By Mike Howell
EASA Technical Support Specialist

Accuracy and precision of measuring & test equipment (M&TE) have been topics of previous Currents articles (November and December 2014). A related topic that was not covered in the previous articles is resolution. The Joint Committee for Guides in Metrology document JCGM 200:2012 defines resolution as “the smallest change in a quantity being measured that causes a perceptible change in the corresponding indication.” Simplified, it’s the smallest difference that can be measured by the subject equipment. The accuracy of the M&TE must be greater than (less accurate) or equal to the resolution. That is, the M&TE must be able to indicate the value that is compared to the standard during calibration.

Accuracy and precision

Let’s briefly revisit the importance of accuracy and precision. When collecting measurement information, service center technicians obtain data consisting of two components: the actual value (true value) of the measured dimension and the error associated with the measurement (accuracy and precision components). Likewise, the smaller the measurement error, the closer the indication or measurement value is to the actual value. The terms accuracy and precision are often demonstrated and differentiated graphically using an archery example as shown in Figure 1.

Precision refers to the degree of repeatability and reproducibility in the measurement system. Repeatability is the ability of a single technician to obtain the same measurement value multiple times using the same M&TE on the same measured item. Reproducibility is the ability of multiple technicians to obtain the same measurement value using the same M&TE on the same measured item. The precision of M&TE is commonly assessed using repeatability and reproducibility (R&R) studies.

Accuracy is the degree to which the measured value agrees with the true value. The accuracy of M&TE is assessed through calibration.

Resolution

Again, we can simplify the resolution to mean the smallest difference that can be measured by our M&TE. Although our M&TE accuracy should be compared to our acceptable tolerance range for any measurement, we tend to quickly see the resolution of a gauge or meter just by observation. For this reason, the resolution makes for a good “first pass” when selecting M&TE for a specific task. That is, if you have a tool with a resolution of 1 cm and you need to measure something that has a nominal size of 1mm+/-0.1mm, you should already know you have the wrong tool for the job.

There are some obvious examples of bad choices we could identify in a typical service center. We would never think of using an industrial floor scale to measure balance weight or using a steel rule to measure the diameter of magnet wire. In these two cases, we know that the resolution of the M&TE is probably larger than the measured value; if the resolution isn’t there, the desired accuracy surely won’t be there. The selection of appropriate M&TE depends on the purpose of the measurement. For balancing, many may find that a digital scale with a 100-gram max and 0.1-gram accuracy would be suitable. But, service centers balancing small spindle rotors or extremely large assemblies may need something different.

For the magnet wire, the resolution and accuracy required to simply identify a wire size when taking data could very well be different than that required for determining if a sample of magnet wire is dimensionally within manufacturing tolerance per NEMA or IEC standards. Additionally, a wire gauge is never a good choice for measuring magnet wire.

The M&TE chosen by each service center will vary based on requirements from several sources such as customers, regulatory bodies or accreditation. Customer requirements should always be evaluated first before any business process decision is made. A service center whose most stringent customer is a local car wash may have much different M&TE requirements than one who repairs safety-related motors for a nuclear generating station. However, all service centers should choose M&TE suitable for giving them reasonable assurance in the process monitoring, test & inspection activities they perform.

When it comes to monitoring of processes, there are many gauges and sensors for most parameters that vary widely by range, resolution and accuracy. For example, if a pressure gauge used on a VPI system where processing is at 80±5 psi (5.5±0.3 bars) is to be calibrated and the gauge has a range of 0-150 psi (0-10.3 bars), it is reasonable to have a limited calibration performed, maybe from 70-90 psi (4.8-6.2 bars). Figure 2 shows a gauge that might be used this way.

Now, let’s look at a different parameter that should be controlled during a VPI cycle - vacuum. During a global VPI process, there is a dry vacuum phase and sometimes a wet vacuum phase as well. Typically, dry vacuum levels should be below 5 Torr (0.007 bars) and achieving a level of 1 Torr (0.001 bars) or less is desirable, especially for form-wound stators. The gauge shown in Figure 2 is ok for some industrial processes but is inadequate for service center VPI system vacuum measurements. Let’s examine the resolution of the vacuum portion of the scale, from 0 to 30 in-Hg vacuum. Table 1 displays unit conversions from in-Hg to Torr. If we are interested in dry vacuum levels in the range of 5 Torr and below, it becomes apparent why the gauge in Figure 2 is inadequate. One could not differentiate a vacuum of 0.5 Torr from a vacuum of 10 Torr.

This does not mean that if your service center has an inadequate vacuum gauge that you are not achieving acceptable vacuum levels – it just means that you have inadequate process control and you don’t know what vacuum level you are achieving. A more reasonable choice for a VPI system vacuum gauge is shown in Figure 3. A gauge like this may have a range of 0.2 to 20 Torr and an accuracy of 20%

Service centers should evaluate every measurement that affects the quality of the service or product provided. For each, consider the range of possible values as well as the accuracy and precision of the M&TE needed to do the job. Even for the most skilled and experienced technician, having the proper M&TE is critical for proper disposition of any component or machine.

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Categories: Technical topics, Repair procedures & tips, Service center equipment

Tags: Test equipment Measurements Calibration

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