Conventional Mass vs. Nominal Mass - Certify the in-house balances

C

calib_eng

#1
I have two weights , 20 g, and 2000 g, which i use to verify the inhouse balances. After getting them calibrated from an external lab. they reported the CONVENTIONAL MASS VALUE ie 20.0001 g and 2000.0001 g. now to verify the inhouse balances what values should i use either 20.0000g (actual value from supplier) or 20.0001 g (calibrated value).
thanks
 
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Tim Folkerts

Super Moderator
#2
Presumably the value from the calibration lab is more accurate. If not, why bother having it calibrated???

Also, if that last decimal place is truly important, then it is important to understand "conventional mass" which includes a correction due to the buoyant force of the air. Since the density of air (which has a surprisingly large value about 1.2 kg/m^3) can change due to pressure, temperature, or humidity, you may even need to consider those factors. As an extreme case, image trying to find the mass of a helium balloon. If you connect it to your scale, you will get a negative value, even though the balloon does indeed have a positive mass.

A google search of "conventional mass" turns up several web pages with good info on the topic.

Tim F
 
P

preetham - 2007

#3
:agree1: Yes i agree with tim's openion.
The gauges are calibarated to check whether the existing one is correct or not.and to determine what is the that we are getting in the perticular period ,so we have to fallow the calibartion reading rather than the master reading.
 
Q

quality.shesha

#4
Tim Folkerts said:
Presumably the value from the calibration lab is more accurate. If not, why bother having it calibrated???

Also, if that last decimal place is truly important, then it is important to understand "conventional mass" which includes a correction due to the buoyant force of the air. Since the density of air (which has a surprisingly large value about 1.2 kg/m^3) can change due to pressure, temperature, or humidity, you may even need to consider those factors. As an extreme case, image trying to find the mass of a helium balloon. If you connect it to your scale, you will get a negative value, even though the balloon does indeed have a positive mass.

A google search of "conventional mass" turns up several web pages with good info on the topic.

Tim F
I too agree Tim,:thanx: for the info

but you need to ensure that the calibration agency should be a certified one.
you can do this by getting their certification copy as well as their standard calibration copy....
I think Tim has given more info.
what I have tried to give is a very basic one...

shesha
 

Hershal

Metrologist-Auditor
Staff member
Super Moderator
#5
If the calibrated value is from an accredited cal lab use it.

The calibrated weights will also have a stated measurement uncertainty. You will need this in order to know what the true value of the weight is. The true value is the stated calibrated value, plus or minus the uncertainty value, with 95% confidence.....or stated another way 95% of the time, the true value is stated calibrated value plus or minus the uncertainty. Down side, by definition you will never know if you are in the other 5% unless you are way off.

That is not the same as the balance verification value, which will have its own uncertainty. For example, if the weight has a stated calibration value of 20.0001g plus or minus an uncertainty value of 0.00005g, then the true value of the weight is somewhere between 20.00005g and 20.00015g, with 95% confidence. Now apply that to the balance with all its uncertainties and you soon see that simply taking 4-5 decimal places into account does not tell you the true value of a measurement on your balance or of your weights.

Hope this helps.

Hershal
 
Q

quality.shesha

#7
Hershal said:
If the calibrated value is from an accredited cal lab use it.

The calibrated weights will also have a stated measurement uncertainty. You will need this in order to know what the true value of the weight is. The true value is the stated calibrated value, plus or minus the uncertainty value, with 95% confidence.....or stated another way 95% of the time, the true value is stated calibrated value plus or minus the uncertainty. Down side, by definition you will never know if you are in the other 5% unless you are way off.

That is not the same as the balance verification value, which will have its own uncertainty. For example, if the weight has a stated calibration value of 20.0001g plus or minus an uncertainty value of 0.00005g, then the true value of the weight is somewhere between 20.00005g and 20.00015g, with 95% confidence. Now apply that to the balance with all its uncertainties and you soon see that simply taking 4-5 decimal places into account does not tell you the true value of a measurement on your balance or of your weights.

Hope this helps.

Hershal
"Measurement Of Uncertainity"
Do you have more data on this.....
do you have any software or excel sheet to calculate this.
I believe this greatly affects when the measurement needs to be too accurate.

I feel he too needs to calculate the "Measurement Of Uncertainity" and get the calibarted value.

Shesha
 

Hershal

Metrologist-Auditor
Staff member
Super Moderator
#8
I can provide a link to an uncertainty calculation package. It is available through Agilent at no cost.

http://metrologyforum.tm.agilent.com/index.shtml

Uncertainty is generally divided into two groups.....Type A or Random, which is statistically derived.....generally the actual readings.....

Type B or Systemic is not statistically derived and includes - but is not limited to - temperature, humidity, uncertainty from calibration of your standards, local acceration of gravity for some measurements, and many other influences.

The best thing to do to begin is to list everything that can affect your measurement, and try to quantify them.

Hope this helps.

Hershal
 
C

calib_eng

#9
thanks shesha and hershal but

the format of my verification form is


applied value 20.0001
measured value 20.0001
error 0.0000


what i wanted to ask was what value should i put in the applied value column, becuase nominal mass was 20.0000 as informed by supplier, and calibrated mass was 20.0001 plus minus uncertanity, now wich way should i go, should i neglect the minor errors after calibration and use the standard supplied value of 20.0000 or should i use the value 20.0001 and calculate the error of balance.
hope i was clear to ask.
thanks
waiting for reply
 

Hershal

Metrologist-Auditor
Staff member
Super Moderator
#10
What this means is your balance has insufficient resolution to see a difference.....that is NOT a bad thing, it is just reality....

Stay with the calibrated value.....in this case the Type B influences likely will drive your uncertainty.

Hershal
 
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