Sandia LabNews

Primary Standards Lab: Sandia’s the word for precision measurements, calibrations

Hy Tran

REALLY ROUND — The image of project lead Hy Tran (2541) is reflected in a polished quartz ball that is the standard for roundness. The Primary Standards Laboratory at Sandia uses a specialized instrument to measure roundness deviation against the roundness standard, which has been certified to national standards to be round within about 20 nanometers. (Photo by Randy Montoya)

You probably never gave roundness a thought.

But when it’s crucial that something be really round, federal labs and agencies can turn to the DOE Primary Standards Laboratory (PSL), operated by Sandia.

Take the standard for roundness, or deviation from a circle. The PSL uses a specialized instrument to measure roundness deviation against a roundness standard, a polished quartz ball nestled in a padded box. That roundness standard has been certified to national standards to be round within about 20 nanometers.

“You can’t measure the actual diameter of a circle in here, but we can measure how far off that circle is from being a perfect circle,” says project lead Hy Tran (2541). “We are capable of resolving a tenth of a nanometer from this piece of equipment. … The resolution is exquisite.”

If you blew up the quartz ball to Earth size, the instrument could detect hills and valleys about 1¼ inch high, he says.

The PSL develops and maintains precision measurement standards, provides measurement assurance training and consultation, provides calibrations and technical support, and performs technical surveys and measurement audits. Its work ranges from doing electrical, physical, dimensional, and thermodynamic calibrations for Sandia organizations to certifying reference standards for Pantex, Y-12, and other DOE sites to doing work for others like NASA.  The consultation work is often overlooked but vital to Sandia’s mission. The PSL is called on to support other nuclear security sites and organizations outside DOE.

Measurement and calibration are critical because they affect the quality of scientific and technical data that’s published, conclusions drawn from data, and certification of product-to-performance requirements. Sandia’s calibrations largely trace to reference standards from the National Institute of Standards and Technology (NIST) for just about anything that can be measured.

“Anything that’s manufactured, you have to be able to measure quantities. This is what we do,” says PSL distinguished technologist Jim Novak (2542). “It doesn’t matter what the discipline is, whether its voltage or mass or pressure or temperature, you have to quantify it and it has to meet some type of standard” and an estimate of accuracy.

Most measurements and calibrations are based on comparison.

“You’re always comparing with a standard,” explains Bud Burns (2541). “You have a standard, you know what that uncertainty is, and you compare an unknown with that standard.”

There’s a gray area with measurements, and instruments must be calibrated to reduce those uncertainties to acceptable tolerances, Jim says. Over the decades, new instruments and techniques have lessened the uncertainties.

There are basically two types of measurement standards, those based on an artifact and those that are intrinsic. Measurements often are based on an artifact — something physical such as that polished quartz ball that could vary in the tiniest way from another object that’s based on it. An intrinsic standard, on the other hand, means you can reproduce a measurement anywhere on the planet and get the same results because it relies on inherent and reproducible properties of a phenomenon or substance.

Specially built for precise measurements

The 45,000-square-foot PSL includes 30,000 square feet of specially designed lab space for measurements and calibrations representing more than 100 metrology disciplines — physical and mechanical quantities such as gas flow, acceleration, or vacuum standards; radiation, including alpha radiation, laser pulse energy, neutron pulse, and solar power; electrical quantities such as DC and AC voltage and current; and microwave electrical quantities.

To ensure calibrations can be done accurately, temperature and humidity are rigidly controlled in each PSL lab, and the building is shielded from radio frequency waves and electromagnetic radiation and isolated from vibration. Even gravity has been calculated at specific locations within some labs because of its importance to precise calibrations. The work requires uncommon equipment, impeccable attention to detail, and experts in metrology, the science of measurement. 

Sandia has performed calibrations since the 1950s, but in 1968, the Atomic Energy Commission, a DOE precursor, designated Sandia to maintain the Primary Standards Laboratory for the weapons complex. That makes the PSL NNSA’s technical arm for measurements, Jim says.

It has unique capabilities to support the nuclear weapons complex, including pulsed neutrons for neutron generators, microwave devices for radar systems, and gas leak measurements for components that must retain seal integrity at different temperatures and pressures, such as from sea level to space.

The PSL also tests how proficiently other DOE laboratories perform their own measurements based on standards provided by Sandia. If a particular laboratory’s core capability is measuring DC voltage, the PSL sends it a voltage source. The PSL knows what the voltage is, but it’s up to the other lab to measure it. “Then we look and see if the results are what they should be,” Jim says.

Looking at capabilities

A snapshot of some individual PSL labs:

  • Length/Mass/Force, which deals with dimensional, mass, and force: One specialized machine, resembling a giant washing machine, compares items up to 64 kilos, about 140 pounds. A similar machine does comparisons from one to 10 kilos, about 2.2 to 22 pounds; another does 100 grams to one kilo. The equipment is so sensitive operators must leave after starting it so their body doesn’t affect the results, Hy says. Comparisons for less than 100 grams are done manually on other equipment. In another area, two interns, an Air Force Academy cadet and a University of Santa Clara junior, helped qualify an interferometer system that measures surface texture in the nanometer range. Anything that’s been machined has texture marks, and that surface affects everything from friction to wear, Hy explains.
  • Microwave, which tests instruments used for radar, guidance systems, and satellites: Arriving at a single precise calibration could require painstaking attention to an extraordinary array of equipment. One particular morning, technician Santiago “Jimmy” Cheykaychi (2542) set up to calibrate power meters and sensors using a collection of electronic equipment: a pulse generator, an amplifier, modulators, a frequency counter, an oscilloscope to make sure the signal is as pure as possible, and couplers and isolators laid out on acoustical foam. Connecting equipment to minimize reflections and noise for the level of accuracy needed requires specialized training and practiced technique.
  • Radiation and optics: The lab supports neutron generator production, builds detectors that measure pulsed neutrons, calibrates that instrumentation in a capability not found anywhere else in the US, supports radiation measurement techniques, and does research and development for other customer needs. Optics work is augmented by Sandia’s photovoltaic and materials groups. The lab houses a low-power laser, but sometimes it uses high-power lasers at other Labs sites for calibrations. “So the optics business is not just in this lab. It’s multi-laboratories throughout Sandia,” Bud says.
  • Temperature: Jobs include calibrating temperature probes using a reference probe for comparison. The standard for primary temperature measurements is a standard platinum resistance thermometer, or SPRT, which looks like a glass wand. SPRTs are calibrated at several temperatures with metal fixed point cells — standards that depend on using the latent heat of the metal transitioning from liquid to solid in the case of a freeze plateau or solid to liquid in the case of a melt plateau, thus maintaining a stable temperature plateau under standard atmosphere. The temperatures are very reproducible and stable, says project lead Lisa Bunting Baca (2541). “It’s kind of a process to get them on a plateau and those plateaus maintain a very steady temperature for a period of hours or days, depending on the exact fixed point. Then we monitor with control SPRTs, so it’s a lot of maintenance, a lot of monitoring equipment, a lot of control charts” to make sure SPRTs can measure precise temperatures to 5 millidegrees, she says.
  • Flow, acceleration, shock, and humidity: Equipment includes an acceleration system, a centrifuge, instruments for measuring the flow of gases, a drop box system for shock, and chambers capable of measuring humidities from 5 to 95 percent and dewpoints from about 25 degrees C to about minus 40 degrees C. Why? Thermal batteries require a certain humidity condition when they’re assembled, for example, and shock is key to figuring out how much force a projectile wields on a target and how well it will penetrate.
  • DC: The lab maintains the ohm — a basic unit of electrical resistance — with an artifact standard based on very accurate resistors housed in a mineral oil bath maintained at 25 degrees Celsius, plus or minus 0.003 degrees. Technicians use instrumentation to transfer a one-ohm value to resistors being calibrated, such as those from other federal laboratories, with uncertainties as low as plus or minus 55 nano-ohms per ohm — 55 parts per billion. The lab also does pressure and temperature tests so the end user can correct for different temperatures or altitude for that particular resistor, Jim says.
  • AC: The lab uses a miniaturized microcircuit to derive the standard for AC current, given that the unknown AC current produces a certain amount of heat. “Then you apply an equivalent amount of DC to produce that same amount of heat, and because DC current is calibrated you know what the AC value is,” Jim says. In another part of the lab, project lead Stefan Cular (2542) explains a pulse high-voltage system capable of operating from about 10,000 volts to 300,000 volts in durations of 2 to 30 microseconds. There are variations for different currents, different voltages, and different ranges for impedance, capacitance, and inductance.