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to liquid metals, with caution due to specific challenges. In addition to the high-
operating temperature, the compatibility with solid materials (corrosion) and high
density of LBE impose additional limitation for the applicability of certain techniques
or materials.
A detailed analysis of measuring techniques for liquid-metal applications is given
in OECD (2015), Schulenberg and Stieglitz (2010) and Buchenau et al. (2011). In the
following sections, an overview of the most relevant practical consideration for mea-
suring flow rate and differential pressure in a liquid-metal facility is presented.

3.3.2.2.1 Flow rate

High accuracy and repeatability at the process operating conditions are the most
important criteria for selecting the instrumentation. In particular, an experimental con-
firmation or calibration of the measured signals is important.
In this context, measurement techniques that can be transferred from water or air
are most attractive because of the accumulated experience and commercial availabil-
ity. A calibration in a water setup can be performed (e.g., at the factory by the supplier)
and scaled to liquid metal considering the relevant nondimensional parameters, for
example, Reynolds number, and physical properties. These off-the-self measurement
techniques can be categorized as follows, each type with advantages and limitation for
application in liquid metals:
2
l Techniques based on a pressure difference, proportional to ρu , such as venturi or Annubar,
rely on a calibration constant and a differential pressure measurement. Venturi nozzles have
low permanent pressure losses, yet the peak velocity must be taken into account for erosion
considerations. Although some industrial standards, for example, DIN-EN-5167, are avail-
able for their construction, it should be noted that they have been originally developed for
application in large diameters and Reynolds numbers. Thus, they should be used with
caution and empirically validated.
l Vortex flow meters rely on measuring the frequency of oscillating turbulent vortices behind
an obstacle, in principle correlated to the Reynolds number. As this physical phenomenon
occurs only beyond a minimum threshold Reynolds number, this instrumentation operates
with a minimum flow rate and is inherently not suitable for measuring at low flow velocities.
According to the operating experience at KIT, these sensors perform well in a liquid-metal
environment. The main practical challenge is given by the material compatibility at high
temperature.
l Coriolis flow meters are based on forces produced by a mass flow rate in an oscillating
geometry. In principle, these forces depend only on the mass and frequency, independently
of physical properties such as density or viscosity. Thus, a direct transfer from water to liquid
metal is theoretically possible, with potentially higher accuracy than the other techniques
mentioned above. In practice, the main technical challenges are related to the material com-
patibility and the high density of LBE. Both are less severe for sodium.

Alternatively, special techniques are under development exploiting the features of liq-
uid metals, mainly their high electric conductivity. Prominent examples are electro-
magnetic techniques based on an external magnetic field and ultrasonic sensors
relying on the effects of velocity on the propagation of a pressure signal. They are