dear all,
really need help with this one.
i’m doing calibration external for pH meter. and i received the calibration data :
pH 4 = 4,1
pH 7 = 6,9
pH 9 = 9,9
before i used it, i always do verification with buffer standard solution that i have (pH4,7&9).the result is :
pH 4 = 3,8
pH 7 = 6,5
pH 9 = 9,1
i was confused which data should i take to become standard for me?the result from calibration or the result from verification?
does anyone know which one…coz i very confused bout that.
thanks:)
I am assuming that the external calibration was performed by a competent calibration laboratory accredited to a standard such as ISO 17025.
Without knowing the method you used, I would say thrust the external calibration if the above is true.
Below are a couple of questions to think about, as the answers could have drastically affected your results.
- Are the PH solutions you used with in date?
- When were the opened?
- Are you taking temperature compensation into account?
- Is temperature compensation turned on?
- Did you wash the PH probe prior to the verification?
- Did you put the probe straight for PH 4 into PH 7?
dear buckled,
the external calibration was performed by a competent calibration laboratory accredited to a standard such as ISO 17025, thats true
but what if i say that the pH meter
about the question you ask,i’ve done everything right.
my first assumed for the result is i the pH meter is broken…(i assumed…i dont know if it right or not).
but if we take the result from the calibration data, then my question is "why does we have to do verification before use?"
i also compared it with another tool (different brand) and the result is :
pH meter that i have is 3,1 and the other pH meter is 4,2.
i become more confused…
thanks so much if anybody could help me…
Dear rika_ndry,
don´t get too worried about these results. As a matter of fact, if the pH-meter reads to ±0,1 pH-units, don´t expect to get a reproducibility better than the uncertainty. Usually a pH-meter is calibrated (contrasted and adjusted) at the moment of use with buffers of pH=4 and 9, and then verified with buffer pH=7, and that is the reason why a calibration with buffers as a scheduled external calibration makes no sense to me. During an external calibration, what should be calibrated is the temperature compensation, and the zeroing (offset compensation) at 0 V input. The adjustment of the electrode parameters is then made at the moment of use adjusting zero and slope with fresh buffers, rinsing the electrode between measurements first with purified water, and then with a small portion of the buffer to be used. Remember that electrodes age, need to keep the right level and concentration of the KCl bridge, and sometimes the junction gets clogged, so there are plenty of factors that could affect the measurements.
Best regards
Alfred
Alfred, I couldn’t have said it better.
Dear all,
I would agree with alfred.
I would like to share some points
1.pH meter calibrated according to usage in some range.
ie for pH measurement in the range 3-7.5 it is to be calibrated with pH 7.0 & then slope control with 4.0 & for higher pH it with 7.0 & 9.2 or 10.6.
also some pH meters are calibrated with all 3 std pH buffers ,so you can use itin any range.
Rika, you can refer to your instrument manual for more details.
vishwesh
dear all,
i think i become more confused…before go further may i ask a view of question for all of you :
- why do we have to do a verification for a tool before we used it? eventhough that tool already been calibrated.
- what i should do with the data calibration result and with the data verification result? should i just choose one of the data to become the standard? if i should choose standard from data calibration result…then what i should do with the data verification result.
thanks by the way…waiting for reply
Dear rika_ndry,
the verification of a measuring tool before usage is a common practice in all cases where this tool can easily be affected by the measurement itself. This is the case, for example, with pH-meters and balances, which need a daily verification too (some companies even make a verification before and after the raw material dispensing of a batch or campaign). In most cases this verification will show that the instrument is in spec, but in some cases the instrument will show a drift and needs adjustment. If you want, you can regard it as an extremely high calibration frequency.
Your working standard is the buffer and as such it needs the certification and care every standard deserves. If you have doubts regarding the buffer, get a new, certified buffer and verify the pH-meter again. In your case, in my opinion the external calibration data are useless (or at least are just one more verification) because the wrong parameter was calibrated. As I said above, the calibration parameters of the pH-meter are the temperature compensation and the offset adjust, performed by the external calibration service. The verification before use with the buffer should be recorded (readings before and after verification) to detect trends. Both (the external calibration, and the verification before use log) together make up for the calibration file of the pH-meter.
Best regards
Alfred
[quote=rika_ndry]dear all,
i think i become more confused…before go further may i ask a view of question for all of you :
- why do we have to do a verification for a tool before we used it? eventhough that tool already been calibrated.
- what i should do with the data calibration result and with the data verification result? should i just choose one of the data to become the standard? if i should choose standard from data calibration result…then what i should do with the data verification result.
thanks by the way…waiting for reply[/quote]
Dear Rika,
Calibration: The process of determining the performance parameters of an artifact, instrument, or system by comparing it with measurement standards. Adjustment may be a part of a calibration, but not necessarily. A calibration assures that a device or system will produce results which meet or exceed some defined criteria with a specified degree of confidence.
Verification: Verification is one aspect of testing a product’s fitness for purpose. verification is the act of proving or disproving the correctness of intended usage of system or equipment underlying a system with respect to a certain formal specification or property.
Those equipment which cannot calibrated means there is no option of adjust there actual position for instance mercury thermometer, you can only verify it but cannot adjust it , so it must be verify and those equipment or instruments which have an option to re adjust according to their master calibrater or standard, they are lying under the heading of calibration.
For example:-
pH meter or conductivity meter, It should be daily calibrated by their standard buffers, so they should be daily calibration.
Balances or weighing scales, It should be daily verified, b/c in most of balances there is no options of readjustment if measuring reponses are drifted from standards. these type of equipment should be daily verified
fitness for purpose.
Your all results are correct, beacuse pH meter, Conductivity meters have a tolerance of measuring outcomes so every times you have to calibrate and instrument will take automatically set the standard buffer value with internal set value, so maintain two records one for yearly or biannualy calibration + service by external agency and one for daily calibration record.
Thanks
[quote=Shahnawaz]Dear Rika,
Your all results are correct, beacuse pH meter, Conductivity meters have a tolerance of measuring outcomes so every times you have to calibrate and instrument will take automatically set the standard buffer value with internal set value, so maintain two records one for yearly or biannualy calibration + service by external agency and one for daily calibration record.
Thanks[/quote]
Dear shahnawaz,
so you saying that i need these three record (one for yearly or biannualy calibration + service by external agency and one for daily calibration record)? then if i have these three record, which result should i took for standard?
and i have one problem again. in my place there is 3 different brand pH meter. (Hanna, Schoot and Ion check).
i calibrate these three pH meter with one standard buffer solution (at same time, same buffer sol - to compare it). and i got this result :
Hanna : pH 4 = 3,9
pH 7 = 6,9
Schoot : pH 4 = 3,984
pH 7 = 6,972
Ion check : pH 4 = 3,96
pH 7 = 7,02
my queston is : which standard should i take from these three result to become pH standard in QC? there two option :
1 . from each result, i use conversion factor from each tool ex:
Hanna : pH 4 = 3,9 cf : +0,1
pH 7 = 6,9 cf : +0,1
so when i used hanna, if i get result for example :
pH 4 = 4,5 then when i use conversion factor the result become 4,6.
same with ion & schoot
or
- from all three pH result i took the minimum and maksimun range, if i took from the data the min-max rang would be :
pH 4 = 3,9 - 3,984 average : 3,942
pH 7 = 6,9 - 7,02 average : 6,96
so for standard whenever we use this three pH meter the standard always :
for :
pH 4 = 3,942 cf : +0,058
pH 7 = 6,96 cf : +0,040
so when i used hanna/schoot/ion check, if the result for example :
pH 4 = 4,5 then when i use the standard above , and conversion factor the result become 4,558.
thanks waiting for reply.
If you are using std buffer from a vender. they will have a COA which may indicating ±0.02 at 25C.
calibration: you standardize the PH meter with them (at lease 2 points cover the range). Which slope must within 95-105%.
Verification: you read the buffer again with the calibrated PH-meter, the value must with ±0.02.
The 3 ph-Meters you mentioned I think the last 2 should be OK.
Dear Rika,
I have sent the answer on your mail address.
If you have further querry, u can have talk on my skypee address.
Thanks
[quote=rika_ndry]Dear shahnawaz,
so you saying that i need these three record (one for yearly or biannualy calibration + service by external agency and one for daily calibration record)? then if i have these three record, which result should i took for standard?
and i have one problem again. in my place there is 3 different brand pH meter. (Hanna, Schoot and Ion check).
i calibrate these three pH meter with one standard buffer solution (at same time, same buffer sol - to compare it). and i got this result :
Hanna : pH 4 = 3,9
pH 7 = 6,9
Schoot : pH 4 = 3,984
pH 7 = 6,972
Ion check : pH 4 = 3,96
pH 7 = 7,02
my queston is : which standard should i take from these three result to become pH standard in QC? there two option :
1 . from each result, i use conversion factor from each tool ex:
Hanna : pH 4 = 3,9 cf : +0,1
pH 7 = 6,9 cf : +0,1
so when i used hanna, if i get result for example :
pH 4 = 4,5 then when i use conversion factor the result become 4,6.
same with ion & schoot
or
- from all three pH result i took the minimum and maksimun range, if i took from the data the min-max rang would be :
pH 4 = 3,9 - 3,984 average : 3,942
pH 7 = 6,9 - 7,02 average : 6,96
so for standard whenever we use this three pH meter the standard always :
for :
pH 4 = 3,942 cf : +0,058
pH 7 = 6,96 cf : +0,040
so when i used hanna/schoot/ion check, if the result for example :
pH 4 = 4,5 then when i use the standard above , and conversion factor the result become 4,558.
thanks waiting for reply.[/quote]
No need to get confused. bear in mind, pH meter cannot be verified. It can only be calibrated.
What you can do is get calibration buffers from reliable source, such as those traceable to NIST and get their CoA. Calibrate your pH meter on daily basis or before use ( on daily basis is more than enough). If You need to show calibration report, then calibrate only the temperature probe by 3rd party (ISO 17025 certified).
If your pH meter cannot show the value of slope, i am truly sorry to tell you that you need to refresh you calibration buffer more often. Of course, electrode aging, contamination or membrane drying may also contribute to the in-accuracy.
Lastly, never use your pH meter to verify the buffer solution (prepared on your own by mixing relevant chemicals) that you are going use for pH meter calibration. It is like create an error - your preparation (uncertainty) to give greater error - your measurement (uncertainty).