Can a Calibrated Digital Thermometer be used to monitor my oven?

N

neoth

#1
hi all,

my dept is an accredited lab doing soil testing and the samples are required to be placed inside an oven.

does the oven need to be calibrated by an accredited lab?

can i just send my digital thermometer to a accredited lab and get it calibrated and then use the digital thermometer to monitor my oven? (my digital thermometer is only 1 channel) does this consider i am calibrating the oven,

will this method be able to pass thru an audit?

regards
 
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D

Duke Okes

#2
hi all,my dept is an accredited lab doing soil testing and the samples are required to be placed inside an oven. does the oven need to be calibrated by an accredited lab? can i just send my digital thermometer to a accredited lab and get it calibrated and then use the digital thermometer to monitor my oven? (my digital thermometer is only 1 channel) does this consider i am calibrating the oven,will this method be able to pass thru an audit?regards
The oven itself does not require calibration. It is the monitoring/control system which maintains the proper oven temperature which requires calibration.

Use of a digital thermometer would suffice if:

- It is accurate & precise enough for the range of temperature you want to control
- The oven is being monitored on a regular basis to ensure that the temperature is correct (these checks would need to be recorded)
- People have the ability to adjust the oven if needed to keep it within the required tolerance

Since you are an accredited lab I would look at the accreditation standards regarding calibration requirements.
 
D

DrM2u

#3
To add to Duke's post ... You are actually using the calibrated thermometer to VALIDATE that the oven controls are accurate and working properly. If you have the knowledge and ability you could even calibrate the oven controls. Either way, the calibrated thermometer provides you the necessary traceability. Do not forget to have a defined process on what you will do whenyou find the oven controls not to be within specifications (i.e. send out for repairs or calibration, replace, etc). No auditor should find a nonconformance with this practice as long as you meet the requirements in the applicable standard.
 

BradM

Staff member
Admin
#4
Good answers all. :agree1:

Kind of :topic: ever so slightly...

Ovens can also have different uniformity. Temperature variations do exist inside the chamber, from the product/ tip of your thermometer, and from the product and the oven controller.

For good process control (not part of a requirement, I don't think) assure that the thermometer tip is reading in a location that is sufficiently uniform with the product/ control probe.

Otherwise.... controller setpoint is 40C, indicator is 38C, thermometer placed in oven reads 35C, yet the product is under/overheating. :tg: Depending on temp/oven size/tolerance, you may need to define the work zone.

Just assure that you have good control over the oven uniformity.:agree1: It may not be perfectly uniform, but as long as you know what it is, you can make the necessary corrections.
 
N

neoth

#5
hi all

thanks for the reply,

i did not include the uncertainty of the chamber into my calculations and since i am using only 1 channel, i did not have a proper mapping of the working area inside the chamber,

will this be an issue?

regards
 

harry

Trusted Information Resource
#6
........................................ will this be an issue?.............
What are the risks involved (or how critical it is)? My understanding of soil testing and analysis is that the oven is used for drying the soil (removing moisture). Go figure the risk!
 
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G

George Weiss

#7
I was wondering what the difference is between calibrating your own oven/chamber and calibrating a customer’s? I am sure temperature and humidity calibrations are beyond-scope of a soil analyst facility.
I have had the privilege to work in a calibration facility which did everything, but how does this map over to a limited scope facility? Operating front panel knobs is OK, but to certifying the chamber?
I am sure the lab is technically capable, and was just refering to the pesky accreditation body's view.
I am in my retentive question hour, and am sorry if this seems off topic.
LIKELY :topic:
 
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G

Graeme

#8
:2cents:
hi all,

... can i just send my digital thermometer to a accredited lab and get it calibrated and then use the digital thermometer to monitor my oven? ...
You may have to do a little more than that ... the digital thermometer (the instrument) is only one part of a system. You also have to consider the other part of the system - the sensor (except in the rare case that the sensor is permanently attached to the meter.)

The most common type of sensor is a thermocouple, so in the absence of other information I will assume that. Thermocouples come in two grades - standard and special. For the most commonly used types, standard thermocouples have a specification of +/- 2.2 °C or 0.75% of output, whichever is greater. Considering that most digital thermometers have a specification 20 to more than 40 times tighter than that, you can see that the SENSOR is actually the major contributor to measurement uncertainty. The special grade thermocouples have a typical specification of +/- 1.1 °C or 0.75% of output, which improves things but still leaves the sensor as the major uncertainty source.

If you are actually using a thermometer with a thermistor sensor or platinum resistance (PRT) sensor your uncertainty may be much better.

In any case, the sensor (as well as the meter) should be calibrated. The minimum would be at 0 °C and the highest temperature you plan to measure; additional points are also desirable. Also, thermocouples in particular should be considered consumable items, and they can change characteristics even from bending in normal use. A regular quality check at 0 °C (in an ice point bath) will help detect shifts in between calibrations. But do not make the common mistake of connecting the thermocouple to the meter and reading that the room temperature is correct. Thermocouples generate a voltage that is a function of the difference in temperature between the closed end and the junction at the meter. If both ends are at the same temperature there is no difference and no output, so the meter just gives you the temperature of the connection socket.

GCP

 

tacom

Involved In Discussions
#9
Dear Neoth


1) İf the oven tolerance is given in the standard test method (e.g 105 C ±5C) you have to not only calibrate the oven but also to monitor oven’s temperature by using thermometer calibrated continously or pre -determinet period
2) İf the oven temperature tolerance is not given in the Standard test method or oven’s temperature is not important for the test result or oven is used only for drying the materials and drynees level is checked by weighing after drying process there is no need for oven calibration. But even so you should determine your own tolerance and monitor the oven. Because over heating may cause the soil material chemical decomposition
3) What you have done is called not calibration but verification.
4) One point check for ovens verification by using calibrated thermometer is generally accepted procedure by auditors for mechanical test laboratory.

TACO
 
D

dv8shane

#10
hi all,

my dept is an accredited lab doing soil testing and the samples are required to be placed inside an oven.

does the oven need to be calibrated by an accredited lab?

can i just send my digital thermometer to a accredited lab and get it calibrated and then use the digital thermometer to monitor my oven? (my digital thermometer is only 1 channel) does this consider i am calibrating the oven,

will this method be able to pass thru an audit?

regards
I think it may be possible for you to measure the sample directly this would mean the controls for the oven would be no longer relevant. You set the oven to obtain the temperature of the sample.
Prior to this you should experiment with the location of the sensor in the sample in the usual racks etc and take measurements to see if there is any major deviation that falls outside your process specifications. It may work if you find and use the coldest spot of the sample and set your oven to obtain the the desired sample temperature. Of course you will have to document everything but it may save you the costs of bringing in an outside calibration laboratory to calibrate the oven
 
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