Hi guys,
We have several ovens here were I work, and we have always used either:
1. A single point calibration because the oven is used at a single point at a particular process (we have a limitation label on the oven stating the exact temperature point we calibrate at)
2. A range calibration, using only 2 points
An internal auditor brought up the point that a calibration is defined as 3 points within a range, so would using 2 points not considered a true calibration?

The temperature ranges for example are:
45-85°C
40-130°C

We have the requester fill out a "Calibration Assessment Form" where they have to document their process tolerance (working range). From this information, we then determine if we have the correct standard (4:1), and to list the calibration range we will be using, taking account the process tolerance.

Example
40-85°C with a process tolerance of ±3°C.
We would then use the following test points for calibration:
37 & 88°C.

Anyone see a problem with analogy? Should we be using 3 points?

I would suggest for single points, just run cal at those points.....

For a range, low and high ends of the range and a mid-point.....if the range is large enough, say 30 C or more, add 1/4 and 3/4 points, just to chart the linear curve.....

Of course, the cal could also be done in accordance with ASTM Standards.....

Hershal

Great,


I think that linearity would be the greatest cause of variation in an oven and needs to be verified. As a simle test at home just put a thermometer in your oven and watch how accurate it really is over its operating range.

Al...

Ihave always used "3 Points is a calibration" for the reason that they equate to offset, gain and linearity. If you only calibrate to 2 points you only have offset and gain, but not linearity.

The other detail I wondered about was chamber profile. Do you profile the oven chamber? That is often more of an issue than controller accuracy. I've calibrated many ovens over the years and seen chamber inaccuracy far exceed that of the controller.

A very loose definition of the profile is verifying uniformity of the physical parts of the chamber you use for your process. The common method is a 9-point profile, where you check the eight corners of a "box" (the cubic area of the chamber used for the process), and a center point.

Hope this doesn't stir up the pot too much.

Thanks for all the replies. I forgot to mention that when the oven is new, we perform a thermal profile per mfg's specifications. We have a procedure that dictates how many sensors to use, and how uniformity is calculated. We also determine the average temp, min, max, variance, greatest difference between all sensors, and finally uniformity. We normally don't do a uniformity check again unless the oven was repaired, or baffles were adjusted.

When we check most ovens, it usually involves placing a single temperature sensor in the middle of the oven. In the past, we have used a type T SLE thermocouple, but we are now moving to using RTD's for better accuracy in order to maintain a 4:1 calibration ratio.

In our plant, we calibrate using a 10-probe hygrothermograph. We check only the temperature/s in which the oven is used. We then check the graph and if the graph is not linear, we subject the oven for troubleshooting.

Thanks for all the replies. I forgot to mention that when the oven is new, we perform a thermal profile per mfg's specifications. We have a procedure that dictates how many sensors to use, and how uniformity is calculated. We also determine the average temp, min, max, variance, greatest difference between all sensors, and finally uniformity. We normally don't do a uniformity check again unless the oven was repaired, or baffles were adjusted.

When we check most ovens, it usually involves placing a single temperature sensor in the middle of the oven. In the past, we have used a type T SLE thermocouple, but we are now moving to using RTD's for better accuracy in order to maintain a 4:1 calibration ratio.

Hi,

I have a small oven that I would like to calibrate it's used to dry parts only the range is 90°F ± 5° . Can some provide information on how to do this correctly to meet ISO requirements?

Thanks
Qulity-Misfit

Hi,

I have a small oven that I would like to calibrate it's used to dry parts only the range is 90°F ± 5° . Can some provide information on how to do this correctly to meet ISO requirements?

Thanks
Qulity-Misfit


Hello there!

There are three different checks you can do:

1. Verify the controller is accurate. Usually, you will hook up to it with a simulator and verify it's accurate. If the controller has been made within the last ten years, it should be fine.

2. Verify the controller with the sensor. You can pull the sensor out of the oven and place it in a calibrating furnace/oil bath, or use ice/boiling water with a standard probe. Although not as optimal, you can use a standard probe and place beside the control sensor while it is installed.

3. Make a performance verification of the uniformity of the oven, or verification of a "sweet spot" in an oven. Say there is one spot that is used, and it is clear to all that products don't go anywhere else, then you can verify that one spot.

All of this will involve having a traceable standard for verification. In specific applications like yours where the oven runs at a constant temperature, many drop a calibrated thermometer in the top of the oven (assuming this is a benchtop or something like that) and have the measuring tip of the thermometer inches within the product to be measured.

Assure the thermometer is calibrated and has acceptable accuracy. I would get a spare.

Hello there!

There are three different checks you can do:

1. Verify the controller is accurate. Usually, you will hook up to it with a simulator and verify it's accurate. If the controller has been made within the last ten years, it should be fine.

2. Verify the controller with the sensor. You can pull the sensor out of the oven and place it in a calibrating furnace/oil bath, or use ice/boiling water with a standard probe. Although not as optimal, you can use a standard probe and place beside the control sensor while it is installed.

3. Make a performance verification of the uniformity of the oven, or verification of a "sweet spot" in an oven. Say there is one spot that is used, and it is clear to all that products don't go anywhere else, then you can verify that one spot.

All of this will involve having a traceable standard for verification. In specific applications like yours where the oven runs at a constant temperature, many drop a calibrated thermometer in the top of the oven (assuming this is a benchtop or something like that) and have the measuring tip of the thermometer inches within the product to be measured.

Assure the thermometer is calibrated and has acceptable accuracy. I would get a spare.

It is a benchtop oven very small footprint. We do have a calibrated Fluke 177 that can measure temperature. Don't I have to write a calibration procedure for this if so what is the format?

Thanks,

Quality-Misfit

I would keep it simple. For a small drying oven you can place a thermocouple in the center and take measurements for 3 cycles to confirm that it stays within the temperature. If you only use it at 90 deg, then do it at that temperature and identify that it is used at 90 deg.

If you are using an electronic controller - especially for cycling temperatures - it is good to use a chart recorder with multiple point recording to look for things like hysteresis and uniformity. It is a condition that can be affected by the controller settings, as well as any inlet/outlet damper adjustments. For this, bringing in a cal tech with the equipment and training is usually a safer bet than doing it yourself. Don't forget, your measurement equipment (thermocouples, readouts, etc.) for evaluating the oven must be calibrated, too. :cool: