Understanding Calibration Part 1 - Aircraft Electronics Association
Contents
1. instruments and it allows for more effective use if you have lesser instru ments slaved to them with internal calibrations It also gives a clearer picture of exactly what was checked on the instruments Cost has become a critical and some times crippling issue for many avionics shops Fees from 300 for a scope to 1 500 for a nav generator are common and a 3 5 digit multi meter can cost 75 to 100 or more for an annual calibration with data A full annual shop calibration can top 5K per RF workstation which can seriously impact revenues especially with those items unavailable for use during calibration These issues require some good planning to minimize their impact on your business The use of redundant key items cross checked and with their cal cycles offset by six months and master slave items that allow some simple inter nally performed cals are becoming an increasingly important strategies to optimize a shop s processes as well as to help control costs and equipment availability O Part II Understanding Calibration which will be published in next month s October issue of Avionics News deals with calibration standards and strate gies If you have comments or questions about this article send e mails to avionicsnews aea net2. sible DC voltage scales and a 1K ohm precision resistor on possible scales Use the 9V battery and resistor in ACCURACY VS PRECISION 1C3 1 25 aa oS a Innacurate Accurate Accuracy to True Reference 62 AVIONICS NEWS SEPTEMBER 2008 Precise to 1 4 Precise to 1 32 series to do a simple 9mA current test and the 115VAC line voltage for an AC voltage test If values are good keep a log and use the same battery and resistor it is good to go for the day with about 95 percent confidence A known good current source to add a better current test a current limited power supply is ideal allows a more precise check if current is an important daily parameter When a value out of the expected range is seen cross check the source with a second instru ment If the source is good note the problem then send the suspect DMM for calibration or repair This prevents the day s work being performed with a bad instrument If your meter shows the Low Battery or similar weak battery mes sage immediately replace the internal battery and re check Normally cali bration is not affected by this battery change but operational accuracy can be affected More complex equipment can be checked together counter to signal generator signal generator to scope etc with a simple test or reference setting to ensure operation before daily work begins Getting into this habit prevents serious problems with the
3. INDUSTRY Illustration by Larry Stewart PART Understanding Calibration Regular Calibration and Repairs Necessary for Accurate Operations BY WALTER SHAWHLEE 2 Editor 5 Note This is Part I of a two part series exploring calibration of equipment Part I of the series focuses on the necessity of calibration and repairs for accurate operations very electronic device ever designed has issues of compo nent drift and change as well as the usual wear out damage and failure modes To avoid these prob lems impacting avionics work regular calibration and repairs are necessary to ensure accurate operation In addition to regular calibration cycles it is important to develop a solid daily self check routine to validate equipment before beginning work This daily check strategy is one of the best contributions of ISO 9000 techniques to the avionics world and it can truly help an operation if imple mented wisely Fortunately an avionics shop does not need to embrace the full nightmare of ISO 9000 certification to get the benefits of a quick daily performance verification To learn more about ISO 9000 techniques visit the International Organization for Standardization at www iso org Simple Daily Checks A simple check for using any DMM digital multi meter is to short the leads and check for zero ohms this confirms the leads are good and con siderable circuitry is working Then check a 9V battery on pos
4. day s work and ensures a higher quality of general operations It also quickly catches equipment defects before they can influence any external avionics system work Installers should perform pull checks on crimped terminals and lugs no wire should come out with their crimping tools to ensure they are not too worn and or use a size gauge to confirm their continuity test gear and any extension cables are fully functional These quick daily checks can save a world of trouble in an installation Understanding Accuracy Precision Resolution Uncertainty Accuracy is a measure of how close you are to a known standard reference it is generally expressed in terms of percent or some unit value tolerance Precision is not the same it is the degree of resolution possible with an instrument or device An instrument can be precise with out being accurate and vice versa as they are measures of different quali ties For example a ruler with divi sions only every quarter of an inch is not very precise but it can be very accurate A digital multi meter with 4 digits of resolution has consider able precision but in fact might have terrible accuracy in terms of the mea sured value rendering its precision irrelevant For a good explanation of the dif ference between these two terms visit http en wikipedia org wiki Accuracy The resolution of a measurement is a reflection of how finely the
5. dis tinct values will be displayed it is the numerical expression of the device s precision For example a simple digi tal meter reading 0 1 00VDC has a resolution of 1 percent or one part per 100 If the display is 1 000VDC the resolution increases to 0 1 percent or one part per 1 000 but it implies nothing about real accuracy only the granularity of the measurement Because all digital measurements implicitly have a quantization error A to D conversion uncertainty of at CALIBRATION Q amp A Q How accurate is my equipment A Some instruments can be inaccurate by design Consider this A 3 2 DMM implies tit can resolve one part in 1 999 or if thought of in terms of 1V one part in a 1 000 or 0 001 which is 0 1 percent of full scale expressed as 1 volt An instrument can be inaccurate in an absolute sense and still be within calibra tion For example a meter with a full scale accuracy of 0 3 percent on DC and a 1 digit uncertainty is still within cal if it reads 1 004 or 0 996 on the 1 volt DC scale This spread of values could be seen within your lab a spread of 008V between different instruments and everything still be within cal for a spec of 0 3 pecent and a 1 digit uncertainty If the scale is actually 3 2V FS the error at 1 volt DC can be 1 010 and still be at the limit of calibration although the error is 1 percent of the displayed value least plus or minus one significant fig ure it
6. e calibrations are charged by time Sometimes a customer s choice is to not adjust anything but rather sim ply note the value with high precision this sometimes is done with frequency standards so aging and drift can be plotted accurately While there is an argument to be made for this in some Q What about test cables A Test cables can be the single largest source of measurement error and repeatability problems in any shop yet they usually are ignored which is a huge mistake Test cables must be functionally tested annually and ideally daily in your check routines and they should be retired as soon as any connector wear or inter mittent operation is found Oscilloscope probes are especially easy to damage and should be confirmed as working before each daily use with the scope cal output and the probe compensa tion adjusted if needed The probe and scope must be matched and compensated correctly for proper operation Oscilloscope probes are not universally interchange able Coax cables for RF jumpers should be of high quality for high flexibility RG58C U for example not RG58A U if 50 ohms with crimped ends and heat sleeve strain reliefs to withstand repeated flexing and bending Higher quality lower loss cable must be used for microwave devices such as DMEs transponders and GPS work These often have solid cores and thus fail more often because of repeated han dling Using a network analyzer or tracking g
7. enerator and spectrum analyzer are the easiest ways to validate coax cables and can give you the exact loss in dB at spe cific frequencies Meter leads also are highly prone to flex related breaks good quality leads pay real dividends here Repairing older intermittent test leads is rarely a good tech nique as the rest of the cable is likely to be on the verge of failure Often the most economical strategy is to toss them when faults are detected unless the fault is easily identified mechanical damage on an otherwise good condition cable The lost labor costs and re work associated with damaged test cables is high so purging anything suspect is a smart and more economical choice Don t forget every lead is a thermocouple dissimilar metal junction so care is required to control low level DC errors caused by lead connections to instruments This is the source of thermal EMF errors and uncertainty at low levels It is not normally a practical issue above 1mV but is very significant in the uV measurement region 66 AVIONICS NEWS SEPTEMBER 2008 special instances virtually no avionics shop wants to receive back test gear that is not as close to the correct value as possible as any other technique becomes hard to correct for in daily work Be certain to discuss with the cali bration facility what you want and expect and be certain you understand all the costs involved Requesting data is an important step in optimizing
8. means those same displays have an implicit error of at least 1 percent and 0 1 percent on top of any basic accuracy problems Accuracy is a measurement of how closely the displayed value is to the known real value Unfortunately it can be expressed in confusing ways From a quantum perspective it can never be any closer than the basic resolution but it can be far worse Common practice is to specify the error in terms of the full scale value such as 0 1 percent FS However this can mean any measured value less than full scale can have a much larger absolute error than expected because the error is not expressed in terms of the value displayed but rather in terms of the full scale range It is not uncommon for low cost 3 5 or 4 5 imported DMMs to have serious accuracy problems on AC measure ments 0 5 percent to 1 5 percent FS is common and errors as large as 0 5 percent FS on DC and 1 to 2 percent FS for current This error effectively makes them two digital meters from a practical perspective These meters also can have huge floor specifications on top of the basic accuracy such as 5 digits which is a complicated way of saying they are just not very accurate Continued on following page The Illusion of Accuracy Accuracy of 0 1 FS Accuracy of 0 5 FS More digits of resolution do not necessarily mean higher measurement accuracy AVIONICS NEWS SEPTEMBER 2008 63 Illustration by Larry Ste
9. n the calibration world unless you fully understand the process It is also possible to have equipment fail calibration and be returned to you uncertified when in fact a tiny adjustment would have prevented this It s all hidden in the definition of cali bration services you request In general most calibration facili ties will not do any adjustment if the measured value falls within the unit specifications no matter how close to the edge of the limit it might be Unless agreed in advance this is the norm not the exception If with adjustments has been specified a limited time will be spent trying to bring the instrument into the calibration limit but not necessar ily optimized If this is not possible usually by a single adjustment the Continued on page 66 CALIBRATION Continued from page 64 instrument typically is said to fail calibration and is shifted to repair status which is a new charge In some cases depending on shop policy and prior agreement this can trigger a full re calibration with new charges In addition measurement data can be provided for incoming as received and outgoing as left measured values and various certifications accredita tion and documentation levels also can CALIBRATION Q amp A be offered Measurement data general ly is offered at an extra change which can be significant in some cases Some instruments have a flat fee for calibration som
10. ot at the end because accuracy can be expected to degrade by that time In addition the FAA has been insisting that standards be segregated from daily use items to preserve their integrity so keep that in mind in your shop planning Most shops have no way to calibrate complex nav system generators spectrum analyzers low level frequency generators or their own primary or transfer standards therefore external calibration is clearly required in these cases Be certain however the cal facility you use actually can calibrate all functions of your nav equipment Because of the obscure nature of many avionics items this is not a given even at high end calibration facilities by staff and equipment shortages Today it is important to understand exactly what you are asking for when sending equipment out for calibration and to understand how it will impact prices and what you will receive back when the calibration work is com pleted In the avionics world the implicit understanding when you have a radio in for service or certification is to return it in as close to factory original accurate condition as possible the customer reasonably expects nothing less It is not common practice in the avi onics industry to return radios espe cially nav gear with errors at the extreme specification limits and for you to do nothing other than check and return the radio without adjustment And yet this routinely happens i
11. tably be further degraded by the inaccuracy of your own specific equipment during the transfer calibration How the Calibration Industry Functions Many shifts have occurred in the calibration industry during the past few years some driven by ISO proce dures some by profit issues and some 64 AVIONICS NEWS SEPTEMBER 2008 CALIBRATION Q amp A Q Can I calibrate items internally A This is a complex issue If you have externally validated suitable standards still in valid calibration status and if you have established valid cal procedures based on the manufacturer s service manual with tolerances for them then yes you can internally calibrate items of lesser accuracy The required accuracy of your standard typically is four times better than what you wish to calibrate 10 times is preferred Many shops own nothing of good enough quality transfer standard to do other lesser items therefore they find external calibration is the best and most cost effective answer However investing in a few good quality instruments can permit you to reliably and internally cal many lesser items counters digital multi meters power supplies Basically any 3 digital multimeter can be a high quality standard for calibrating a power supply which typically has only a 2 to 3 percent tolerance Good practice dictates anything used as an internal cal standard should be used in this function primarily at the beginning of its cal cycle n
12. wart CALIBRATION Continued from page 63 Read all the specs carefully before buying any test equipment and be certain you really understand its capa bilities Many digits do not ensure accuracy only more resolution Keep in mind during calibration you have to deal not only with the errors of the item being calibrating but also the errors of your own refer ence standards as well as how they were calibrated and the physical con nections temperature and other factors involved in making the measurement These collective factors are referred to as the uncertainty of a measure ment Calibration is really about the reduc tion and control of errors and uncer tainty not their complete elimination which generally is not possible Uncertainties in calibration mea surements basically are composed of two types of variables Type A data is generally composed of statistical vari ables such as the ability to reproduce the results and to repeat them Type B data is usually the assigned statistical probability distribution from specifica tions such as accuracy resolution and transferred measurement uncertainties Type A and Type B are combined for the total uncertainty of the mea surement An example of how a cal lab could specify its best measurement capability as an uncertainty for 10 volts DC is 10V 12 uV per Volt 0 5 uV This represents the best effort possible with its equipment and tech niques and will inevi
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