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FAQ – Calibration of Coating Thickness Gauges and XRF Spectrometers

For decades, Fischer has stood for measuring instruments of the highest quality and accuracy. We use the accumulated know-how in the field of measurement technology not only in the manufacture of the measuring instruments, but also for relevant services such as measuring instrument calibration. With regard to the correct calibration, we have summarized important questions and answers for you in the following.

In the following, you see important questions about the calibration of coating thickness gauges and the calibration of xrf spectrometers.

Calibration of Coating Thickness Gauges

Which statistical parameters should be taken into account for a comparison of measurement values?

The following parameters are important for a comparison of measurement values: arithmetical mean value, standard deviation, amount of single measurements.

Without the relevant standard deviation and number of single measuremenents, the mean values cannot be compared with each other in a  meaningful manner.

Why do I need to calibrate?

Each physical measuring method is influenced by the parameters of the coatings and the substrate. These parameters are: parts geometry, electrical conductivity, magnetizability, density of the coating, measuring surface etc.

Each time, when these parameters of coatings or substrate have changed, it is necessary to recalibrate the measurement instrument. 

I am calibrating my magnetic inductive or eddy current measuring instrument on a flat plate. Now I want to measure on a turned part with a small diameter. Will it be OK to proceed, without any further calibration?

No, the calibration on the flat plate causes a systematic measuring error on a curved surface. This means the measuring values will  be too large. This is because the instrument rates the measuring signals of the sample (here: the curved object) in such a manner, as if they were from a flat sample. Therefore, regular calibrations are usually necessary when the shape or geometry of the parts or the measuring surface changes.

Why may two parties receive different measurement results? What could be the reasons for that?

Possible causes could be that 2 measuring instruments with different calibrations (characteristics) are used or that measurements were taken with the same measuring instrument but on different measuring surfaces. The correctness of measured values obtained with measuring instruments is always ensured via calibration standards. In the case of magnetic induction and eddy current measuring instruments, calibration must be carried out on the measuring surface of the real, uncoated test objects, on which the coating thickness must also be measured for the coated parts. Furthermore, it must be ensured that measurements are taken at the same location or on the same measuring surface and that a sufficient number of measured values are recorded for a meaningful mean value as well as a meaningful standard deviation. Only in this way, comparable measurement results can be achieved.

How to verify a calibration for DFT measurements?

One checks the calibration by remeasuring the calibration foil on the uncoated sample with several measured values (usually 5 to 10). It has to be this measuring position, where you measure later on as well. Fischer Base calibration plates are not useful for this calibration. Subsequently, the user must decide which deviations from the nominal value of the foil and the measured mean value he will allow, so that the measuring instrument is still considered to be sufficiently well calibrated. The assessment of the calibration of a measuring device in the context of statistics and with regard to the uncertainty of the measured film thickness is provided, for example, by the standards DIN EN ISO 2178: 2016 "Non-magnetic coatings on magnetic base metals – Measurement of film thickness – Magnetic method" (Chapter 8) and DIN EN ISO 2360:2017 "Non-conductive coatings on non-magnetic metallic base materials – Measurement of film thickness – Eddy current method" (Chapter 8).

Calibration of XRF Spectrometers

When to use a normalization with a Fischer XRF instrument?

In the field of measurement technology, the normalization causes an adjustment of the measurement task for current settings or for new current substrates. This has to be conducted in case of modifications of primary filters or anode currents or collimators. It is also necessary, if alloy compositions or substrates of the sample have changed.

My Fischer XRF instrument seems to measure implausible values. How can I ensure, that I am measuring correctly?

You should then choose measuring equipment monitoring. You will check the XRF measurement instrument by remeasuring the calibration standards. If there is a significant deviation between the measured value and the nominal value of the standard, adjustments are necessary.

What does a reference measurement signify, with a Fischer XRF instrument?

A reference measurement is a new calibration of the energy axis. This is relevant for the correction of XRF instruments with proportional counter, on the influence of temperature.

How to check a calibration on the Fischer XRF instrument?

In order to check the calibration, it is necessary to remeasure the calibration standards within the menu position 'Product', 'Measure CAL. standards'. If you find there is a deviation, the XRF instrument needs to be recalibrated.

How often do Fischer X-RAY calibration standards need to be recertified?

It depends on how often calibration standards are used for measurements with the measuring instruments. So the customer can determine by himself. A typical value would be every 1 – 3 years.

Is it possible to stack the X-RAY calibration foils during the calibration?

Yes, it is possible. There is a rough rule for usage: for proportional counter instruments one may use 2 – 3 foils. For measuring instruments with a PIN detector or SDD it is possible to use 1 foil.

Does the plate for pure elements for the FISCHER XRF instruments need to be recertified ?

This not necessary. You don't need to recertify the plate because the elements are saturation thick and therefore very stable.

What should be done if the XRF instrument asks for β€˜Base material of cal. std. set’ and β€˜ProductΒ΄s base material’ during normalization or calibration?

Here, WinFTM is asking for the substrate material. Please place and measure the uncoated base material from the calibration standard set and the uncoated base material of the objects being measured. Caution: if the wrong parts are placed on the XRF instrument, this can seriously impact the correctness of the results!

What’s the difference between a manufacturer's certificate and an ISO 17025 certificate for calibration standards?

Calibration standards with the ISO 17025 certificate are measured according to a strict procedure as defined by the accreditation bodies; they have less measurement uncertainty than calibration standards with a manufacturer's certificate.

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