Measurement Uncertainty in Extensometer Calibration

M

midhun926

#1
hi
i have devised extensometer calibration apparatus using a electronic probe( works like a plunger type dial guage, but gives a voltage out put based on the displacement of the plunger) and DRO(Digital Read Out, it is didgital indicator). I have used a coaxial spindle in the apparatus.The working is simple.Extensometer is clamped on the spindle.When i rotate a screw , spindle moves upward, which in turn presses the probe. the the DRO ( indicator) will show the displacement in microns. It has got a resolution of 0.1 micron.The apparatus has got an uncertainty of 0.0004mm (k=2, 95.45% confidence level, )...........


1.My question is what is the uncertainty budget for the extensometer calibration? i need information about the various sources of uncertainty that can contribute to the above mentioned calibration.

2.Suppose if resolution of extensometer is 0.001mm, and if it is found to be an uncertainty source , How cal i find out the limit? how do i know which probability distribution it follows?


Thanks in advance
 
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M

midhun926

#2
Extensometer Calibration Uncertainty

I have some doubt on evaluation of uncertainty in calibration of extensometer. I have mentioned it in the attachements. Please read the attachments and give me suggestions......thank you.... I am new to this forum:)
 

Attachments

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Hershal

Metrologist-Auditor
Staff member
Super Moderator
#5
Re: Uncertainty in extensometer calibration.

This was a bit of a challenge to discern.

Each reading point had only two readings. At least three should be taken to get a good standiviation, then divide that by square root n, where: n=the number of readings for that point, and you have Type A for each reading point.

Break the Type B contributions out and list each. There is the uncertainty of each gage block used, the uncertainty of your calibrator, if you join gage blocks then there is wring, resolution of the calibrator is typically listed at half the smallest resolution, thermal coefficient of expansion or contraction needs to be included also. Any of those that are calibrated are typically going to be normal distribution so divide by 2 to get back to standard uncertainty. The others will likely be rectangular so divide by square three.

Now take your Type A and Type B contributions, square each, add them, then take square root and you are at standard uncertainty. Calculate your degrees of freedom and find the multiplier to get to 95% confidence, multiply your standard uncertainty times that, and you have expanded uncertainty.

Hope this helps.
 

Marc

Hunkered Down for the Duration with a Mask on...
Staff member
Admin
#6
Re: Uncertainty in extensometer calibration.

Thank you, Hershal!
 
M

midhun926

#7
Re: Uncertainty in extensometer calibration.

First of all thank you for your response, Hershal ..I think you can help me better if i tell you little more about my calibration methode.I have a electronic probe( just like a plunger type dial gauge,but there is no dial in that.When the plunger moves upward, it gives voltages which is propotional to the displacement of the plunger. The electronic probe is connected to indicator which displays the displacement.It has got a resolution of 0.0001mm(.1 micron)....is int it clear? ....ok ..then , i have a calibration apparatus which consists of two coaxial spildles( that is two spindles, second one kept above the first one, in vertical direction.At the top of the i keep the electronic probe such that the plunger comes in contact with the end face of the second spindle.At the bottom of the first cylinder i have a screw,which ,if rotated in clockwise direction ,will move the second spindle upwards, without moving the first spindle. When the spindle moves upwards , the plunger also moves upward and the corresponding displacement is displayed in the indicator.The apparatus is constructed such that it has a resolution of 0.0001mm.....I hope it is clear....Then i will tell you something about extensometer. It has got a movable lever. It can move upto 1mm.If it moves the corresponding displacement will be displayed in its own indicator...I will clamp the extensometer in such way that its lever comes in tight contact with secod spindle of my calibration apparatus. So when i rotate the screw,both the lever of the extensometer and plunger of my electronic probe moves same distance. The extensometer's indicator shows the displacement of the lever while my indicator shows the displacement of plunger,which is the master reading. This is how i do the calibration.Then if you go through the attachements which i have posted earlier, you can help me better....I will list down my doubts

1. You suggested me to take three readings.But I am following IS 12872/ISO 9513 which says " clause NO:5.52 Verification consists of one series of at least ten readings( that is,.1mm,.2mm,.3mm........1mm).and another set of reading after removing the extensometer and placing it back....It does not say anything about 3rd or 4th set sof reading.So type A calculation may be imposiible as it requires atleast 3 sets of reading. What do you say?

2. You said " there is an uncertainty of each gauge block used".Iam not using any gage blocks.

3. I apologise I could not get what you meant by the following sentences. Please explain it.
1."if you join gage blocks then there is wring,"
2."resolution of the calibrator is typically listed at half the smallest resolution"
3.You have advised me to take the thermal coefficient of expansion into
consideration. It must be for thatcoaxial spindle right?

I appreciate your patience and great mind which you have shown to me..Please give me your valuable suggestion
 

Hershal

Metrologist-Auditor
Staff member
Super Moderator
#8
Re: Uncertainty in extensometer calibration.

I appreciate the further explanation, it helps.

It appears you are taking measurements in a parallel arrangement, using a calibrator and extensometer in a side by side format.

If so, then the least resolution may be taken only once, that is, 0.1 micron as least resolution, take 0.05 micron as the uncertainty for the resolution, but only once if the master and UUT are in parallel.

Each of the reading you take, such as 1 micron, should be taken at least three times. Ten is better and more preferred. That is needed for development of Type A as I previously described.

You are not using gage blocks, so the wringing can be ignored.

As for a thermal coefficienct of expansion or contraction, you - MAY - be able to ignore that for now and develop your uncertainty. However, you should over the next year examine whether the items used have a similar thermal coefficient as a gage block and next year revisit the uncertainty and then include that influence.

Hope this helps.
 
M

midhun926

#10
Re: Uncertainty in extensometer calibration.

Thank you Hershal...Your reply has cleared 70% of my doubts.Just like you said , I am taking measurements in parallel arrangement, keeping extensometer and calibrator in side by side format.I have an extensometer at my office which needs to be calibrated. I will use the method suggested by you, for evaluating the measurement uncertainty. I will send you the detailed calculation...once again i thank you for your valuable instructions
 
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