In the shop -- measuring PCB temperatures in the cooker

My cheezy Chinese temperature meter ($9 on eBay, but it looks like a Fluke* from a distance so it must be good) seems to do a pretty good job
of measuring the temperature of my skillet when I'm doing skillet reflow, but not so good at measuring the temperature of the actual board.
I'm doing an experiment with soldering two-sided boards of soldering all the little components on the back side, then spacing the board up 1/8" and doing the components on the front side, while the back-side components stay stuck with surface tension. It's worked well once (and maybe twice -- that board is cooling as I type).
But both times the temperature reading on the circuit board itself have obviously been wildly off -- seeing a reading of 120C on a board with melted solder is a clear indication that the meter and the solder have different opinions of the board temperature.
The thermocouples themselves are the welded-bead type, and seem to track fairly well when they're both in good thermal contact with the skillet -- although I'm wondering if those temperature readings are really accurate.
So -- any suggestions on accurately measuring board temperatures in a reflow oven? Is it just that a welded-bead thermocouple, even with some thermal grease, is not the right choice? Is there some other thermocouple that will do, or should I try measuring the thing with an IR thermometer or other means?
I'm open to suggestions.
Thanks.
* "If it works, it's a Fluke!" I'm not sure why they never adopted that as a company slogan.
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Tim Wescott
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On Fri, 22 Jan 2016 14:54:12 -0600, Tim Wescott wrote:

Argh -- it was NOT that hot. I'm not sure how I hallucinated a good solder joint (distance and geezer eyeball may have contributed), but I hit on the notion of clipping a bit of solder off of the roll and leaving it on the board -- when it melts, I'll know.
Oh boy -- thrice-baked circuit! I wonder how it'll be?
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Tim Wescott
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On Fri, 22 Jan 2016 16:04:05 -0600, Tim Wescott wrote:

Updates in real time! The thermocouple read somewhere between 180 and 190C at the point where I noticed that the solder lump had melted. Since I'm being a bad boy and using 63/37 solder I guess that it's reasonably accurate.
I need to cobble a fan onto my entirely cobbled-together apparatus -- with the board spaced off of the skillet it seems to heat faster if the air is moving around.
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Tim Wescott
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"Tim Wescott" wrote in message
On Fri, 22 Jan 2016 16:04:05 -0600, Tim Wescott wrote:

Updates in real time! The thermocouple read somewhere between 180 and 190C at the point where I noticed that the solder lump had melted. Since I'm being a bad boy and using 63/37 solder I guess that it's reasonably accurate.
I need to cobble a fan onto my entirely cobbled-together apparatus -- with the board spaced off of the skillet it seems to heat faster if the air is moving around.
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Tim Wescott
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On Fri, 22 Jan 2016 17:31:15 -0500, "Carl Ijames"

"Heat quicker" doesn't matter. Cast iron in the oven and Al in the oven still come up together because the heat the oven makes gets infused evenly across the entire mass, and the dwell of the solder process itself is longer. Al is good for moving thermal energy, but not retaining it uniformly through a mass. The cast iron is better because it RETAINS the heat more uniformly better. He could get creative by cutting the apron form the skillet and leaving the bottom plate and perhaps a 3/8 inch side apron (1cm or so).
The oven is a good idea because it would heat from the top as well as the bottom. He could thermal epoxy the thermocouple to the skillet plate or even drill a hole in it from the side and insert the TC all the way into the hole. Placing one more in the air above the PCB assembly will likely yield what temp the PCB has been brought to/near.
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The best way I know of to measure temperature on a board is to borrow a diode junction for the sensor: http://www.ti.com/lit/an/sboa019/sboa019.pdf
Transistor base-emitter junctions work fine and the rest of the circuit doesn't need to be powered.
You can use the 183C melting point of 63/37 eutectic lead solder to check the calibration.
Measuring correctly with thermocouples is difficult unless they are embedded within the material. I clamp a thermocouple between two copper washers under the lid knob of my Farberware pots, to know when they reach boiling on the woodstove downstairs. The readout is about 10C lower than the same probe immersed in the pot.
-jsw
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On Fri, 22 Jan 2016 19:50:18 -0500, Jim Wilkins wrote:

So do you think a diode would do better than a thermocouple as far as actually being the temperature of the board?
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On Fri, 22 Jan 2016 19:36:49 -0600, Tim Wescott

The thermal response of diodes is *extremely* linear within a range of sensible temperatures. I made a tire thermometer out of one when I was racing, on the recommendation of an electronics engineer. Checked against a precision scientific mercury thermometer, it was +/- about a half-degree C over a two-decade range.
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On 01/22/2016 08:47 PM, Ed Huntress wrote:

Two decades is e.g. 4.2K to 420K. Pretty good quality mercury, that. ;)
Cheers
Phil Hobbs
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Phil Hobbs wrote on 23/01/2016 08:29:

How many years are there in a decade and in a century in Phil's world?
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On 01/23/2016 08:49 AM, jnLsS? ?????? ? ??????? ?dRIfd wrote:

I was making a funny--note the smiley. There are two competing uses of 'decade', as in "the Seventies", i.e. 10 of something, as in "decade box", i.e. powers of 10.
Cheers
Phil Hobbs

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On Sat, 23 Jan 2016 08:29:35 -0500, Phil Hobbs

Two decades of Celsius from 1 deg. C, , not of Kelvin. <g>
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On Sat, 23 Jan 2016 09:50:48 -0500, Ed Huntress

Ok, you mean from 274K to 27400K? ;-)
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Jeez, everybody's a critic. <g> My old scientific thermometer, which I broke years ago when I left it in my freezer and mistakenly threw a frying chicken on top of it, ran from something like -30C to 250C, or thereabouts.
My analog electronic tire thermometer was calibrated in deg. F, from 90 deg F to 220 F, but I tested it down to 32 F.
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On 01/23/2016 09:50 AM, Ed Huntress wrote:

Wow, if you went down to 0.001 C, you could have five decades.
Cheers
Phil Hobbs
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On Sat, 23 Jan 2016 12:21:38 -0500, Phil Hobbs

You could have lots of things -- except common sense. d8-)
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On Sat, 23 Jan 2016 12:50:57 -0500, the renowned Ed Huntress

Is this the part where the physicist who wandered into the wrong coffee shop gets roughed up?
--sp
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Best regards,
Spehro Pefhany
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On Sat, 23 Jan 2016 14:27:44 -0500, Spehro Pefhany

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Spehro Pefhany wrote:

Every coffee shop I've ever been in has mostly college-age women all hours of the day. I think you mean maybe he was at the wrong bar?
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It would likely do better at actually being AT the temperature of the board instead of at some average of the board and the thermocouple's wire leads.
I have better luck measuring surfaces with 30 gauge thermocouples. Try measuring the external temperature of a boiling pot of water with a thermocouple to see how tricky it can be.
-jsw
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