American Magnetics AMI Liquid Helium Level Sensors User Manual

American Magnetics, Inc.

P.O. Box 2509, 112 Flint Road, Oak Ridge, TN 37831-2509

Phone: [phone] Fax: [phone]

Internet: http://www.americanmagnetics.com
E-Mail: [e-mail]

AMI LIQUID HELIUM LEVEL SENSOR

INSTALLATION, OPERATION, AND

MAINTENANCE INSTRUCTIONS

I. INTRODUCTION

The AMI liquid helium level sensor uses a small Niobi um-Titanium (NbTi) wire as the
detector element. A heater creates and helps maintain a norma l zone in that portio n of
the wire above the liquid helium level while that portion of the wire below the liquid
helium level remains superconduct ing. The output voltage of the sensor vari es linearly
with a change in liquid level.

The AMI liquid helium level sensor is designed to operate with an AMI liquid helium
level meter. Operation of the sensor with other level meters or operation of different
length sensor with a meter calibrated for a specifi c length may void the sens or
warranty.

AMI

II. SPECIFICATIONS

Diameter:. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1/4"

Active lengths:. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 to 80 inches

Overall length: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . usually 1 inch longer than active

length (1/2 inch at top and bottom)

Sensor current: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 milli-amperes (nominal)

Sensor voltage: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0-60 V DC depending on sensor

length

Nominal sensor resistance:. . . . . . . . . . . . . . . . . . . . 4.5 ohms/cm (11.6 ohms/in.) @ 20K

5.4 ohms/cm (13.7 ohms/in.) @ 300K

Maximum magnetic field:. . . . . . . . . . . . . . . . . . . . . 10 Tesla

Caution symbol: necessary instructions in this document in order to protect
against damage to the product.

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AMI

III. INSTALLATION

A. Carefully remove the sensor from the shipping tube and remove all packaging

material.

NOTE
and contact the shipping representative to a file a damage claim. Do not
return the instrumen t t o AMI unless prior authorization has been
received (refer to Section VIII).

B. The sensor must be mounted with the electrical leads at the top.
C. For minimum losses, mount the liquid helium sensor so that warm helium gas rising

from the sensor can pass di rectly ou t o f th e dewar without contac ting surface s at 4.2K.
Do not mount the sensor in restricted areas (tubes, etc.) where the liquid level around
the sensor might be depressed by pressure differences in the gas. Do not cover the
holes in the sensor.

D. The sensor may be mounted by taping or clipping it to an appropriate support

structure. Do not exert excess pressure on the sensor with the mounting device to
avoid crushing the tube. Avoid constraining both ends of the sens or and allow for
contraction of the sensor during cooldown.

NOTE
possibility of cracking or breaking the sensor or wire insulation.

CAUTION

sensor in a vacuum may cause thermal damage and/or destructi on of
the superconducting filament sensor. Do not inadvertently turn the
instrument on with the sensor in an evacuated chamber. Operation in
pumped liquid helium environments is acceptable to 1K as long as
liquid helium is present.

: If there is any shipping damage, save all packaging material

: Avoid bending the sensor or lead wires when cold to avoid the

: Do not operate the sensor in a vacuum. Operating the

E. Avoid installing in a location where icing (frozen water or gas) may occur since ice

formations may cause erratic operation. Ice format ion on the NbTi filament may stop
the propagation of the normal (resistive) zone before it actually reaches the liquid/gas
interface. This will give an indication of a higher helium level than actually exists.

F. Ensure the level meter is de-energized (unplugged) and connect the sensor to the

meter. The liquid helium level sensor leads are color coded:

Red......

Blue......V +

Yellow.... V -

Black..... I -

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I +

AMI

IV. OPERATION

A. The liquid helium level sensor is designed to work with all AMI liquid helium level

instruments. The level meter will be calibrated for a specific length level sensor
(calibrated length will be marked on the calibration label of the level instr ume nt).

NOTE
the top and bottom of the sensor.

Further information on the helium level instrument is contained in the Installation,
Operations and Maintenance Instructions for the particular model instrument you
have purchased.

Helium consumption is a function of the power input to the sensor and wi ll vary with
the current, temperature (resistance) and the length of the sensor. AMI has, under
ideal laboratory conditions, measured the helium consumption for a typical sensor to
be as low as 20 milliliters per hour. This was measured in an open dewar when the hot
gas did not contact the dewar walls. However, in typical installations the helium
consumption will be somewhat higher. The maximum helium consumption (at 70 milli­amperes and 4.5 ohms/cm) would be 30 ml/hr/cm of active length. To minimize helium
consumption it is recommended the sensor be installed in accordance with the
installation instructions and the power to the sensor tur ned off at the level i nstrument
between measurements.

Liquid helium losses due to superconductive helium level sensors can be quite
variable. These losses, due to current in the sensor, are generally a function of physics
and not the manufacturer.

The sensor element is a very small diameter NbTi wire held in a vertical position. The
top of the wire has a small heater attached to initiate a resistive zone. If the current is
adjusted properly, the resistive zone will propagate from the heater area down to the
liquid helium level and will stop with out penetrating below the liquid. It takes a r ather
large amount of heat to maintain the filament in the resistive state in opposition to the
cooling effects of the surrounding helium gas. In the best case, the heated gas leaves
the system without transferring heat to the li quid he lium. In the worst c ase such as in
a completely closed dewar, all of the heat from the sensor eventually finds its way to
the liquid and causes evaporation.

: All sensors have a nominal one-half inch non-active portion at

If the current in a sensor is left on continuously, large losses can occur. It is usually
only necessary to turn the electronics on when it is desired to know the level and then
turn it off. This procedure will minimize the helium losses. For those who want this
process automated, AMI has developed patented "S ampl e and Ho ld" instruments.
These instruments combine analog and digital electronics to measure the level on a
periodic basis. The measurement is made by turning on the sensor current and
monitoring the progress of the resistive zone. The instant the resistive zone is
determined to have reached the liquid helium level, the current is turned off and the

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liquid level is saved and displayed. The whole process is repeated at intervals selected
by the customer. The liquid losses increase as the sample frequency increases.

The losses for worst case conditions can be estimated if all the parameters are known.
Parameters are defined as follows:

Q

= Power produced in watts by the heater at the top.

h

= Power produced in watts during the growth of resistive zone t owards liquid

Q

v

level.

Q

= Power produced in watts after static conditions are reached, i.e. after the

s

resistive zone reaches the liquid surface.

I = Sensor current (0.075 amperes).

= Heater resistance (approximately 5 ohms).

R

h

= Normal state resistance/length of NbTi filament (approximately 4.55 ohm/

R

s

cm @ 20K).

v = velocity of propagation of resistive zone (approximately 20 cm/second @ 75

milli-ampere sensor current).

L

= length in cm of sensor active region NOT submerged in liquid helium.

G

t = amount of time the current is on in seconds.

= time at which normal zone starts propagating in seconds.

t

0

= time at which resistive zone stops at liquid level in seconds.

t

1

There are three regions where heat is produced:

III

Power

II

I

time

Region I. The heater region
Heat is produced as long as the current is on.

Q

=I2 • Rh • t

h

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Region II. The transition region
The normal zone is assumed to propagate at a constant velocity (20 cm/second). The

heat produced in the NbTi f ilament dur ing th e ti me requi red fo r the r esi st ive regi on to
reach the liquid surface is:

=I2 • Rs • LG • (t1 – t0) / 2

Q

v

2

=I

• Rs • L

Region III. The steady state region
After the resistive zone reaches the liquid surface, the filament becomes a simple

resistor with constant resistance. The power produced in this steady state is:

=I2 • Rs • LG • t

Q

s

EXAMPLES

2

/ (2 • v) since (t1 – t0) = LG / v

G

Let's estimate the loss for an ext reme cas e o f a 6 0 inch ( 152. 4 cm) l ong sens or in a MRI
system with all of the sensor length above the liquid helium level ( L

= active length).

G

All other cases are better than this and can be easily calculated . The res ults are:

Q

= 0.028 watts • t

h

= 14.86 joules

Q

v

=3.9 watts • t

Q

s

Since the heat produced in region III (s teady state) is wasted and no t require d for level
sensing, AMI has developed and patented a sample-and-hold system which red uces the
third term (Q

) to zero because it automatically turns t he current off when the resistive

s

zone reaches the liquid helium.
The sample time for this example of a 60 inc h sensor is approximately 7. 6 seconds. The

total energy input for this sample is thus:

Q

= 0.028 watts • t

h

where t = 7.6 seconds
= 0.21 joules
= 14.86 joules

Q

v

Consequently, total heat input (Q

+ Qv) is approximately 15.1 joules. The latent heat

h

of evaporation of liquid helium is approximately 21 joules/gm. So in this case we have
evaporated 0.72 grams of liquid helium (about 5.7 ml) for one sample.

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If only 10 inches (25.4 cm) of the sensor is above the liquid helium level then:

Q

= 0.028 watts • t

h

= 0.431 joules

Q

v

= 0.143 watts • t

Q

s

The third term, Q

, is again reduced to zero (due t o sample-a nd-hold funct ionality) a nd

s

the sample time is approximately 1.27 seconds, thus the total heat input for a sample
is Q = 0.45 joules

The helium loss is 0.45 joules / (21 joules / gm) = 0.021 grams or approximately

0.166 ml.
The velocity of propagation is the most uncertain term in the calculat ion. These

calculations are intended to give you an idea of what the worst case helium losses are
and to demonstrate helium loss variability.

V. TROUBLESHOOTING

A. No level readin g:

1. Ensure level meter is plugged in.

2. Ensure the lead s a re connected to th e proper inst ru ment termin al s.

3. Ensure all lead wires are secure and are not broken.

4. Ensure the vessel is cold and capable of collecting helium.

B. Erratic or erroneous level reading:

1. Ensure there is no ice formations around sensor.

2. Ensure sensor is not installed in a res tricted area.

NOTE

: Anomalous behavior of the sensor may be seen, under some

conditions, at the lambda point of helium.

If the cause of the problem cannot be located please call an AMI Technical Support
Representative at [phone].

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VI. MAINTENANCE

The helium level sensor will provide years of useful service and require no
maintenance if installed and operated in accordance with these instructions. The
sensor is a sealed unit and internal repair or service is not feasible.

VII. WARRANTY

All products manufactured by AMI are war ranted to be free of d efects in mat erials and
workmanship and to perform as specified for a period of one year from date of
shipment. In the event of a failure occurr ing during normal use, AMI, at its op tion, will
repair or replace all products or components that fail under warranty, and such repair
or replacement shall constitute a fulfillment of all AMI liabilities with respect to its
products. All warranty repairs are F.O.B. Oak Ridge, Tennessee, USA.

VIII. RETURN AUTHORIZATION

Items to be returned to AMI for repair (warranty or othe rwise) require a return
authorization number to ensure your order will receive the proper attention. Please
call an AMI representative at [phone] for a return authorizat ion before
shipping any item back to AMI.

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