In our day to day job, we may come across many many machines and instruments. Sometimes you may be operating them for a long time.
 But do you know, 
1. What is the operating principle of that machine/instrument? 
2. What are the limitations of that machine/instrument? 
3. Under what conditions you should operate the machine/instrument and under what conditions you should not? 
Please list the instruments and machines you operate or you are responsible for and try to answer the above questions.

 Now there will be a question 'why I should know all these things?'. 

  It is to avoid mistakes in the inspection/operation process. The necessity of knowing the principle and limitations of the machine/instrument you are using can be explained with some examples.
 Digital thickness meter: This instrument may be working on the principle of ultrasonic testing. That means we have to change the velocity of sound depend on the material we are checking. For example, if we set the velocity as 5900m/s and checked the thickness of an aluminium piece the readings will be wrong because the speed of sound in aluminium is not 5900m/s. That is the speed of the sound in iron. If a person doesn't know the principle of working and operating the machine the situation will be pathetic. Similarly the probe, a straight probe is used in UT thickness machine then it will give only readings perpendicular to that of the probe. There are curvature limitations for the probe. All probes may not be able to be used for the thickness checking on a 0.5"NB pipe. To know the limitations on the probe on curved surfaces we have to go through the catalogue of the digital thickness meter

 Portable XRF PMI machine: Suppose there is a mix-up of 304 and 304L pipe. You are the engineer responsible to segregate it. No identification markings are available. You called PMI for the segregation. What will happen? They are working on X-ray fluorescence (XRF) spectrometer technology. The inherent limitation of XRF portable machine is that it cannot detect the element, carbon. So if you are asked to differentiate between 304 and 304L material it is not possible. Portable XRF PMI machine cannot detect SA106 Gr.B and LTCS material. The same chemical composition will be shown in both cases.
 Digital portable hardness tester: You are asked to check the hardness in HB. The normally available hardness machines can set at different measuring standards. Your default setting was in Vickers hardness and wrote the reading as HB. These types of situations will always be there.
 Poldy Hardness tester: In Poldy hardness tester, the poldy bar and the conversion chart are different for different materials.
 Know the purpose of use and the limitations of machines while you purchase. There are many situations where issues arise due to purchase of a wrong machine /instrument.

 Welding Machine: For a welding machine the duty cycle is an important factor. You have purchased a SMAW welding machine having a duty cycle of 40 and you have welded a 60" OD pipe joint. The welding machine will get overheated and burned during welding or the welder has to stop many times for cooling the machine while doing the job. For some welding machines, it may not even capable to produce current for a 4mm of welding electrode.
 Ferrite meter: Ferrite meters are available with different working principles. Suppose as per client specification you have to check the ferrite of a pipe joint (can be checked only from outside) and you have purchased Banana Gauge type ferrite meter. What will be the use?
 Radiography X-ray machine: We have to do the RT of a pipe joint. An X-ray machine is available. But the joints are in the pipe rack. In a pipe rack, the distance between the two pipes will be very less. So the positioning of an X-Ray tube is nearly impossible. So the X-ray machine will be of no use for that situation. We have to use a Gamma radiography camera as a practical solution.

 Understand the principle, the limitations of the machine and instruments before purchase and while using. The instrument is an asset. The wrong purchase will create a financial loss and of no other use. The wrong usage with out knowing the machine will cause accept the inspections which are actually not acceptable and will cause rejections which are actually acceptable. This will also cause financial loss in terms of rework.