4.2 Instrumentation: Pressure, Flow, & Level
4.2 Instrumentation: Pressure, Flow, & Level
4.2 Instrumentation: Pressure, Flow, & Level
2 Instrumentation:
Pressure, Flow, & Level
Pressure
• Piezoresistive transducers
– Resistance bridge – 4 active arm
strain-gauge
– Calibration required at temperature
– Example: Endevco 8510B
– Typical price: ~ $1K per each
• Pressure capillary extension
– Extend capillary from cold
environment up through cryostat to
room temperature environment
– Ensure leak-tight
– Check mean free path length for low
pressure (vacuum) applications
Pressure
• Variable reluctance
transducers
– Magnetically permeable
stainless steel diaphragm
clamped between inductive
pick-up coils
– Diaphragm displacement
changes induction of both
coils
– AC bridge / amplifier circuit
converts inductive change
to proportional DC output
voltage
Cryogenic flow metering techniques
Single phase flows
1. Pressure drop devices based on Bernoulli Principle
a) Venturi
b) Orifice plate
c) Pitot tube
2. Friction pressure drop (packed screens)
3. Hot wire anemometers based on h = f(v)
4. Acoustic flow meters based on Doppler effect
5. Turbine flow meters where frequency ~ velocity
6. Optical techniques (Laser Doppler)
These techniques are for
the most part all used in
Two phase flows classical fluid flows.
1. Void fraction measurement (Av/A) The unique “cryogenic”
a) Capacitance measurement features have to do with
b) Optical characterization instrumentation used to
2. Quality measurement (mv/m) detect signal and need
for low heat leak.
Pressure drop devices
Δp
Δp
Venturi Orifice
• Venturi flow meters have advantage over orifice plate due to low loss coefficient
where β = Dt/D
.V
Two phase flow measurement
• Measurement of flow quality (mv/m) in a two phase mixture (liquid +
vapor) is difficult.
– Vapor velocity and liquid velocity may be different
– Flow regime is not known
• Measurement of void fraction (Av/A) is more straightforward
– Capacitive meter based on different dielectric constant
Co-axial capacitor
– Optical techniques
• Total mass flow rate can be determined in some part of the circuit
where the fluid is single phase using a conventional flow meter
RF Void Fraction Measurement
Liquid Helium Flow Visualization
Optical cryostat
Optics
camera
Laser
Normal fluid convection around cylinder
Diameter = 6.35 mm
2-phase Helium Flow Visualization
2-phase Helium Flow Visualization
CHF Investigation: modeling
• A physical description of void fraction growth or force balance requires
knowledge of bubble size, frequency, spacing and velocity
θ
Visualization: Optical fibers
– Fiber bundle: 40,000
20 µm strand bundle
chosen over solid core
• Avoid multi-mode distortion in
larger diameters
• Maximum flexibility
SLR:
• Ektachrome P1600,
Digital camcorder ‘still’
pushed to 6400.
• 1/250 s shutter speed • 1/3000 s shutter speed
• halogen lamp illumination • halogen lamp illumination
• horizontal channel - slow • black line spacing in upper right is 1 mm.
bubble motion • vertical channel - ‘fast’ bubble capture
Visualization: Image capture
• CCD Images
Normal
Developed by Efferson
Zone
(1970), but now a commercial
product
Heater drives the normal LHe level
zone of SC wire to the liquid
interface, where it stops due
to improved heat transfer
Units are most often
calibrated in LHe at 4.2 K
Variable performance in He
II due to improved heat
transfer
Some SC level meters based
on HTS materials have been
developed for LN2
Capacitive Level Gauges
Most are custom, some are available
as a prototype commercial units,
particularly for high dielectric constant
fluids (e.g. LN2)
Measurement Methods:
• AC Bridge
• High frequency oscillator
• Time constant method
• Phase-lock loop technique
Sensitivity =
Differential pressure (head) gauge
Q Requirements
• No liquid in vertical
leg of lower capillary
tube
• dp/dL = Δρ g
= 1.06 (Pa/mm)helium
• Heat load may be
large to keep vapor
line dry
He H2 Ne N2 O2 Ar
ρl 125 70.8 1240 807 1141 1394
ρg 16.7 1.33 9.4 4.6 4.47 5.77
Ultrasonic level measurement
Signal travels at sound speed
Acoustic oscillation
changes frequency &
amplitude when capillary
leaves liquid
Heat Pulse Mass Gauging
Measurement of He II volume (mass) by heat pulse technique
mass = Q/∆h
Technique used extensively for space based He II cryostats but
also pressurized He II systems for superconducting magnets
From Volz, et al
Advances in Cryo. Engn.
Vol 35 (1990)
Summary of Level Measurement
Techniques
Availability Readout Range of heat Deposition
Liquid-Vapor Detectors
SC wire Development Voltage On the order of mW’s
Mass gauging
Internal energy change Development Temperature On the order of 1 Joule