Black Test Corner for Temperature Testing

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D_Wood

First off, I am not certain if this is the correct location for this thread. I am responsible for maintaining the accreditation of our testing lab as it pertains to various client-test data programs issued by various NCB's, which are all governed by ISO 17025.

I would like to discuss the requirement of using a Black Test Corner to perform temperature testing.

In the world of compliance testing per International Safety Standards, it is becoming the norm to see reference to a "Black Test Corner" as a requirement when performing temperatrure testing (heating). Typically, this relates to wording such as "the product (appliance, etc.) and its surrounding area shall not attain excessive temperatures during normal use". The key word that seems to come into play here is "surrounding area". I'll come back to this in a moment.

For reference, a Black Test Corner consists of a draft-free corner with 3/4" (20mm) plywood on the joining walls, painted flat-black. A grid of holes, 7mm dia., spaced 100 mm apart covers the entire surface of the plywood, making it resemble pegboard. The holes are used to place blackened copper or brass discs with thermocouples attached to them. These discs are placed in only enough holes needed to cover the surface area ralative to the product being tested. The number of discs to be used in itself has led to many heated discussions throughout many industries. The thermocouples are then connected to a recording/measuring system, either a DAQ module & computer or a handful of meters.

Now, back to the "surrounding area". My arguement is that thermocouples placed on the outer surface of a product would reveal the maximum temperature rise expected from the product during normal use. These recordings would address whether a surface was too hot-to-the-touch, or hazardous to nearby walls, ceilings, etc. Thus, why would the Black Test Corner and an array of thermocouples strategically placed on a wall be necessary to determine this?

A study was performed and documented by G. Beges (and others) back in 2000. Their arguement leans toward the quest for a perfect "repeatable and traceable" testing system for accredited testing laboratories, and highly recommends the Black Test Corner along with a computer monitored DAQ setup. To me this is all fine and dandy, however, the intent of the test should not be overlooked. If there are other test methods that produce valid results and the same conclusion, then why shouldn't they be accepted as well?

I am interested in comments from others who have encountered the need to construct a Black Test Corner, and any circumstances in which your test data may have been accepted using alternate methods; such as thermocouples attached to the outer surface of the product, etc.

Thank you,
D_Wood
 

Hershal

Metrologist-Auditor
Trusted Information Resource
This test is very common for labs that test wood fired and even gas fired heaters and similar items.....the reason for the thermocouples on the wall as opposed to the heater (or whatever device is being tested) is to determine the radiated heat as opposed to a direct thermal transfer which would happen if the thermocouples are directly on the item. As you have no doubt seen, the corners are not immune from catching on fire.

I can't disagree that a combination of thermocouple arrangements may give a truer picture, but currently the ASTM, CSA, and other standards that descibe this test method do not specify that.

That does not mean it can't be done by your lab.....just that now it would be a standard method used outside the intended scope (ISO/IEC 17025 Clause 5.4.5.2), or the other option is to have that specific part as a lab-developed method (Clause 5.4.3). Either way, validation must be performed and the customer must sign off on the validation and use of the test method (Clause 5.4.5.1, 5.4.4). Also, part of the validation should include an uncertainty study (Clause 5.4.6.2) so the lab knows what the potential error is.

Hope this helps.

Hershal
 
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D_Wood

Good reply Hershal.

Since my post I have submitted my point to a member of a safety standards work group. To be a little more specific, I have tailored my point to address the requirements from the "General Requirements" for household appliances (60335-1) and the need for an addition to the particular standard 60335-2-2.

I agree with the requirement as it would pertain to a stationary appliance, but present my point as it pertains to a portable appliance or hand-held appliance that is intended to be moving during normal use. I suggested a proposal to be considered that would address a more specific situation. As you know, "general standards" have to cover such a wide range of products, thus the reason for the particular standards or "Part-2's" as they are sometimes called.

Also, it is not that I am so much against a draft-free test corner, it is just the relevancy of the complex grid of thermocoupled discs on the wall as it pertains to some product types.
-D_Wood-
 

BradM

Leader
Admin
OK, this black test corner intrigues me.

So you are placing the discs around the item of interest to measure the heat generated by the device. So, you would take a toaster, for example, place the tool and sensors around the outside of the toaster (like sitting on a countertop) and measure the heat being dispersed.

This is more of a safety/environmental test regarding the product, and not any uniformity/performance factors of the product.

Is this accurate?
 
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D_Wood

You are correct Brad. This is a safety test.

The "discs" are placed in the holes of the plywood mounted on the wall. Your toaster would be placed close to the wall and operated. The temperature rise recorded would be compared to the requirements depending on the particular safety standard for the product.
-D_Wood-
 

BradM

Leader
Admin
I did want to follow up on this.

First, I would suggest they let YOU write the procedure for this. By reading your posts, you have a real talent for writing.

Obviously by my last question, I am unfamiliar with this test. However, several things did come to mind.

A quick review of the internet provides the various problems typically associated with thermocouples. It has been taught to me that thermocouples are not good temperature measurement devices at or around room temperature. The millivolt potential is very small.

I would think using thermistors or RTD sensors would be more advantageous.

Also, I would see having the exposed thermocouple bead showing, so as to maximize sensitivity to temperature changes. Placing them in discs seems they would greatly slow down any sensitivity to temperature change.

As far as how many thermcouples to use, I would be totally guessing at that. I know there are specifications for mapping ovens and chambers (based on cubic feet), but nothing like this setup.
 

Hershal

Metrologist-Auditor
Trusted Information Resource
BradM, you are correct that RTDs are more precise, however they are also much more expensive and not quite as robust as thermocouples.....this test can easily use up to 100 thermocouples, depending on the corner size.....and only the corner is enclosed.....unless it is the full room version of the test, the corner is exactly that, a corner.....the rest is basically open.....

Just a little background.....

Hershal
 
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David Cailler

Does anyone know where you can purchase the 15mm X 1mm brass/copper discs off the shelf? I've been looking but so far had no luck.
 

Jim Wynne

Leader
Admin
Does anyone know where you can purchase the 15mm X 1mm brass/copper discs off the shelf? I've been looking but so far had no luck.

There's this place where it appears that the 1/2-inch by 18 gauge would be pretty close to your dimensions. Click on "Metals" on the left side of the page, then look under "Copper."
 
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