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WHY CALIBRATE your TEST EQUIPMENT? During preparation of the Precision Voltage Reference project elsewhere in this issue, we consulted with Trio Smartcal on the subject of calibration. This article gives a brief overview. By TONY TONG You have invested in test equipment to allow you to measure something. It could be a multimeter to measure a power supply voltage, an oscilloscope to measure waveform amplitude or a spectrum analyser to measure RF power at a given frequency, etc. In any of these cases the big question is “how important is the result you get from your test equipment to you and your company?” Many people consider calibration to be a necessary evil. However the implications and cost of using an inaccurate item of test equipment can be much greater. For example, let’s take a production maintenance technician who is troubleshooting electronic control systems. Typically his multimeter has not been checked since new (several years ago) and has had a hard life in his toolbox, being dropped a few times. It now reads 5% low. He is checking a control board for the manufacturing line and is checking the 5V board supply which has a tolerance of ±5% but it happens to be faulty and is high by 7%. The board is acting erratically but his meter tells him the supply is OK, comfortably inside tolerance. The technician then spends another four hours looking 70  Silicon Chip elsewhere for the problem. Not only does this cost the company money for his time but production has been down for four more hours and this cost the company $40,000 in lost output. In a recent case here at TRIO Smartcal, we had a customer who was setting up CCTV systems. He owned a well-respected brand of TV signal generator, about three years old. He never suspected it to output anything other than the specified signals. He had set up several hundred systems in the last year and felt that his waveform monitor (not the signal generator) was out of spec because it was not showing the picture he expected. We performance-verified the monitor and found it was well within spec. It turned out that the signal generator was out of spec instead and he had probably set up every system incorrectly for at least the last year; not what he wanted to hear! Any measuring instrument should have its performance verified regularly if it is going to be used to measure something that you rely on. And regularly does not mean annually, as many people believe. However most test equipment manufacturers do recommend annual performance verification. The cost of verifying performance and calibrating can be quite small compared to the cost of rectifying problems caused by faulty measurements from inaccurate equipment. So, how do we minimise the cost of calibration? There are several ways and one or more may be applicable to your situation: (1) IDENTIFY WHICH items of your equipment need to be calibrated. This sounds obvious but often equipment that has been superseded or not required for current operations is still being calibrated every year, just because no-one has looked into what is required. Items that are not to be calibrated should be labelled “Accuracy Not Verified – Do not use to verify performance”. (2) YOU CAN USE your calibrated equipment to check your non-calibrated equipment, provided that the calibrated equipment specifications are typically 4-10 times better than the un-calibrated item. This is called the Test Uncertainty Ratio (TUR). Measure the parameter with the calibrated equipment and then verify that the non-calibrated units indicate similar readings. This will increase confidence in the non-calibrated equipment without increasing calibration costs. More info on TUR can be found at http://www.agilent.com/metrology/ uncert.shtml (3) FIND OUT if your calibration provider offers various levels of calibration and select the best fit for your equipment. If you run a laboratory doing calibration work for your customers or your lab is NATA-accredited, then you will need siliconchip.com.au a full NATA report. This is an expensive option but you get a test report from an accredited lab. If you need to comply with ISO9001, then a traceable calibration with data is all you need. For those instruments where you like to know they are within specification but don’t need the full traceable calibration with data, then a Performance Verification or “Mini Cal” is a lower cost option. (4) YOU CAN EXTEND the calibration interval. This applies if you have an instrument which has been calibrated annually for many years with good tracking records to its performance. Then you can make use the past data reports, to identify several key measurements, extract them from the report into a spreadsheet and graph them against the published specification limits. This will allow you to predict the rate of drift. Armed with this information, you can confidently set extended calibration intervals. An example is a meter with a specification limit of 1% as a 1-year specification. Its initial cal data shows it to be -0.5%, the next cal shows the error to be 0%. The third cal shows the error as +0.5%. Hence, the drift is +0.5% per year so we can expect it to be out of specification at siliconchip.com.au the next calibration interval. If the drift was only .025%, we could predict six years (from -0.5% to +1.0%) drift and extend the calibration interval accordingly. If you buy an instrument that comes with a factory “cal-report” you already have one of the reference points needed to predict drift. Doubling the calibration interval would halve your costs for calibrating this item without impacting on the quality of the measurement. (5) TAKE OUT a calibration contract. It is often possible to take out a calibration contract with your service provider. This is normally taken for 1-3 years and discounts are available for equipment quantity and for length of contract. You may also get additional benefits, including fixed prices for the contract period. You also reduce the delays and internal cost of creating multiple purchase orders and billing events. This also improves the turnaround times due to the elimination of delays for quoting and sending purchase orders. It also allows the annual calibration costs to be budgeted more accurately, which improves your expenses forecasting. (6) TAKE OUT LIMITED calibration. This allows you to match your instru- ment’s verified performance to only that which you need. You do not pay for calibration tests you do not require. A case in point was an RF signal generator used in a laboratory. It was used in a test rack to perform only one test yet the user was getting a full calibration done every year. When the generator was tested for only the required signal, the cost dropped by 75%. This was without compromising the traceability or quality for the measurements needed for the specified task. All that was required was a label indicating that the instrument had a limited calibration. Supplementary information concerning limited calibration of the unit should be made available for a person who uses it. (7) FINALLY, YOU CAN arrange to have an expert from your calibration provider visit your facility. He can recommend ways to reduce your calibration costs and to improve measurement quality. There is usually a charge for this service which is waived if you proceed with a calibration contract/ plan. For further information or a quote for calibration contact Trio Smartcal on 1300 134 091. www.triosmartcal. SC com.au May 2009  71