Could we calibrate a digital thermometer like this way?

Hi All,

For some kinds of digital thermometer (with probe to measure internal temperature and infrared to measure surface one) we could check if they are accurate (such as checking with melting ice) but we can not calibrate them because there is no adjusting part on them. Could we indirecly calibrate like this way?

For example: if temperature measured by probe is 1.5 oF higher than that of melting ice (32oF). A note "substract 1.5 oF" is attached on the thermometer and close to the probe for easy remembering.

When this thermometr is used to measure a temperature of a product, the valule of the temperautre to be written down on the monitoring form is the displayed value minor 1.5 oF (such as displayed temperature is 34oF, the temperature to be written down on the monitoring form is 32.5 oF).

For the infrared, the same way is used.

I thought of sending back to the manufacturer but they advised me to buy the new one because the calibration cost is higher than that of buying the new one. I have asked some regulatory food inspectors and they do not like this idea.

Please advise if you have any experience on this issue.

Thank you

Dear Quang Nguyen,

I used to calibrate digital thermometer against master MIG thermometer. Since there is no adjustment, I give correction factor to the reading. In my experience the best digital thermometers are in Japan. They were as good as master thermometer. I donot have the brand name.


Regards
Sujit

Hi All,

For some kinds of digital thermometer (with probe to measure internal temperature and infrared to measure surface one) we could check if they are accurate (such as checking with melting ice) but we can not calibrate them because there is no adjusting part on them. Could we indirecly calibrate like this way?

For example: if temperature measured by probe is 1.5 oF higher than that of melting ice (32oF). A note "substract 1.5 oF" is attached on the thermometer and close to the probe for easy remembering.

When this thermometr is used to measure a temperature of a product, the valule of the temperautre to be written down on the monitoring form is the displayed value minor 1.5 oF (such as displayed temperature is 34oF, the temperature to be written down on the monitoring form is 32.5 oF).

For the infrared, the same way is used.

I thought of sending back to the manufacturer but they advised me to buy the new one because the calibration cost is higher than that of buying the new one. I have asked some regulatory food inspectors and they do not like this idea.

Please advise if you have any experience on this issue.

Thank you

dear Quang,

i have seen people using the same procedure with HACCP certification for their digital equipments calibration. but this needs to be calibrated against the standard thermometer. the difference in reading you find can be mentioned and On Job Training can be carried out for monitoring staff and record to be kept for auditor review.


regards.


Riz

Agree with RIZ.. calibration thermometer should be compared by Standard Thermometer, so you can get valid data for error value and you can use this value to adjustment your temp. read.

We check the temperature of the water. The water is used to clean the tankcars. The water needs to be > 74 C otherwise the system will be shut down by the computer. To check if the temperature on the digitale display is ok wel check the water with a different thermometer (infrared, which is calibrated). All the values are kept in a log to see if there is any difference. The digital thermometer in the installation is also checked yearly by the manufacturer.

I have asked some regulatory food inspectors and they do not like this idea.

The problem is that inspectors expect to see a "official" standardised method despite the fact that the result compared to short-cut methods is probably not much different operationally speaking. They are aware that official methods involve items like traceable master thermometers . Personally, I invested in one master digital one (doesn't hv to be incredibly expensive) which can be externally re-calibrated anually (> traceable). The choice of buying a zero resettable one to be adjusted (or not) at same time is up to you. It is certainly easier to use resettable units for routine use however i hv used yr method after comparing against a master unit with no auditor trouble. (It is not so difficult to adjust the formula for the routine unit "calibration factor" if the master thermometer is not reset to zero.)
Another limitation in ice method only is that it is sometimes necessary to measure temperatures substantially away from 0degC. Inexpensive digital units are often unfortunately not that accurate if you look at their usage specifications.

Rgds / Charles.C

Thanks to you guys

Actually, when I check my digital thermometer in melting ice, I do use a master thermometer. The reason why regulatory inspector does not like is the reading value is not the value recorded in the monitoring form when it is used to check temperature of a product as I gave an example.

Is there any different solution for that issue?


Regards,

Quang

I

s there any different solution for that issue?

It's called educating the inspector.

I wrote a SOP to explain the correction, ie presented the formula.

Then I validated it.

Then I referenced it on the monitoring form.

Rgds / Charles.C

I don't like this method because it makes a big assumption; that is that melting ice will melt at zero celcius. It only will if the ice is pure. How certain can you be of the purity of the water used to make the ice? Not very I would imagine. Any solutes will depress the freezing temperature of ice (and you should note that solutes also raise the boiling temperature of water). Why don't you try it by dissolving some salt in water and seeing the effect?

That means you can't take off a constant nor add a constant by using this method. (I would also argue that you can never make a correction by measuring a single point anyway.) So I completely disagree that it's about educating the auditor!

I would argue that you could go to a different manufacturer and purchase some test caps to be able to do internal calibrations, say once a week and then get them externally calibrated once a year vs. a reference probe. I took this approach and whole kit probably cost around £180 and then there's approx a £40 cost a year. If you're using it for CCP monitoring, I would say it shows lack of management commitment not to spend that kind of money!

Dear GMO,

I think you misunderstood or, more likely, I was insufficiently clear.

Procedure is to use master thermometer, do external calibration which gives a (hopefully) constant correction for use if no reset master to zero. Then use this master to measure temp. of ice water at same time as insert thermometer "X".
I agree with you, the ice is never zero hence you get a (variable) correction for this parameter. The external calibration people I use claim they use specially crushed ice from distilled water but frankly I don't believe them. I hv also seen other external companies use a purely electronic simulated system, ie black box, which is pre-calibrated.

In the end, I am still not sure it makes that much practical difference to assuming ice is zero degC.
IMEX, the 100degC point is more tricky, heat losses down lines etc.

Rgds / Charles.C

To make things simple, on whatever form you record your temperatures, you could just include a line or spot for a "correction factor" and an "adjusted temperature." The daily temperature monitoring forms for our incubators have these boxes included. That way it's clear both what your correction factor is, and what the final, recorded temperature is.

I still disagree that you can correct for a miscalibrated thermometer in this way as the offset is not constant over the range of use.

Why do you not believe the calibration company? If you are that unsure why are you using them?!!!