PML Workshop Leads to Safer Flow Calibrations

A sailor assigned to Aircraft Intermediate Maintenance Department tests an aircraft jet engine for defects aboard the USS Abraham Lincoln. Credit: Brandon C. Wilson/U.S. Navy.

On September 27, 2011, NIST PML hosted a workshop on turbine meters and hydrocarbon liquid flow measurement to:

• promote the replacement of toxic and flammable calibration liquids with benign ones
• discuss the effects of liquid properties on the performance of turbine meters, and
• report the results of a NIST-piloted comparison between 12 labs in the Department of Defense (DoD) and private industry.

The aerospace industry and DoD use NIST calibration services for hydrocarbon liquid flow to ensure agreement and quality of measurements of jet fuel flow and hydraulic oil flows. They require accurate flow measurements to evaluate jet engines on test stands and actuators for ailerons on airplanes and turrets on tanks.

The workshop was a unique occasion for in-depth discussion among manufacturers, calibration laboratories, and end users of the flow meters for these applications. At the workshop, NIST presentations demonstrated that non-flammable, non-toxic surrogate liquids can be used for flow calibrations without loss of accuracy, provided that the meter is calibrated and used within its viscosity-independent range or what is commonly referred to as the linear range of the meter. 

Today, most laboratories that calibrate turbine meters for jet fuel applications use a surrogate fluid called Stoddard solvent. This surrogate resembles kerosene insofar as it is less flammable and toxic than jet fuel; however, it still presents fire and biological hazards.

Recently, Arnold Air Force Base Calibration Laboratories and NIST replaced Stoddard solvent with biologically and environmentally benign mixtures of propylene glycol and water mixtures. A mixture of 1 % (molar) propylene glycol in water matches the kinematic viscosity of jet fuel, and pure propylene glycol has a kinematic viscosity of approximately 50 × 10^-6 m²/s, which matches the middle of the range of hydraulic oils at 21 ^oC. 

Calibration laboratories that replace Stoddard solvent with mixtures of propylene glycol and water will reduce inhalation danger to workers, eliminate fire danger (At Arnold Air Force Base, a fire truck had to stand by during Stoddard-solvent calibrations.), and decrease the cost and risk of disposal of Stoddard solvent. NIST calibrations of turbine meters using flows of propylene glycol and water mixtures agreed with NIST’s Stoddard solvent calibrations within 0.02 %. 

That result validated the theory for the dependence of turbine meter calibrations on the fluid’s density and viscosity. NIST’s theory for turbine meters also incorporates the effects of bearing friction and fluid drag; then, it correlates data spanning a 200:1 flow range with liquid mixtures spanning a 42:1 kinematic viscosity range. Based on NIST’s results, the workshop consensus supported a transition by the calibration community from Stoddard solvent to propylene glycol and water mixtures.

Comparison among the 12 calibration laboratories showed agreement within the uncertainty expectations of all the participants. However, the uncertainties related to the transfer standard were as large as 0.2 %, four or more times larger than nine of the 12 participants’ uncertainties. This prevented a critical evaluation of the uncertainty statements of nine participants and indicates a need for improved transfer standard flow meters for jet fuel and hydraulic oil applications.