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

curt@xxxxxxxx (Curt Welch) wrote:
"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(FlukePro
du 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.

Ah, I just realized something else.

Without knowing the physics, I was thinking the IR radiation from objects
to be be fairly constant based on their heat. So if there was an issue
with shinny material reflecting IR light, that would only add to the amount
of radiation coming from the material and cause an IR thermometer to
register too high. But then reading about it, I learn that different
objects give off different amounts of radiation directly linked with how
much they reflect. So the more IR radiation an object will tend to
reflect, the less it will also tend to radiate from its internal heat by
the same factor. So aluminum, which has a very low emissivity value,
reflects a lot of IR, but also, doesn't give off much when it's heated.
And duh, that's what just clicked with me. Unlike iron, which glows very
nicely when it's heated, aluminum doesn't glow when you heat it (which
makes it that much harder to figure out when you are about to melt a big
hole in it).

But, according to what I've read today, it should glow with basically the
same frequency spectrum of iron, but with just far less intensity (that is,
adjusted by it's emissivity value). I don't think I've ever noticed any
glow from aluminum when I've welded it. Is this just because I was always
in too bright of a light when heating aluminum to notice any glow from
it???

Now I feel the need to go do a science experiment and heat up some aluminum
and turn off the lights! :)

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

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