrated, shielded instrument.  spend some bucks on that and let me know the results.  then we’ll have a better answer on this global warming thing.

the straight dope on tony kordyban

in those twenty-odd years tony has come to the conclusion that a lot of the common practices of electronics cooling are full of baloney. he has run into so much nonsense in the field that he has found it easier to just assume "everything you know is wrong" (from the comedy album by firesign theatre), and to question everything against the basic principles of heat transfer theory.

tony has been collecting case studies of the wrong way to cool electronics, using them to educate the cooling masses, applying humor as the sugar to help the medicine go down. these have been published recently by the asme press in a book called, "hot air rises and heat sinks: everything you know about cooling electronics is wrong." it is available direct from asme press at 1-800-843-2763 or at their web site at http://www.asme.org/pubs/asmepress order number 800741.

this has been followed by his best-selling sequel, aptly titled "more hot air," from asme press. (it is the best-selling sequel he has ever written.)

this advice column is an extension of that educational effort. if you have a doggone thermal design question that you'd like tony to answer in this column, please send it to:

unlike dear abby, though, he won't necessarily limit himself to questions from actual people, but might resort to making some up to teach a lesson once in a while. engineers are sometimes too shy to share their embarrassing dumb questions in public.

copyright by tony kordyban 2005 electronic publishing by coolingzone.com with permission of author. all other rights reserved by tony kordyban.