Re: ??? How to Make Long Welds Without Expansion Cracking ???

"Bob La Londe" <nospam@xxxxxxxxx> wrote:
"Curt Welch" <curt@xxxxxxxx> wrote in message news:20080915190805.275
The cheaper ones don't have as large of a heat range. You have to
spend more money to get ones that go to higher temperatures. I think I
spent around $100 for one to go up to around 1000 deg.

http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=96451

HF has one that claims a range of -4°F/-20°C to 968°F/520°C on sale right
now for 29.99. I just happen to see it in the store the other day for a
lot more. I don't know if it would hold up very well, but you never can
tell. It might give me some idea of where I am at for temp anyway.
Should be enough since 5052 melts at around 1200°F anyway. Some HF stuff
seems to hold up and others fail before you get it out of the package.
They have a good return polciy though.

I have the Fluke 62 mini which sells for about $90 and a temp range to 500C
so it's basically in the same class as the harbor freight unit. The
accuracy and range seem to be about the same except the Fluke seems to go
to slightly lower temps (-20F vs -4F).

http://www.amazon.com/gp/product/B000MX5Y9C

Here are the specs:

http://us.fluke.com/usen/products/specifications.htm?cs_id=37965(FlukeProdu
cts)

The fluke also has a max temp feature so as you scan an area, the max temp
seen will be saved and displayed at the same time it's showing the current
temp. But other than that, the units look basically the same and the HF
unit is less than half the price.

Another thing I've used it for that you might be interested in is checking
the temp of my oven when powder coating.

In reading about these units to make this post, I've learned something new.

The shinier the material, the less accurate the reading. Measuring some
materials like aluminum, can be tricky because of this. I've seen some odd
numbers with my unit in the past for materiel like aluminum but didn't
understand what was going on. After some experimenting with the over this
morning, I think I have a better understand of the problem.

There's a parameter of material called the emissivity which is a measure of
how much radiation it will give off relative to a perfect black body. It's
a number less than 1. A value of 1 is a perfect ideal back body which
absorbs 100% of the light that hits it (and looks black because of that).

http://en.wikipedia.org/wiki/Emissivity

Most typical non-shinny materials have an emissivity over .90. Here is a
list of some values.

http://www.engineeringtoolbox.com/emissivity-coefficients-d_447.html

If the material you are trying to measure has an emissivity near 1, these
IR temperature units will give a fairly accurate reading. But for values
much less than 1, the accuracy will not be very accurate. The more
expensive Fluke units allow you to enter the Emissivity of the material you
are trying to measure so you can compensate for it. The cheaper units like
Mine and the one from HF don't adjust. The HF manual says that to measure
the temperature of shiny object, put duct tape on it and measure the
temperature of the duct tape instead (one more use for duct tape!). The
Fluke manual suggest you paint it black. :)

But none of the manuals talked about how much error to expect, or what the
error would be like. So I did some reading to try and understand the issue
and I did some testing with my unit.

Walking around with my unit measuring the temp of dark objects and shinny
metal objects, I couldn't see any real errors. The shinny objects were
within a degree or two of all the other objects around them.

So I put a few objects in the oven and heated it up. I put in an
alumuminum cookie tray, a piece of very shinny aluminum foil, and a cook
tray from teh toaster over which seems to be bare aluminum on one side, but
blackish non-stick coating on the other side. Heating the over to around
350, I opened the door and measured the temp of these items as there were
sitting inside the hot oven. Again, no real error noticed. There was
temperature differences of about 30 degrees depending on what part of the
oven I was pointing or what item I was pointing at but that seemed mostly
normal temperature variations - and because the door was open, the temps
were dropping fairly quickly at the same time. Nothing really indicated to
me that the shinny aluminum was creating any big error. With the exception
however that as I scanned over the shinny aluminum foil, the max reading of
my IR thermometer would sometimes catch an oddly very high reading (like
500+ when everything else was in the 340 range).

The emissivity of Aluminum can be as low as .04 from the table on the above
web page, and less shinny aluminum is in the .1 range. Which means that
the amount of radiation aluminum gives off due to it's heat is 99% less
than a black body should. If the IR thermometer is measuring this
radiation, and calculating the temp based on what it reads, then the
indicated temperature it seems to me should have been 99% below the real
reading (relative to absolute zero!!!). So why wasn't I seeing a huge
error????

After more reading, I finally grasped what was going on.

The more reflective a surface is, the more it will reflect the radiation
from _surrounding_ material, but the less it will radiate due to it's own
internal heat. How much it radiates due to it's own heat, is exactly
inverse to how much it reflects. So material with a .1 emissivity
coefficient will radiate 1% based on it's own heat, and 99% based on the
the heat of the radiation that is falling on it from surrounding objects!

So here's what is happening as I now understand it. When you measure
material with a high emissivity, you are mostly measuring the heat of that
object. That is, the IR radiation it is giving off, is mostly due to it's
own surface heat. But when you measure a material with .5 emissivity, half
the radiation you measure is based on the internal heat of the object, and
half is just reflected radiation from the objects around it. So if the
object you measure is roughly the same heat as all the objects around it,
you will notice no error at all.

This is why when I measured the objects while they were in the oven, the
temperature looked all about the same. And it's why when I walked around
the room, where everything in the room was about the same temp, the shiny
stuff and the non-shiny stuff all measured about the same.

So, to test this, I take the oven tray which is has the dark coated
non-stick coating on one side, and bare metal on the other, out of the oven
(holding it in a oven mitt) and measure the temp of both sides.

The dark side measures 327 F (basically correct for something just coming
out of an oven set to 350 F), but the shinny side measures 145 F! This was
in a room where the temp was around 75 F. The temp was dropping as I
measured it, but as I flipped it back and forth and checked the temps, the
dark side was consistently high where as the shinny side was consistently
far lower.

A little math based on the temp diff shows the shinny side of that tray had
an emissivity of about .3.

This explains why at times I've measured the temperature of a hot piece of
aluminum and got numbers far lower than expected (I think I burned myself
once when the IR therm showed something in the 150 range so I thought it
would not be too bad to touch - but it was probably more like the 300+
range and wasn't so nice to touch).

So, the whole trick is that when you try to measure something shinny (like
aluminum - even dull dirty oxidized aluminum like my toaster oven tray),
you end up measuring more of the reflection of the stuff around it, instead
of the temp of the actual unit. So if it's surrounded by material of the
same temp (like in the oven), you get a fairly accurate number, but if it's
temp is very different than what's being reflected from around it, like
when trying to measure preheat temp of a piece of aluminum sitting outside
in the open air, there's going to be a lot of error.

I guess as long as you are aware of how the error works, and can
approximate an emissivity value for the material, you can just adjust based
on the temperature of the stuff around you. So, for really shinny material
with an emissivity of .1 (clean aluminum sheet) the difference from room
temp to the temp indicated will only be 1/10 what the difference should be.
So if it measures 10 degrees above room temp, the real temp will be more
like 100 degrees above room temp.

Some experimenting by painting the surface flat black (or using duct tape)
and measuring the temp of the flat black part (the real temp) vs the rest
will be an easy way to figure out the error for any material.

--
Curt Welch http://CurtWelch.Com/
curt@xxxxxxxx http://NewsReader.Com/
.

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