Oven Temperature Calibration
Oven Temperature Calibration
(OP)
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
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





RE: Oven Temperature Calibration
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
RE: Oven Temperature Calibration
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
RE: Oven Temperature Calibration
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
RE: Oven Temperature Calibration
Our shop uses a Honeywell paperless recorder, model QX, for surveys. It meets military, pharmaceutical, & aerospace (NADCAP) accuracy requirements.
http://ww
RE: Oven Temperature Calibration
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
RE: Oven Temperature Calibration
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Oven Temperature Calibration
m777182
RE: Oven Temperature Calibration
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
RE: Oven Temperature Calibration
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
RE: Oven Temperature Calibration
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
RE: Oven Temperature Calibration
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
RE: Oven Temperature Calibration
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
RE: Oven Temperature Calibration
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