Oven Temperature Calibration 2

[jdonville]

My lab has a homebrew forced-air oven (normal operating temp 110C) that I would like to test for temperature uniformity. The relevant spec is ASTM E 145. The spec indicates the use of 9 type J or T thermocouples to sample at each of the 8 corners and geometric center of the oven.

We have other ovens that we would like to test operating up to maybe 200C.

I have Googled extensively, but have yet to find much guidance on the type (termination) and placement (e.g., orientation, method of attachment) of the thermocouples for this application. Note that there are no ports that I am aware of (other than the doors) in the sides of the oven for accomodating the leads.

I would appreciate suggestions, drawings and sample written procedures that would help me specify the termination (exposed, ribbon, sheathed, etc.) and perform this testing.

Jeff

 

[IRstuff]

Your thermocouples should be so large that the orientation would matter. Unless you have another access port, the leads are simply going to have to come through the door seal. You'll need to make sure that the resultant air gap is minimized.

As an aside, if you did have an oven that was used for active devices that generated their own heat, the ASTM method mentioned herein and and same MIL-STD-883 method will give irrelevant results, since the affect of local heat generation on the local temperature is now highly dependent on the overall air flow and distribution as well as the temperature of the incoming air.

TTFN

 

[danw2]

It is not unusual to have a jig custom welded in the shape of a cube with an additional center bar (missing from drawing) from steel angle iron. The jig is sized to fit the internal oven dimensions and to be inserted into the oven. The survey T/Cs are attached to the jig frame with a wire tie. The thermocouples can be attached so that they are either reading air temp or frame temp.

Survey thermocouples are frequently made from a roll of certified insulated thermocouple wire, with the junction twisted and welded and cut to length. The insulated wire is generally small enough to readily fit through the door seal.

Dan

 

[jdonville]

Thanks for the replies so far.

To provide additional information, the oven in question is not large - maybe 4' x 4' x 3' - and is used to dry soil samples for moisture content determination (ASTM D 2216 in case you are curious).

Questions that I still have (lack of example guidance for how to perform the test in the ASTM spec is especially frustrating) are:

1) how should the thermocouples be attached to the frame (we actually have SS wire shelves inside, so I would likely use those) to keep it in the specified position? My understanding is that the thermocouples need to be measuring the air temperature at the specified location, and I would think that the shelving/frame would conduct heat differently than the air, and I don't want the results to influenced by improper contact between the shelving and the thermocouples.

2) Are there any specific recommendations for a data collector? I have been looking at a model that has 10 channels (as I would want to monitor the ambient temperature in the room where the oven is during the test) so I don't have to fool around with moving thermocouples around inside the oven, and the test is specified to run for 24 h - which can be done during the weekend.

Looking forward to your continued good responses.

Jeff

 

[danw2]

Tie the T/C to the shelf frame with a piece of wire. Keep the junction of the T/C away from the frame a couple inches so it remains in the air.

Our shop uses a Honeywell paperless recorder, model QX, for surveys. It meets military, pharmaceutical, & aerospace (NADCAP) accuracy requirements.

http://www.lesman.com/unleashd/catalog/record/record_minitrendqx.html

 

[IRstuff]

Ultimately, it shouldn't matter if there are no other heat sources, since the mounting substrate should eventually reach the temperature of the ambient, particularly if the distribution does wind up being uniform.

Since the test, presumably, is done in an equilibrium condition, i.e., the temperature has stabilized, the wireframe and the air should be at the same temperature.

The only caveat with multiple sensors is to ensure that they are all reading consistently, otherwise there is an additional uncertainty source from the different temperature readings from the different thermocouples.

TTFN

 

[itsmoked]

Don't forget that you will need to calibrate the TCs regularly so you don't want them in mounts or wired in semi-permanent positions or you will regret it.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[m777182]

If your homebrew forced-air oven is really what you say, namelly a device with forced circulation and you want to make measurements in stationary state then there are no temperature gradients there. You will be probably confused by the noise and short term drift of thermocouples. By transients you will make your life additionally harder because of different dynamics of your thermocouples. To what precision and repeatability would you like to measure?
m777182

 

[jdonville]

m777182,

The specified operating temp of the oven (once calibrated) should be 110C +/- 5C, therefore I would need the uncertainty of the measurement to fall within the +/- 5C. From what I have read, type T thermocouples have an error on the order of +/- 1C, so there shouldn't be a problem.

itsmoked,

The E145 spec permits the use of uncalibrated TCs if they are made from the same wire lot and give the same reading when placed in the same location as each other. However, your point is appreciated.

Jeff

 

[danw2]

Commercial thermocouples or thermocouple wire is sold as "tandard limit of error" or "special limit of error"

The "special limit of error" version is generally half the error of "standard limit". Well worth the additional couple bucks.

For type T,
standard limit of error is ±1°C for the 0-133°C range
special limit of error is ±0.5°C for the 0-133°C range

Dan

 

[IRstuff]

Actually, you're just barely there. The "desired"
test accuracy ratio (TAR) is 10:1. If your thermocouples are ±1 ºC, then you have a TAR of 5:1, which is still pretty respectable.

One additional note of caution, if I was doing this, I would load the oven with same form factor and thermal characteristic objects to duplicate the air flow conditions under actual usage.

My personal opinion is that empty oven uniformity tests are marginally useful. The only things it might tell you is if the temperature controller is working properly and if the blower is marginally working properly. It positively will not prove that you have uniform temperature under use.

TTFN

 

[jdonville]

IRstuff,

Well, about the TARs, the E145 specifies TCs made from "iron or copper-constantan" - which is type J or T, by my understanding.

I had the same thought about empty vs. working conditions. Our soil samples are normally put into the oven on baking sheets to hold the individual metal containers or sometimes in metal mixing bowls. My thought was to only remove what samples I needed to in order to place the TCs. This would also minimize interference with the operation of the lab, since samples are routinely left in over the weekend.

Naturally, I would hope to verify the proper operation of the temperature controller during the test.

Many thanks for your help,

Jeff

 

[IRstuff]

As a matter of due diligence, I suppose that you might also run a drying uniformity test, i.e., you instrument different samples in different spots and determine the time it takes to dry out the samples.

While the temperature might appear to be uniform, you can't tell anything about the actual air flow, so some samples might dry faster than others.

TTFN

 

[jdonville]

IRstuff,

It's not really a concern. The ASTM D 2216 method presents guidance for determining when samples have dried sufficiently to meet the stated accuracy of the test. We generally have the specimens in the oven for at least 16 hours, which is usually more than sufficient. Thanks, however, for your suggestion.

Jeff