Emissivity Factor of Corderite

[Matthew]

Hi All,
I was just wondering if anyone knows the emissivity factor of corderite.   The reason that I ask is that my new infrared thermometer has an emissivity setting on it. 

Matt

[November]

The emissivity coefficient will depend on what you're aiming at.  Pure cordierite can be as low as 0.67, but just as soon as you begin to use it to bake on and carbon builds up on its surface, the coefficient will increase.  It could increase to as high as 0.96.  There is little point in calibrating your infrared thermometer for emissivity unless you are dealing with an unchanging pure substance, and only that substance.  You wouldn't want to keep changing it for reading other surfaces such as the oven itself, for a container of water, or for your dough.

- red.november

[Matthew]

Thanks RN,
I'm not really calibrating it, I have to pick a value before aiming.  It ranges from 75-96.  I wonder what the default of regular IR thermometers are set at?

[November]

I'm not really calibrating it, I have to pick a value before aiming.

It sounds to me like you're calibrating it.  I've never heard of a thermometer that you have to choose an emissivity coefficient before each use.  I would use 0.95 since carbon is in and on most things you want to measure the temperature of in the course of baking.

[Matthew]

RN,

The Features/Specifications are as follows: (Sorry I made an error in my previous post, the emissivity goes to .99 not .96)
As you can see I must select the emissivity.  When I turn the IT thermometer on it automatically goes to .75 & then I have to scroll to find my desired factor.

*selectable Emissivity: 0.99, 0.95, 0.89, 0.85, 0.79, 0.75
*  Field Of View: 12: 1 (The distance-to-spot ratio (D:S) is the ratio of the distance to the object and the diameter of the temperature measurement area. For instance if the D:S ratio is 12:1, measurement of an object 12 inches away will average the temperature over a 1-inch diameter area.)
*  Single dot laser targeting
*  Non-contact Measuring
*  Built-in Laser Targeting With On/Off Switch (inside battery compartment)
*  With White-Gray Back Light
*  Max/Min//AVG indicating
*  With C / F Selecting Switch (inside battery compartment)
*  Auto Data Hold
*  Auto power-Off after 15 Seconds
*  Low Battery Indicating
** Suitable for Food, Safety Inspect, Fire Inspect, Power Vehicle And Diesel Engine Plastic Factory etc.

Specifications:

** Temp. Range: -25 ~ 560 Deg. C (-13 ~1040 Deg. F)
** Resolution: 0.1 Deg. C/ 0.5 Deg. F
** Accuracy: -25 ~ - 10 Deg. C: +/- 3 Deg. C (+/- 5 Deg. F)
-10 ~ + 30 Deg. C: +/- 2 Deg. C (+/- 4 Deg. F)
+30 ~ +560 Deg. C: 1% +/- 1 Deg. C (2% +/-2 Deg. F)

** Response Time: 0.5 Sec
** Laser Output: <1mW
** Spectral Response: 8-14um
** Size: 180 x 96 x 48mm

[November]

The Features/Specifications are as follows [...] As you can see I must select the emissivity.

A list of features and specifications does not suggest a device's operation.  What brand and model of thermometer are you using?

[Matthew]

A list of features and specifications does not suggest a device's operation.  What brand and model of thermometer are you using?

It's exactly this unit.
http://cgi.ebay.ca/ws/eBayISAPI.dll?ViewItem&ssPageName=STRK:MEWAX:IT&item=400008752315

If you scroll to the bottom, you'll see a copy of the manual.

In doing a little more research it seems that most infrared thermometers (including Raytek) have a fixed, preset emissivity of .95.

[November]

The operating instructions do not indicate that the emissivity mode must be set with each use.  It simply provides for multiple emissivity mode options.  Are you saying that upon selection of a desired mode, and subsequently turning the device off and on again, the emissivity mode indicated on the display in position 5 (bottom) is not what you selected?

[November]

When I turn the IT thermometer on it autamatically goes to .75 & then I have to scroll to find my desired factor.

Here's a video of its operation.  The video shows the device starting at an emissivity of 0.99 after being turned on.  So either it remembers the mode last selected by the user, whenever that might have been, or it starts at 0.99 by default.  I should think it would remember the user selected mode.  If it doesn't, you might want to check the battery's voltage.

<a href="http://www.youtube.com/watch?v=vFz13Ji5jLw" target="_blank" class="aeva_link bbc_link new_win">http://www.youtube.com/watch?v=vFz13Ji5jLw</a>

[Matthew]

The operating instructions do not indicate that the emissivity mode must be set with each use.  It simply provides for multiple emissivity mode options.  Are you saying that upon selection of a desired mode, and subsequently turning the device off and on again, the emissivity mode indicated on the display in position 5 (bottom) is not what you selected?

No, it automatically defaults back to to the last setting that was selected before turning it off.  All I was really asking was; I have to pick a value before measuring, what should it be?  I'm going to take your advice & use .95 for this purpose since that's what most IR thermometers are set at anyway.

Thanks for you help RN.

By the way,  I followed your lead & coated the bottom of my corderite shelf with a sugar water solution & then baked it.  I have done it twice so far.  How many times should I do it?  Will it get darker each time or will it eventually stay the same? 

Have you noticed a significant difference by doing this?

I measured the stone yesterday while baking in the water/sugar solution & after about an hour it was at about 650 deg about 50 deg hotter than before.  Both times the stone was placed on the bottom rack of my 36" gas oven.  I read in a past post that you can trap more heat into your stone by covering the left & right sides of your oven rack with aluminum foil.  Have you tried this?  Does this theory hold true?

Matthew


 

[djones148]

Have you tried taking a temp reading of the stone at .95 and then taking another reading at .75? I'm curious to see what the difference would be.

[November]

How many times should I do it?

As many times as you want.

Will it get darker each time or will it eventually stay the same?

Until it reaches the color of carbon, black.

Have you noticed a significant difference by doing this?

Significant enough to mention it on this forum.

I read in a past post that you can trap more heat into your stone by covering the left & right sides of your oven rack with aluminum foil.  Have you tried this?

No.

Does this theory hold true?

It depends on the accuracy of your description and whether there are missing details.  As described, no, you will not store more heat in your stone as a result.  In fact, the stone would store less heat because the foil is reflecting radiation away from the rack, so instead the oven walls get hotter and the stone has to transfer some of its heat to the rack.  You would either have to reflect the radiation toward the stone or reduce the reflectivity of the foil in order to direct more heat toward the stone.  Your only chance at a hotter stone in the configuration you described is if the temperature sensor was being tricked in some way.

EDIT: Of course if your oven racks are super reflective already, then yes, wrapping them in aluminum foil would increase the rack's heat and subsequently the stone's heat.  I'm not used to seeing really shiny oven racks.  It would still make more sense to wrap the rack in something much less reflective.

[Matthew]

EDIT: Of course if your oven racks are super reflective already, then yes, wrapping them in aluminum foil would increase the rack's heat and subsequently the stone's heat.  I'm not used to seeing really shiny oven racks.  It would still make more sense to wrap the rack in something much less reflective.

Yes, my racks are in fact chrome, which would make them super reflective.  What do you suggest that I use to wrap the rack?

Matt

[November]

If you're really obsessed with trapping every photon, you can use a product called BlackWrap, Cinefoil, or just generically, black aluminum foil.  Here are some examples:

http://www.filmtools.com/blacfoilblac.html

You can come up with homemade solutions at very little cost, but these commercial products are for the serious application.  Keeping a roll of this around might even be as useful as keeping a roll of normal aluminum foil depending on whether one is trying to reflect or absorb heat.  Using a layer of it bellow one's stone, for instance, could be worth trying.