NIST Traceability

Calibration means the comparison and adjustment (if necessary) of an instrument’s response to a stimulus of precisely known quantity, to ensure operational accuracy. In order to perform a calibration, one must be reasonably sure that the physical quantity used to stimulate the instrument is accurate in itself. For example, if I try calibrating a pressure gauge to read accurately at an applied pressure of 200 PSI, I must be reasonably sure that the pressure I am using to stimulate the gauge is actually 200 PSI. If it is not 200 PSI, then all I am doing is adjusting the pressure gauge to register 200 PSI when in fact it is sensing something different.

Ultimately, this is a philosophical question of epistemology: how do we know what is true? There are no easy answers here, but teams of scientists and engineers known as metrologists devote their professional lives to the study of calibration standards to ensure we have access to the best approximation of “truth” for our calibration purposes. Metrology is the science of measurement, and the central repository of expertise on this science within the United States of America is the National Institute of Standards and Technology, or the NIST (formerly known as the National Bureau of Standards, or NBS ).

Experts at the NIST work to ensure we have means of tracing measurement accuracy back to intrinsic standards, which are quantities inherently fixed (as far as anyone knows). The vibrational frequency of an isolated cesium atom when stimulated by radio energy, for example, is an intrinsic standard used for the measurement of time (forming the basis of the so-called atomic clock). So far as anyone knows, this frequency is fixed in nature and cannot vary: each and every isolated cesium atom has the exact same resonant frequency. The distance traveled in a vacuum by 1650763.73 wavelengths of light emitted by an excited krypton-86 (86Kr) atom is the intrinsic standard for one meter of length. Again, so far as anyone knows, this distance is fixed in nature and cannot vary. This means any suitably equipped laboratory in the world should be able to build their own intrinsic standards to reproduce the exact same quantities based on the same (universal) physical constants. The accuracy of an intrinsic standard is ultimately a function of nature rather than a characteristic of the device. Intrinsic standards therefore serve as absolute references which we may calibrate certain instruments against.

The machinery necessary to replicate intrinsic standards for practical use is quite expensive and usually delicate. This means the average metrologist (let alone the average industrial instrument technician) simply will never have access to one. While the concept of an intrinsic standard is tantalizing in its promise of ultimate accuracy and repeatability, it is simply beyond the reach of most laboratories to maintain.

In order for these intrinsic standards to be useful within the industrial world, we use them to calibrate other instruments, which are then used to calibrate other instruments, and so on until we arrive at the instrument we intend to calibrate for field service in a process. So long as this “chain” of instruments is calibrated against each other regularly enough to ensure good accuracy at the end-point, we may calibrate our field instruments with confidence. The documented confidence is known as NIST traceability: that the accuracy of the field instrument we calibrate is ultimately ensured by a trail of documentation leading to intrinsic standards maintained by the NIST. This “paper trail” proves to anyone interested that the accuracy of our calibrated field instruments is of the highest pedigree.

Thompson Equipment Company's calibration lab is ISO/IEC 17025 Accredited and NIST traceable.

Reprinted from "Lessons In Industrial Instrumentation" by Tony R. Kuphaldt – under the terms and conditions of the Creative Commons Attribution 4.0 International Public License.

Process Instrument Calibration

Calibration is an essential part of keeping process measurement instrumentation delivering reliable and actionable information. All instruments utilized in process control are dependent on variables which translate from input to output. Calibration ensures the instrument is properly detecting and processing the input so that the output accurately represents a process condition. Typically, calibration involves the technician simulating an environmental condition and applying it to the measurement instrument. An input with a known quantity is introduced to the instrument, at which point the technician observes how the instrument responds, comparing instrument output to the known input signal.

Even if instruments are designed to withstand harsh physical conditions and last for long periods of time, routine calibration as defined by manufacturer, industry, and operator standards is necessary to periodically validate measurement performance. Information provided by measurement instruments is used for process control and decision making, so a difference between an instruments output signal and the actual process condition can impact process output or facility overall performance and safety.

In all cases, the operation of a measurement instrument should be referenced, or traceable, to a universally recognized and verified measurement standard. Maintaining the reference path between a field instrument and a recognized physical standard requires careful attention to detail and uncompromising adherence to procedure.

Instrument ranging is where a certain range of simulated input conditions are applied to an instrument and verifying that the relationship between input and output stays within a specified tolerance across the entire range of input values. Calibration and ranging differ in that calibration focuses more on whether or not the instrument is sensing the input variable accurately, whereas ranging focuses more on the instruments input and output. The difference is important to note because re-ranging and re-calibration are distinct procedures.

In order to calibrate an instrument correctly, a reference point is necessary. In some cases, the reference point can be produced by a portable instrument, allowing in-place calibration of a transmitter or sensor. In other cases, precisely manufactured or engineered standards exist that can be used for bench calibration. Documentation of each operation, verifying that proper procedure was followed and calibration values recorded, should be maintained on file for inspection.

As measurement instruments age, they are more susceptible to declination in stability. Any time maintenance is performed, calibration should be a required step since the calibration parameters are sourced from pre-set calibration data which allows for all the instruments in a system to function as a process control unit.

Typical calibration timetables vary depending on specifics related to equipment and use. Generally, calibration is performed at predetermined time intervals, with notable changes in instrument performance also being a reliable indicator for when an instrument may need a tune-up. A typical type of recalibration regarding the use of analog and smart instruments is the zero and span adjustment, where the zero and span values define the instruments specific range. Accuracy at specific input value points may also be included, if deemed significant.

The management of calibration and maintenance operations for process measurement instrumentation is a significant factor in facility and process operation. It can be performed with properly trained and equipped in-house personnel, or with the engagement of subcontractors. Calibration operations can be a significant cost center, with benefits accruing from increases in efficiency gained through the use of better calibration instrumentation that reduces task time.

TECO's calibration lab is ISO/IEC 17025 Accredited. Many calibration houses can only verify calibration within the manufacturer's specifications, and it's a myth that they can fix anything that is broken.

Services include:

• ISO/IEC 17025 Accredited Calibrations
• NIST Traceable Calibrations
• Live test flows on every meter repair
• Calibration and Repair of most types of flowmeters including mass meters
• Flowmeter calibration history files for future comparison
• Calibration accuracy to factory specifications
• Calibrations to multiple secondaries and across OEM product lines
• Multiple test points available

For more information visit http://teco-inc.com or call 800-528-8997 for immediate service.

NIST Certification of Flow Instrument Calibration and ISO/IEC 17025 Accreditation

The Thompson Equipment Co., Inc., (TECO) facility is used for performing flow calibration for magnetic flow meters as well as other primary liquid flow measuring devices. It is equipped with both mass and volumetric transfer standards.

The output of a customer owned meter is correlated with the output of a standard volumetric meter over a range of its flow capabilities. Each six months, each volumetric standard meter is calibrated against a mass standard. Each year the mass standards are calibrated against the Louisiana Department of Agriculture Standards Laboratory. 

Also on a periodic basis, standards owned by the LA Dept. of Agriculture are calibrated against the National Institute of Standards and Technology (NIST) Each standard device carries with it the appropriate certificate identifying when its last calibration was performed, and by ID number, which standard device it was calibrated against. 

The NIST Traceable Calibration Certificate from TECO documents this trail of calibration so that the calibration of any flow meter can be confirmed all the way back to the NIST Laboratories as is often required by regulatory agencies, ISO-9000 procedures, etc. This provides the user with a high level of confidence in the readings from his instrument.

ISO/IEC 17025 Accredited

TECO's calibration lab is also ISO/IEC 17025 Accredited, meaning it is in accordance with the recognized International Standard ISO/IEC 17025:2005 General requirements for the competence of testing and calibration laboratories. The TECO laboratory also meets any additional program requirements in the field of calibration. This accreditation demonstrates technical competence for a defined scope and the operation of a laboratory quality management system.

Unlike many calibration houses can only verify calibration within the manufacturer's specifications, TECO can provide a wide range of fully accredited flow calibration services to meet virtually any need.

For more information, contact TECO at 800-528-8997 or visit http://teco-inc.com.