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.

Paper Production: Measuring Freeness Produces More Salable Product

Measuring Freeness improves quality in paper production.

Better quality and more salable paper is the outcome of accurately measuring freeness at the beginning of the manufacturing process. Controlling freeness makes production lines more efficient and capable of producing better quality paper at a lower cost per ton.

According to the North Carolina State Mini-Encyclopedia of Papermaking Wet-End Chemistry, freeness is defined as “a measure of how quickly water is able to drain from a fiber furnish sample. In many cases there is a correlation between freeness values and either (a) a target level of refining of pulp, or (b) the ease of drainage of white water from the wet web, especially in the early sections of a Fourdrinier former. Standard tests of freeness are based on gravity dewatering through a screen. The devices are designed so that an operator can judge the speed of dewatering by observing the volume of liquid collected in a graduated cylinder. Freeness tends to be decreased by refining and by increases in the level of fines in the furnish. Freeness can be increased by use of drainage aids, removal of fines, or enzymatic treatments to convert mucilaginous materials into sugars."

TECO (Thompson Equipment Company) has been serving the pulp and paper industry for over 60 years, and has helped hundreds of clients with their unique Drainac^® Drainage Rate Indication System. The Drainac^® is an on-line instrument that continually measures the drainage rate of pulp and provides a proportional 4-20 mA DC signal. The unit consists of two major sub-assemblies; a detector and a detector control cabinet. It has earned a reputation as the fastest, lowest cost, and most pain-free device of its kind for measuring freeness.

Basic Applications

Closed Loop Refiner Controls – On-line freeness measurement is commonly used to control the final freeness target (setpoint) for the refiners by cascading the freeness measurement output directly to the horsepower tons / day controller.

Basic On-line Freeness Measurement – Basic on-line freeness measurement is used by production managers and paper machine operators as a “speedometer” of fiber quality enabling them to make real- time decisions that effect final production quality and paper machine run-ability.

Stock Blending – Used for monitoring the fiber characteristics of individual furnish streams so that optimal stock blending can be accomplished on a real-time basis. In this manner, the lower cost furnish stream can be maximized without sacrificing final product quality.

Please watch the video below for a better understanding of why its important to measure freeness for improved paper quality. For more information on freeness measurement, visit http://www.drainac.com or call TECO at 800-528-8997.