Electronics > Metrology

poor man's calibration? (calibrate 5.5 digit with 6.5digit dmm)

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mendip_discovery:

--- Quote from: DH7DN ---Calibration is actually a two-step procedure (shamelessly stolen from BIPM VIM)  ;D
1) establishing a relation between the quantity values with measurement uncertainties provided by measurement standards and corresponding indications with associated measurement uncertainties
2) using this information to establish a measurement result

What Kleinstein mentioned ("checking of the readings") is not a calibration, it's a verification.

--- End quote ---


Calibration,
https://jcgm.bipm.org/vim/en/2.39.html

--- Quote ---operation that, under specified conditions, in a first step, establishes a relation between the quantity values with measurement uncertainties provided by measurement standards and corresponding indications with associated measurement uncertainties and, in a second step, uses this information to establish a relation for obtaining a measurement result from an indication.
NOTE 1 A calibration may be expressed by a statement, calibration function, calibration diagram, calibration curve, or calibration table. In some cases, it may consist of an additive or multiplicative correction of the indication with associated measurement uncertainty.
NOTE 2 Calibration should not be confused with adjustment of a measuring system, often mistakenly called “self-calibration”, nor with verification of calibration.
NOTE 3 Often, the first step alone in the above definition is perceived as being calibration.
ANNOTATION (informative) [4 September 2015] The VIM3 definition of calibration extends the previous (VIM2) one-step definition. For practical reasons a second operational step is introduced in order to facilitate assigning a measured value and measurement uncertainty to an item being measured by the calibrated measuring instrument. In the past this second step was usually considered to occur after the calibration. Both steps together facilitate in a practical way the demonstration of metrological traceability of measurement results (measured values and associated measurement uncertainty) obtained when using the measuring instrument after it has been calibrated.
ANNOTATION (informative) [4 September 2015] Calibration should not be confused with verification or adjustment. Calibration is a prerequisite for verification, which provides confirmation that specified requirements (often maximum permissible errors) are met. Calibration is also a prerequisite for adjustment, which is the set of operations carried out on a measuring system such that the system provides prescribed indications corresponding to given values of quantities to be measured, typically obtained from measurement standards.

--- End quote ---

Verification,
https://jcgm.bipm.org/vim/en/2.44.html

--- Quote ---provision of objective evidence that a given item fulfils specified requirements
EXAMPLE 1 Confirmation that a given reference material as claimed is homogeneous for the quantity value and measurement procedure concerned, down to a measurement portion having a mass of 10 mg.
EXAMPLE 2 Confirmation that performance properties or legal requirements of a measuring system are achieved.
EXAMPLE 3 Confirmation that a target measurement uncertainty can be met.
NOTE 1 When applicable, measurement uncertainty should be taken into consideration.
NOTE 2 The item may be, e.g. a process, measurement procedure, material, compound, or measuring system.
NOTE 3 The specified requirements may be, e.g. that a manufacturer's specifications are met.
NOTE 4 Verification in legal metrology, as defined in VIML [53], and in conformity assessment in general, pertains to the examination and marking and/or issuing of a verification certificate for a measuring system.
NOTE 5 Verification should not be confused with calibration. Not every verification is a validation.
NOTE 6 In chemistry, verification of the identity of the entity involved, or of activity, requires a description of the structure or properties of that entity or activity.
ANNOTATION (for Example 1) (informative) [5 June 2014] By "measurement portion" here is meant amount of material, of proper size, for measurement of any quantity of interest, removed from the reference material.

--- End quote ---

I quite like how they say not to confuse them but given the number of people that get confused on the matter, it is clearly not as straightforward as they would like.


Getting back to the original subject, I am guessing you just need confidence in your meter is reading roundabout the right numbers so checking a meter against the other is fine.  The issues are that you have a very big uncertainty (how sure you are of the readings) and the stability of the readings you have taken. But go for it, its 1 better than nothing. As you head deeper into the bit of despair or rabbit hole you will gradually start chasing ppm[1]s and the ppb.


[1] a recent clamp down from ILAC means UKAS are steering away from ppm as its meaning isn't clear  :-//  so now I am using µV/V and eventually X to 10-9

Kleinstein:
When you send in a meter (especially a commercial one) for calibration, the main part you get is a verification - that is the statement that the meter is still within specs. In addition there can be some explaination for the specs and reference uncertainties used - this may be important if the standards are not very good or the meter is quite good.  There is a little extra paper-work that comes with the calibration, but for hobby use this is of very limited value.
It is only if you also ask for measured values that you get more: a few point with actual data.
If required that can be an adjustament as an extra step. Normally, if the meter is well within specs there will be no adjustment.

Using a 6 digit meter to check a 5 digit meter is not so bad. One will see of the source is stable enough to get usefull readings - in most ranges I would expect the SMU to get stable enough to get a good reading fast and with reading from the display, with no need to use extra measures like recoring of both readings and than check afterwards.

bdunham7:

--- Quote from: Kleinstein on November 27, 2021, 04:56:38 pm ---Using a 6 digit meter to check a 5 digit meter is not so bad. One will see of the source is stable enough to get usefull readings - in most ranges I would expect the SMU to get stable enough to get a good reading fast and with reading from the display, with no need to use extra measures like recoring of both readings and than check afterwards.

--- End quote ---

It will depend a bit on which 6.5 and 5.5 digit meters are involved, but yes--with some care, you can do a decent job on a 100ppm meter using a 35ppm meter as a reference, at least on the lower DC ranges.  As for the stimulus, a good quality SMU will typically be excellent for this, even a very good PSU may do.  My old Power Designs 5020 is more stable and less noisy than typical mid-to-lower end calibrators.

DH7DN:
Well, now we know what "calibration" and "verification" means according to VIM  ;D

I'm just telling because I spent few hours looking for measurement uncertainties in DMM datasheets and I couldn't find them (only accuracies). They have to be evaluated separately or they are stated in a (accredited) calibration certificate. That's the part with "going down the rabbit hole"  :-//



--- Quote from: mendip_discovery on November 27, 2021, 03:28:39 pm ---
[1] a recent clamp down from ILAC means UKAS are steering away from ppm as its meaning isn't clear  :-//  so now I am using µV/V and eventually X to 10-9

--- End quote ---

Interesting! Is there any publication on this matter? Accredited calibration laboratories are obliged to use legal units for reporting their measurement results. I'm not sure about ppm or µV/V, haven't seen it yet...

mendip_discovery:
Uncertainty is a combination of various elements. Spec, imported unc, temp conditions etc etc. I did start a topic on the subject once before, I am willing to pass on my limited knowledge, I learn by making mistakes and learning from it.


I will see if I can find any reference to how unc are to be expressed on the scheduls. I get a feeling it's been a rule for a while but UKAS have just made a decisions on it. Some of them prefer ppm, and I got a larf when I suggested that the dimensional should then all meters and none of this microns and stuff like 10 + (8 × L in m).

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