The Duncan Download Blog: Business Aviation Advice & Observations

Contributed by James Hood, Team Leader Calibration Services

The 4:1 Test Accuracy Ratio is a guideline to minimize the tolerance errors.

Occasionally here at Duncan Calibration Services we need to inform our aircraft tool calibration customers their tool’s Test Accuracy Ratio (TAR) is less than 4:1 and ask if they are willing to accept a calibration with this decreased test accuracy ratio. The most common response we hear is, “What is a test accuracy ratio?”

Test Accuracy Ratio

TAR is a ratio of the accuracy of a tool, or Unit Under Test (UUT), and the reference standard used to calibrate the UUT. Metrology labs strive for a minimum 4:1 TAR. Simply put, this means that the standard is 4 times more accurate that the tool being calibrated. A test accuracy ratio of 1:1 indicates the UUT and the standard have the same tolerances. A TAR of less than 1:1 (0.5:1 for example) indicates the UUT is more accurate than the standard.

TAR Example

For the sake of simplicity, if the accuracy of a pressure gauge at 100 psi is +/- 4 psi. The standard pressure gauge needed to test the UUT would have to have an accuracy of +/- 1 psi to maintain a 4:1 TAR. If exactly 100 psi of pressure is applied to both gauges the UUT can read anywhere from 96 psi to 104 psi and still be in tolerance. The standard pressure gauge would have to always read from 99 - 101 psi to be considered in tolerance.

The 4:1 TAR is a guideline to minimize the tolerance errors a reference standard may have, such as being near the edge of its specified limits. The higher the TAR the more errors from the standard are reduced. Most of the time, this is not a problem. Under normal circumstances our standards are able to achieve a 10:1 or sometimes a 1000:1 TAR.

The test accuracy ratio or TAR can be a simple calculation to verify if the standard used to perform a calibration is accurate enough to calibrate the UUT.

Duncan Aviation's Calibration Services has test and calibration capabilities for a multitude of aircraft tools and instruments that routinely require calibration. Contact us for all your instrument and tool calibration needs. Save money on your instrument calibrations by bringing your tools along on your next Duncan Aviation maintenance event.

James Hood is the Team Leader for Duncan Aviation Calibration Services. He specializes in keeping all the tools utilized by Duncan Aviation’s aviation technicians and customers calibrated and ready for service. His aviation career began in 1999.

Contributed by James Hood, Team Leader Calibration Services

At the Duncan Aviation Calibration Lab we are often asked to clarify the difference between aircraft tool precision and tool accuracy.

Precision

Precision is the measure of exactness. It refers to how closely each unit of measure agrees with another. A precise tool will get the same measurement each time.

Accuracy

Accuracy is a measure of rightness or how close a measured value agrees with the correct value. An aircraft tool is accurate if it correctly reflects the size of the unit being measured.

A tool can be precise and not accurate, accurate but not precise or both. For example: a tape measure with increments of 1/32^nd of an inch is very precise. The distance between measure marks is small. However, when measuring a bolt known to be exactly one inch, a tape measure with 1/8^th inch increments may be more accurate.

It is important to note that just because an aircraft tool is more precise does not mean it is more accurate and accurate tools do not have to have the best precision. The best tools are those tools that have the precision for the job with a good enough accuracy.

Duncan Aviation's Calibration Services has test and calibration capabilities for a multitude of aircraft tools and instruments that routinely require calibration. Contact us for all of your aircraft instrument and tool calibration needs. Save money on your instrument calibrations by bringing your tools along on your next Duncan Aviation maintenance event.

If you have any questions or comments about this, contact Duncan Calibration Services at 402.479.1698.

James Hood is the Team Leader for Duncan Aviation Calibration Services. He specializes in keeping all the tools utilized by Duncan Aviation’s aviation technicians and customers calibrated and ready for service. His aviation career began in 1999.

Contributed by James Hood, Team Leader Calibration Services

Calibrated tools can and do wear out over time. How quickly this happens depends upon the quality and type of tool along with storage practices and environmental conditions. Physical or dimensional tools have moving parts that degrade or become distorted and electronic tools have components that change value.

Spring & Pivot Block—Quality makes a difference

The quality of materials used to manufacture the spring and block inside a click-style torque wrench can influence the accuracy of the wrench over its lifetime. The spring can lose its shape or compress differently depending on usage and storage. The highest torque setting keeps the spring in a compressed state. When the setting is lowered the spring may not return to its original shape, therefore causing incorrect readings. The pivot block inside the unit is a rectangular block of metal. Over time, usage will cause the edges of the block to round, causing the torque wrench to click at an incorrect setting.

Tool Storage Recommendations

When calibrated tools are not in use, the recommended setting for storing a torque wrench is at the 20% setting. If torque wrenches are stored on their highest setting for an extended period of time the spring may not fully decompress. On the other extreme, if the torque wrench is stored at the 0 setting, the pivot block can become tipped or roll. The slight tension on the spring is necessary to keep the pivot block in place without distorting the spring.

Electronic Test Equipment

Environmental conditions (temperature and humidity), internal heat, severity of abuse and age will cause electrical components inside electronic test equipment to deteriorate and change in value. This change in value changes the overall accuracy of the test equipment. However, during calibration, internal adjustments in most electrical test equipment can bring them back to their nominal settings. A low battery can also have a negative impact on the accuracy of meters. When the low battery indicator is on (most meters have these), the battery should be changed soon.

Most calibrations are performed annually to verify normal deterioration of the tools do not change the readings so far that they are out of the published tolerances. If the unit is near or outside the tolerance, the calibration technician can make the adjustments to bring the readings back to the nominal settings. If the tool is at or near the tolerance for multiple calibrations, the tool may need to be calibrated more than once a year. At Duncan Aviation, we see many tools with reduced cycle times. Calibrating them more than once a year limits the number of times the unit is found out of tolerance.

When the tool passes calibration it is given a clean bill of health in the form of a calibration certificate. The calibration certificate states the tool was found “in-tolerance”, “out-of-tolerance” or if any limitation tolerance exist. The certificate should also have a date scheduled for its next check up.

Limitation Tolerance

Sometimes, even with the best efforts, the tool cannot be brought into the specified tolerance. At this time the tool can be given a limitation. A limitation says the unit passes the calibration, but within a new tolerance. The limitation is noted on the certificate and the tool so the owner is aware of the tool’s limitation.

If you have any questions or comments, contact Duncan Calibration Services at 402.479.1698.

Contributed by James Hood, Team Leader Calibration Services

With the staggering variety of pressure guages available (we have capabilities for more than 400 types), our calibration technicians have fielded questions ranging from reading discrepancies to which pressure guage is right for a job.

1. How can two similar 1000 psi pressure gauges calibrated at the same time have different readings and both be right?

Many of the pressure gauges calibrated in our lab are Grade B with an accuracy of ±3%, 2% and 3% of span. This means that the first and last quarters of the gauge have an accuracy of 3% while the middle half has an accuracy of 2%. On a pressure gauge with a range of 1000 psi, the upper and lower quarter readings can be ±30 psi of the reading, while the middle half is ± 20 psi of the reading.

In this example, if you are measuring a pressure of 500 psi, the tolerance is from 480 to 520 psi. One gauge could be indicating a pressure of 495 psi, which is well within the tolerance, while a second gauge indicates a pressure of 505 psi and also be within tolerance. Both are within tolerance although there is a 10 psi difference between the two gauges.

2. Why isn’t the gauge aligned to its nominal setting?

Because an alignment affects the entire span of the gauge, any adjustments to the readings in the center will affect the zero and full scale settings. An alignment attempts to minimize any discrepancies between the zero, midrange and full scale settings.

There is an inherent drift in different types measuring equipment. If a pressure gauge is reading slightly low during a calibration, a subsequent calibration may show the gauge reading slightly high. If the unit is aligned for nominal at each calibration, the drift would cause the readings to possibly be even further off the nominal readings.

Another factor in calibrating pressure gauges is repeatability. Due to needle drag and overshoot during the calibration process, identical input pressures will read differently on the pressure gauge being tested. Every attempt is made to minimize these discrepancies during the calibration process, but they are still an inherent characteristic of pressure gauges.

3. What grade pressure gauge should I be using?

The accuracy of the reading is typically better in the middle portion of a gauge. For this reason, you should always select a gauge with a range that is about double your maximum anticipated pressure. Continuing with our example above, if you need to measure a pressure of 500 psi ± 10%, a Grade B, 1000 psi gauge will do the job. If you need to measure a pressure of 500 psi ± 1%, a Grade B, 500 psi gauge will not be accurate enough.

Another reason to have your pressure readings towards the middle of the range is to avoid damage if there is a sudden pressure spike. If you are measuring a pressure of 500 psi with a 500 psi gauge and the pressure spikes to 600 psi, your gauge just became over pressurized and is probably damaged. If you used a 1000 psi gauge in this same scenario, the pressure spike is still within the range and will not damage your gauge.

Duncan Aviation Calibration Services calibrates over 1,500 pressure guages every year. For more information, contact us at 402.479.1698.

Contributed by James Hood, Team Leader Calibration Services

Just like ancient egyptian tool calibration, all certificated repair stations must ensure all test and inspection equipment and tools used to make airworthiness determinations on articles are calibrated to a standard acceptable to the FAA. This tool calibration is performed according to FAA Regulations.

The National Institute of Standards and Technology (NIST), formerly called the National Bureau of Standards, is one of many non-regulatory federal agencies within the U.S. Department of Commerce that produces, maintains and supplies Standard Reference Materials (SRMs) to primary calibration laboratories. These SRMs are certified as having specific characteristics or component content to be used as calibration standards for measuring equipment.

At Duncan Aviation Calibration Services, when a torque wrench is received for calibration it is calibrated using a torque cell. That torque cell is calibrated using calibrated weights and a calibrated moment arm, each of which are calibrated by other calibration labs. Subsequent calibrations of those weights are eventually traced back to a SRM from NIST and for those of you who enjoy trivia, even further back to the international prototype of the kilogram.

The kilogram is an artifact made of platinum-iridium which is kept at the International Bureau of Weights and Measures (BIPM) located in France under the conditions specified in 1889 when the prototype was sanctioned. The weight of the kilogram equals exactly one kilogram.

With NIST and its international partners, a unit of measure can be the same from one country to the next. This holds true for other measurements beyond just the kilogram for mass. Time, voltage, length, resistance and others all have SRMs to allow calibration labs to have consistently accurate calibration standards across the world.

If you have any questions or comments about this, contact Duncan's calibration lab at 402.479.1698.

Contributed by James Hood, Team Leader - Duncan Aviation Calibration Services

Using tools while out of proper calibration can compromise aircraft performance and safety.

It happens. You are returning a tool to the toolbox and you miscalculate your reach and it drops to the floor. Does it need to be recalibrated? The short answer is yes. But do you know why?

When a measuring tool experiences a sudden deceleration from a fall, a quick visual inspection could lead to the conclusion there is no concern. But be warned, it isn’t what you see that’s important, it’s what you don’t. The damage doesn’t have to be great in order to cause an improper reading from a calibrated tool. 

A torque wrench dropped from an elevated surface to the hangar floor, could have internal parts broken. For instance, on a click-style torque wrench, if the wrench is set to its lowest setting the drop could cause the pivot block to shift out of position. And it only takes a slight bend in the rod of a micrometer or caliper to cause the readings to be off because mating surfaces don’t match.

Other popular tools that could be damaged in a fall are pressure gauges. Some have fragile insides that are easily broken or warped. When crimpers fall to the ground, the dies could be damaged or misaligned.

Whether it’s calipers, micrometer, digital level or torque wrenches, using tools that are not within calibration tolerances compromises aircraft performance, component life and in some cases safety.

After dropping your tool on the hangar floor, err on the side of safety and have it properly tested for calibrated performance before you put it back in your toolbox.

Questions? Call Duncan Calibration Services at 402.479.1698.