Date: - 20-10 -2004
OM-GEN-
Introduction
To
Cryogenic Instrumentation in SM18
Prepared by –
Prashant K. Awale and Bangalore Arunkumar, BARC
Department of Atomic Energy, India
Operation Methods & other General Documents are used for testing the LHC
Magnets in Cryogenic Conditions in building 2173 (SM18) and are the working
documents produced for that purpose only. These Methods are step-by-step
procedures that may be followed by the Shift Operators of SM 18 for carrying out
different power- tests of the super-conducting magnets.
Index / Contents
Chapter No
Chapter Name
Page No.
1.
Introduction to Cryogenic Instrumentation ………………03–06
2.
Sensors used in SM18 Cryogenic system ………………07-09
3.
Temperature Instrumentation ……………………………10-17
4.
Flow Instruments ……………………………………….17-18
5.
Level Instrumentation ……………………………………19
6.
13 KA Current lead Instrumentation ……………………..20
7.
Cryo OK for 1.9K/4.2K/HV@ Cold ……………………...21
8.
References ………………………………………………...21
9.
Acknowledgement ………………………………………..21
2
Chapter 1
//Introduction to Cryogenic Instrumentation//
Abstract:
Cryogenic Instrumentation is the vast and unique filed of measurement
and is one of the many challenges for the cryogenic system of SM18 magnet test
facility as well as for LHC. This write-up/document is written in order to understand
the cryogenic instrumentation aspects pertaining to SM18 magnet test facility and also
addressed is some of the low temperature instrumentation aspects both for LHC as
well as for SM18. Study of various low temperature instrumentation sensors
conducted by the experts from CERN is also discussed here in brief. Also enclosed
here is the list of various instruments and sensors (along with their range, make and
location) that are being use in cryogenic system of SM18 magnet test facility. TT821
Cernox temperature sensor that is installed in each magnet is also discussed.
Introduction:
For testing of approximately 1800 superconducting magnets for the LHC machine at
CERN prior to their installation inside LHC tunnel, an extensive Magnet testing set up
is available in SM18. This set up comprises of total 6 test clusters with 2 test benches
per cluster. i.e. overall 12 test benches are available in SM18 and all this test benches
are in operation. Magnets are subjected to both warm as well as cold tests. For cold
tests these magnets are cooled down to 1.9K. For the controlled cool down and warm
up of these magnets, an extensive cryogenic system is installed and is in operation in
SM18 (1) (Refer reference 1 “Cryogenic Infrastructure for Testing of LHC Series
Superconducting Magnets” further details).
This document/write-up is written in order to understand the Cryogenic
Instrumentation aspects pertaining to SM18 Magnet test facility.
Cryogenic instrumentation may be regarded as a unique field of measurement
requiring the development of new techniques. It should be considered a separate field
of effort because of increasingly higher accuracies required (like in case of cryogenic
temperature control at 1.9K for LHC), the inherent remoteness of the measurements
and the peculiarities of the cryogenic fluids themselves. The latter consideration is
among the strongest in setting cryogenic instrumentation apart.
In addition to the obvious characteristics of low boiling points, cryogenic fluids are
characterized by extremely low heat of vaporisation. Cryogenic fluids like liquid
helium require much less energy for vaporisation. The combination of low boiling
point and low heat of vaporisation increases the possibility that the cryogenic fluid
will become boiling, two phase system. The influence this has on pumping, liquid
density and level determination is quite obvious. Any sensor adding energy to the
system is in fact creating a vapour/liquid interface at the very point of measurement.
An added consequence is that when the system is at equilibrium in the two phase
region, the measurement of both temperature and pressure is redundant because the
system has only one degree of freedom. (2) (e.g. TT 147 and PT147 in CFB / SSL test
at 4.2K with helium pressure of 1350millibar).
3
Instrumentation for the cryogenic system of LHC is one of the many challenges and
most of these is overcome by rigorous research and development, proper design,
selection of commercially available instruments and then tailoring it to meet the actual
system requirements. Instrumentation plays a major role in terms of monitoring the
healthiness of the process, measurements of various process parameters (like
Temperature, Pressure, Flow, Level, Conductivity etc.), controlling the desired
process parameter within the specified set limits and to take care of the safety aspects
in case of control failure (e.g. Safety relief valves etc).
For these type of critical cryogenic applications, a huge number of cryogenic probes
are required and some of the important features these probes must have for the trouble
free operation of the system are: Good accuracy, repeatability, long term stability (i.e.
to keep the specifications over the lifetime of the equipment and under its
environmental conditions), maintenance free with very very high MTBF so that the
probe can last till the life time of the equipment, rugged as far as possible and most
important is to withstand all the extreme process and ambient conditions like
cryogenic temperature of around 1.8K, Magnetic field of the order of 9 tesla
(wherever applicable), ionising radiation field and temperature and pressure cycling.
Over and above this “Redundancy and Diversity” are the principles employed in
instrumentation design, especially wherever accidents and failures are envisaged due
to pressure build up etc. e.g. Multiple safety devices (for relieving the excess pressure
build up) are installed in a single header where pressure build may take place due to
quenching of magnets etc. From diversity point of view, Bursting Discs (Rupture
discs) are also provided in parallel.
Before going to instrumentation aspects, it is good to know the SM18 cryogenic
process. (3) (refer to Basic Cryogenic Document- Cryogenics for LHC Dipole
prepared by Mr Uttam Bhunia- Reference #3). Refer below the block diagram of the
cryogenic system layout for SM18 (Fig#1)
Liquid Super fluid helium is used to cool magnets to 1.9K as helium is the only gas
that makes a good super-fluid due to its very weak intermolecular forces. Helium
condenses to liquid at 4.2 K and turns into super-fluid at 2.17 K. Super-fluid helium
has very high thermal conductivity and hence is a very good coolant, has very low
coefficient of viscosity and can penetrate tiny cracks, deep inside the magnet coils to
absorb any generated heat.
As seen from the Cryogenic system layout below, total 12 CFB’s (Cryogenic Feed
Box) are installed between Cooldown Warmup line (CWL) and Cryogenic Compound
Line (CCL). This CFB is a system that enables a superconducting magnet to be
completely connected up, cooled to 1.9K and maintained at that temperature whilst
the magnet performance is being tested.
As cryogenic Instrumentation is the vast field and therefore this write up/document
talks mostly of low temperature measurement techniques that were studied and
implemented in CERN for LHC / SM18 operations as well as the cryogenic system
level and flow measurements. Also discussed in brief about the various instruments
those are used in SM18 cryogenic system.
4
Fig#1: Block diagram of the cryogenic system layout for SM18:
COMP
1
CWU 2
C only
COMP
3
(2005)
COMP
2
HP GHe
Heater
30 kW
LP GHe
Heater
200 kW
CWU 1
C only
COOLDOWN-WARMUP SYSTEM (CWS)
2 GHe compressors 100g/s 2-12 bar
LN2 distribution
2 Cooldown Units 120 g/s (2x140 kW)
1 LPGHe Heater 200 kW
1 HPGHe Heater 30 kW
-
Cooldown Warmup Line, 12 valve boxes (CWL)
CFB
CFB
CFB
CFB
CFB
CFB
CFB
CFB
CFB
CFB
CFB
CFB
Cryogenic Compound Line, 12 valve boxes (CCL)
WPU 1
Other Utilities required
for Magnet Tests
WPU 2
GHe
Recovery
GHe
GHe
40 bar
GN2
LN2
LHe
GHe <90 K
GHe
INTERFACES WITH CRYOGENIC FACILITIES OF ZONE 18
Heater 1
30 kW
Heater 2
30 kW
CCU2
Linde
CCU 1
(IHI)
(2005)
GHe PUMPING SYSTEM
5
Fig.2 Actual magnet cooling circuit for power test
(Courtesy: Cryo-operation group, SM18)
………………………………………………………………
6
Chapter 2
List of some of the important Sensors used in
SM18 Cryogenic system: (Refer Figure# 2) Table#1
Temperature sensors
Sl.
Sensor Parameter
Range
Location
Make
No
No.
and Type
1
TT821
Temperature
inside magnet
1.6 to 300K
Inside
magnet on
SS collar
In CFB on
line M2
2
TE147
1.4 to 2.4K
3
TE148
Temp on line
M2 (Temp.
refered for
Cryo OK of
1.9K)
Temp. on line
M2
0 to 20K
In CFB on
line M2
4
TE149
I/L temp of
magnet in CFB
1.6 to 100K
In CFB on
line N
5
TE150A
O/L temp. of
magnet on line
M2 in CFB
73K to 300K
In CFB on
line M2
6
TE150B
O/L temp. of
magnet on line
M2 in CFB
1.6 to 100K
In CFB on
line M2
7
TE103
Temp in CFB
on line E
73K to 300K
In CFB on
line E
8
TE161
He. Gas temp
73K to 300K
9
TE130A,B
20K to 300K
10
TE129
Temp of 13KA
current leads
Temp at the
bottom of CL
On line
coming
from CWS
to CFB
On CL
20K to 300K
On CL
CERNOX™
(CX)
Helium 3
bulb Vapor
Pressure,
Ingovi
make.
Carbon
Resistance
(CRT), AB
make
Resistance
carbon glass
(Lake shore
ref: CGR-11000-1).
Pt100
Class#A
RTD, Make
ABB
Automation
Resistance
carbon glass
(Lake shore
ref: CGR-11000-1).
Pt100
Class#A
RTD, Make
ABB
Automation
-do-
-do-do-
7
Pressure Instrumentation
Sl.
No
Sensor
No.
Parameter
Range
Location
Make
and Type
1
PT147
0 to 300mbar
2
PT102
Pr. Of helium
bulb on line
M2
Helium pr. At
CFB Top
CFB
(Refer
TE147).
CFB
3
PT121
Helium Guard
Pressure
0 to 2 bar
4
PT142
0 to 60mbar
5
PT143
0 to 2 bar
-do-
-do-
6
PT151
0 to 5 bar
CFB
-do-
7
PT162
0 to 20 bar
CFB
-do-
8
PT181A
Pr Upstream of
FV142
Pr Upstream of
FV143
On BP return
line to CWS
Helium pr. At
CFB Top
Pr. In Vacuum
vessel
Inside
Helium
gaurd
-do-
Druck
LPX2380
SPL.
Rosemount
Model:3051
TA2A2B21
CQ4
-do-
9
PT181B
-do-
Vacuum
(10-10 mbar to
1 bar)
0 to 1 bar
Top
platform of
CFB
-do-
10
PE126
Pr(Vacuum)
downstream of
valve FV126
0 to 1 bar
-do-
11
PT127
0 to 1 bar
-do-
12
PE182
Pr. (vacuum)
upstream of
FV127
Turbo Pump
P002 suction
pr.(vacuum)
Inficon,
pirani/penni
ng gauge
Rosemount
Model:3051
TA1A2B21
CQ4
Inficon
(vacuum)
Type:PSG
400 (ref:
350-000)
-do-
-
-do-
Ingovi,
vacuum.
14
PDT239
0 to 80mbar
-do-
Rosemount
Model;
3051CD2A0
2A1CS5Q4
15
PDT265
ΔP across
FE239 (warm
He gas flow
from CWS)
ΔP across
FE265 (cold He
gas flow from
CWS)
0 to 300mbar
-do-
-do-
16
PT185
0 to 2 bar
-do-
8
Flow Instrumentation
1
FT239
2
FT265
3
FT136A/B
4
5
FT160
FT145
Warm helium
gas flow in
CFB from
CWS
Cold helium
gas flow in
CFB from
CWS
He. Gas mass
flow for
cooling the
resistive upper
section of
current leads
0 to
100gm/sec
In CWS
-do-
-do-
-
On CFB top
plarform
Brooks
mass
flowmeter
ref: 5863
-do
-doNot in use
Liquid He.
Flow in the
magnet for
cooling to
1.9K
Differential
head type
V- Cone
meter
-do-
Level Instrumentation
1
LE100A
Liquid Helium
level in CFB
Helium vessel
Lact = 300mm
Inside CFB
AMI make,
Superconducti
ng Nb-Ti level
sensor.
Φ = 6.35mm
2
3
LE100B
LE140
-doLact = 85mm
-do-do-
-do-do-
4
LE148
-doLiquid helium
level in X+Y
line.
Liquid helium
level in line
M2.
Lact = 40mm
-do
-do-
Most of the Important Cryo Valves (e.g.
CV145,CV150,CV103,CV104..) are of Velan make Model: SCGR-DN
6-32.
Most of the security (safety valves- Pressure relief valves) are of Circle
Seal controls Corona, California make.
Helium gas leak detector used for detecting the helium leak if any
from the system is of Pfeiffer make.
......................................................................................................
9
Chapter 3
Temperature Instrumentation:
Temperature measurement is a key issue in the Large Hadron Collider (LHC), as it
will be used to regulate the cooling of the superconducting magnets. The compromise
between available cooling power and the coil superconducting characteristics leads to
a restricted temperature control band, around 1.9 K.
The various components of the LHC cryogenic system work at temperatures from
ambient down to 1.6 K. Depending on the actual temperature value, different
accuracies are required on its measurement. Between 300 K and 25 K, an uncertainty
of 5 K can be tolerated to monitor the warmer components and the general cooldown. However, at the nominal operation of superconducting magnets (below 2.2
K, i.e. at 1.9K) only 10 mK inaccuracy is allowed, to give enough room for the
regulation band of the cryogenic controller, while avoiding magnet quench and
minimizing the cooling effort of the cryogenic system. Table 2 below shows the
allowed uncertainty on temperature measurement. The aimed resolution has to be ten
times better than the overall accuracy (dT < 1mK, below 2.2 K temperature
measurement on the LHC machine).(4)
Table#2: Required overall Temp. accuracy and resolution:
Temperature Span (K)
Accuracy (mK)
Resolution (mK)
1.6 to 2.2
10
1
2.2 to 4.0
20
2
4.0 to 6.0
30
3
6.0 to 25
1000
100
25 to 300
5000
500
Also, the accuracy budget is to be evenly shared between the sensor and the signal
conditioning.
CERN has done extensive and elaborate study of various temperature sensors
available for cryogenic temperature measurements like CERNOX TM (CX), TVO,
RhFe (Rhodium Iron Resistance Temperature Detector), AllenBradley(AB) carbon
resistor, Pt100 (Platinum RTD with 100 Ω resistance at 0◦C) etc. Other temperature
sensors that are available for low temperature measurements are Silicon diodes,
germanium sensors, helium three isotope gas bulb vapor pressure measurement etc.
The various guidelines for selection and usage were:
1.
2.
3.
4.
5.
6.
7.
8.
9.
Single sensor covering full temperature range from 1.6K to 300K.
Sensor should be able to withstand the thermal and pressure cycling.
High magnetic field environment ( around 9 teslas).
Radiation field.
Accuracy (inclusive of signal conditioning electronics and hardware)
shall be better then the required specification as mentioned in Table#1
above.
very good long term stability and very very low long term drifts.
Maintenance free as most of these sensors will be inaccessible while
the system is in operation.
Very good sensitivity specifically at low temperatures (around 1.9K).
self heating effect…
10
CERNOX™ (CX), TVO®
and RhFe listed above cover the full temperature range with a
single sensor. AllenBradley® (AB) and Pt100 can be combined to cover respectively
low and high temperature scales, or used alone in applications not requiring full range
measurements.
In terms of resistive values, CERNOX (CX’s) span is the largest among all sensors (3
decades), requiring wide dynamic range signal conditioning. Also covering the whole
temperature range, RhFe spans over only 2 decades of resistance, with the advantage
of less demanding dynamic range, but with the consequence of limited sensitivity.
Sensors with negative dR/dT, like CX, TVO® and AB, show high resistance and high
sensitivity (dR/R / dT/T) at low temperatures, where measurement accuracy has to
be at its best. This semiconductor behavior relaxes the constraints on signal
conditioner accuracy for low temperature measurement. On the other hand, at low
temperature metallic sensors like RhFe exhibit a sensitivity one order of magnitude
worse, demanding much more accurate signal conditioning.
Typical characteristics of cryogenic temperature sensors
Sensor type
T Span (K)
R Span (Ω)
dR/dT (Ω/K)
(dR/dT)/(dT/T)
CERNOX(CX) 1.6 to 300
30000 to 30
-40000 to -0.1 -2.7 to -1.0
TVO
1.6 to 300
9000 to 900
-7000 to -0.7
-1.3 to -0.2
RhFe
1.6 to 300
6 to 110
+0.7 to +0.4
+0.2 to +1.0
AB
1.6 to 100
10000 to 100
-12000 to -0.3 -3.0 to -0.2
Pt100
73 to 300
18 to 110
+ 0.4 to +0.4
+2.0 to +1.0
Of almost all the sensors mentioned above, CERNOX™ (CX) is the sensor best suited
for LHC application. CERNOX™ (CX) temperature sensor possess many attributes
desirable in a temperature sensor for LHC type of project application including high
sensitivity, excellent short- term and long term stability, small physical size, fast
thermal response and very very low calibration drifts (almost negligible) when
exposed to magnetic fields and ionizing radiation. Details of this sensor is discussed
in following sections:
TT821:
TT821 is CERNOX TM (CX) temperature sensor manufactured by M/s Lakeshore.
Sensor Model No: XCX-1050-SD-30 and is referred as Short Thermometer. This
sensor is used to monitor magnets internal temperature and same is mounted inside
the magnet on the SS nonmagnetic collars. For proper mounting of the sensor, a
support block made out of PCB (developed by CERN) is used. CERNOX sensor is
push fit mounted in a groove provided in this short thermometer block. The 4 lead
wires of the CERNOX sensor are soldered to the 4 soldering points provided on the
block. This block is then screwed on the SS collar inside magnet and a polymide foil
is sandwiched between thermometer and mounting surface to avoid electrical damage
of the sensor in case the surface is under high electrical potential. (5).
11
There is one drawback of this sensor.i.e. these sensors are not directly
interchangeable. Because each individual sensor has it’s own specific calibration
curve, it is strictly forbidden to interchange thermometers. Each thermometer has got
its own fit and coefficients and same is available in MTF for the corresponding
magnets.
The resistance of this temperature sensor is measured by four wire technique in order
to get rid of lead wire resistances. Therefore, a 4 –wire twisted thermometer cable is
soldered to the thermometer. To minimalise heat flow from ambient environment to
the sensor by conduction of electrical leads, thin wires are used ( silver plated copper
wires of AWG30 with polyimide insulation with ρ(300K) of 0.32 Ω/m is used). Stress
on these thin wires is avoided by more robust extension wires, which are
mechanically fixed (i.e. by a knot) close to the connector. Extension wires are also
silver plated copper wires of AWG24 with polyolefine insulation and ρ(300K) of
0.07Ω/m.
Fig#4: installation of TT821 (Cernox) inside magnet- wiring methodology.
What is CERNOX™ (CX) ? (8)
The “CERNOX™ (CX)” temperature-sensing element is used in the magnet assembly
(TT821). “CERNOX™ (CX)” (short for Ceramic Nitride-Oxide) is a thin film resistance
temperature sensor commercialized by M/s Lakeshore Cryotronics,Inc. The sensor is
fabricated from zirconium reactively sputtered in a nitrogen-oxygen atmosphere. The
resulting thin film is comprised of conducting zirconium nitride embedded within a
zirconium oxide non conducting matrix. This material has a negative temperature
coefficient of resistance making it useful as a temperature sensor. To tailor the sensor
to a given temperature range the ratio of conducting to non conducting material is
varied. The main advantage is a single device can be fabricated for use from
below 0.3 K to 420 K. Cernox temperature sensors also possess many attributes
desirable in a temperature sensor including high sensitivity, excellent short-term and
long-term stability, small physical size, fast thermal response and small calibration
shifts when exposed to magnetic fields or ionizing radiation. It should be noted that
each fabricated sensor has a typical characteristic polynomial curve. The typical
resistance values are around 45000 Ohms at 1.6K and around 60 Ohms at Room
temperature (300K).
Each sensor that is used in magnet is independently and individually calibrated in a
lab. and its coefficients (fit) are made available for future reference. Coefficients are
normally stored in MTF in components folder under the corresponding magnet in
12
which this particular sensor is going to be used. Typical curve fit equation for a
Cernox sensor is:
T = 10 ^∑{A(i) x [1/log10I] ^i }, where i = 0,…..,9 and A(i) are coefficients.
R is resistance in Ohms and T is temperature in K.
e.g.: For a Cernox thermometer calibrated in a lab following are the typical
observations:
Thermometer CX_LS_X09273.
Range: 1.615725 to 290.5813K.
(Table# 6) Coefficients:
A(0)
41.44669
A(1)
-1192.59
A(2)
14621.59
A(3)
-101618
A(4)
443679.2
A(5)
-1262277
A(6)
2342678
A(7)
-2739332
A(8)
1834192
A(9)
-536570
Fig# 5: Resistance as a function of temperature for CERNOX family
of temperature sensors:
Stability: Stability is one of the most important characteristics for a temperature
sensor. Short term stability for Cernox sensors is tested during manufacturing and is
found to be better than ± 3 mK repeatability at 4.2 k upon repeated thermal cycling.
Long term stability data is available in terms of “ Mean deviation from original
calibration after 5.8 years as a function of temperature for 39 Cernox sensors chosen.
At temperature of 1.4 K, the mean deviation was + 0.05 milliKelvin and at 4.2K
temperature the mean deviation observed was -0.17K.
For checking the effects of radiation effects on these Cernox sensors (Prior to their
selection for LHC Project), experts and researchers at CERN has thoroughly
investigated the radiation tolerance of Cernox temperature sensors. In one study, 66
Cernox sensors at 1.8 K were irradiated with neutrons to total fluences of 3 x 1014
13
n/cm2 to 1 x 1015 n/cm2. The mean calibration shift at 1.8K was +1mK. No signs of
thermal annealing were observed. In a second experiment, cernox temperature sensors
from two different lots were irradiated at 4.2 K by a neutron source to a total fluence
of normally 1 x 1015 n/cm2. Sensors within each lot behaved in a similar manner. The
first set had the lower sensitivity and showed a continually decreasing resistance
throughout the irradiation with an equivalent temperature variation of 4mK. At 4.2K.
The resistance of the second set initially increased slightly and then decreased
showing an equivalent temperature variation of about 2.5mK. These data evidence the
cernox’s insensitivity to radiation.
Signal Conditioning for Cernox Temeprature sensor: In order to meet the stringent
accuracy requirements, experts at CERN had explored all the possibilities of various
signal conditioning circuits that can be used like Logarthmic conditioner, Linear
Multirange conditioner etc. The Linear multi-range conditioner was found to be
satisfying all the accuracy requirements. Figure #7 below shows the front end block
diagram of this signal conditioning circuit.. It is well known that self heating effects
cannot be neglected when using resistive type sensors at low temperatures and hence
they impose the use of relatively low exitation currents. Therefore, the input span of
30 Ohms to 30 Kohms for this cernox sensor is divided into three parts (3 decades) so
that the sensing current is kept lower for high R values therby reducing the self
heating of the sensor, and higher for small R values, increasing the voltage developed
at the sensor and its signal to noise ratio. Refer Table# 7 . (4)
Table# 7
Resistance (Ω)
Current (µA)
Typical Temperature (K)
30 to 300
100
50 to 300
290 to 3000
10
5 to 50
2900 to 30000
1
1.5 to 6
The front end design is based on comparison bridges in which the comparison signal
is generated by means of a radiation hard reference resistor. This reference resistor is
a high accuracy (0.01%) and low thermal drift (10ppm/K) thick film NiCr metallic
resistor. Refer figure below. This comparison bridge permits to compensate variations
in excitation current supply and variations in amplifier gain. In order to get rid of
thermoelectric/thermocouple voltages AC measuring technique (current inversion –
bipolar sensing current, oscillating at 2 Hz) is used. Amplifier offset is compensated
by means of the inverting switch at the front end amplifier input. Voltage developed
across the reference resistor and the Cernox temperature sensor is measured in order
to compute the R value and thereby the temperature as sensed by Cernox sensor. { R
= (V/Vref) * Rref.} where V is the voltage measured across the sensor, Vref is the
voltage developed across the reference resistor, Rref is the value of reference
resistance and is a known constant and R is the sensor resistance. The actual
temperature value is computed by inserting this R value in the Curve fit equation for
the cernox sensor (for which coefficients were obtained at the time actual
temperature calibration).
14
Fig: 7. Signal conditioning front end for Cernox tempetaure sensor.
The signal conditioning circuit shown above is the one that will be used for LHC. One
which is used in SM 18 Magnet test set up is more or less same but with minor
differences. Instead of field bus output, the one used in SM18 has two types of out
puts, i.e. 1)0 to 10V DC analog output as the signal corresponding to the resistance
value and 0 to 6V DC as the out put corresponding to Range of resistance that is being
measured. 2) 4 to 20mA DC analog output as the signal corresponding to the
resistance value and 2 bit digital value indicating the Range of resistance that is being
measured. Out of these two available options, the second one is used in SM18. The 0
to 10V DC output and 0 to 6V values for indicating range is fed to a LF data
acquisition system for further computations and indication of the actual temperature
value.
Redundancy of TT821: Each magnet is provided with only one TT821 Cernox
temperature sensor. The total cost for each Cernox sensor inclusive of the calibration
cost works out to be much higher and hence its not economical to have back up sensor
in the magnet. Then how about redundancy? What happens when this sensor fails? If
sensor fails when in use in LHC, the back up to this will be available from the
adjacent (previous or next) magnets TT821 (cernox) temperature indication. Thus the
redundancy aspect is taken care of.
TE147 is a Helium 3 bulb Vapor Pressure temperature sensor. The pressure exerted
by a saturated helium vapor in equilibrium with its liquid is a very definite function of
temperature and same principle is used to measure the temperature of the liquid
helium in line M2. This temperature indication is also used to check whether the 1.9K
cryo OK condition has come or not. The most common form of vapor pressure
equation are :
LogP = A + B/(C + T) where A, B and C are the coefficients and are constant.
T is the temperature and P is the pressure. For temperature measurement, this helium
vapor pressure bulb is inserted in the helium line whose temp is to be monitored and
15
the pressure generated by this filled bulb system is monitored in order to compute the
corresponding temperature. This helium vapor pressure is monitored by using Druck
make absolute pressure transmitter LPX2380 SPL.
TE-149 and TE150B: Sensor :Resistance Carbon Glass manufacturd by M/s LakeshoreRef:CGR-1-1000-1.
Carbon-Glass RTDs (CGRs) have the longest history of use of any sensor suitable for high
magnetic fields and wide range temperature sensing. These resistance temperature sensors
are highly reproducible and can be used from 1.4 K to 100 K and in magnetic fields up to 20
tesla. Their extremely high sensitivity at liquid helium temperatures makes them very useful
for submillikelvin control below 10 K. CGR sensors are monotonic in resistance temperature
characteristic between 1.4 K and 325 K, but their reduced sensitivity (≈0.01 /K) above 100
K limits their usage at higher temperatures.
Features:







Low magnetic field induced errors
For use in magnetic fields up to 20 tesla
Reproducible in the 1.4 K to 100 K range
Monotonic R vs. T and dR/dT vs. T response curves
High sensitivity provides submillikelvin control at 4.2 K and below
Usable sensitivity over the broad range of 1.4 K to
325 K
Good resistance to ionizing radiation at low temperatures
Typical Carbon-Glass Resistance
Values
Typical Carbon-Glass Sensitivity
Values
Typical Carbon-Glass
Dimensionless Sensitivity Values
Fig# 8
TE 148: (Carbon Resistance Thermometer- CRT): This is the Carbon resistance
thermometer and is referred as CRT. The one used here and for LHC is “AllenBradley (AB) make” Model CRT_AB(100 Ω, 1/8 watt). CRT has high sensitivity at
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low temperatures and it is one of the low cost sensor. This particular sensor is most
suitable for temperature range below 20K. At higher temperature it has decreasing
sensitivity, stability problems and also competition with the metallic resistors like
platinum(Pt) etc. The CRT sensors have NTC.
The typical interpolation equation that fits the data in 2K to 20K range is:
[(LogR)/T]1/2 = A + BLogR,
Where, R is the resistance at temperature “T” and A,B are the experimentally
determined constants.
{e.g.: A typical 0.5Watt, 220Ω resistance will measure roughly 1KΩ at 1K, 20KΩ at
0.1k and 300KΩ at 0.015K.}
…………………………………………………………………………………………..
Chapter 4
Flow instruments—
a) Thermal mass flow meter is mainly used for low gases flow rates at ambient
temperature. (Brooks make mass flow meter)
b) Large liquid flows at very cold temperature are measured by drop in level method
by “Hot Superconductor Wire”. The principle of level measurement is described
under level heading.
c) Large gas flows are measured by V-cone meters. The differential head developed
in the V- cone is measured by standard industrial differential pressure transmitter.
FT239 (Warm helium gas flow in CFB from CWS) and FT265 (Cold helium gas
flow in CFB from CWS): (Fig# 9). These flows are measured by V- Cone (√CONE)
meters Model VB. V – Cone meter is a differential pressure type flow measurement
device. A SS cone is positioned in the center of the pipe to increase the velocity of the
flowing fluid and create a differential pressure across the two taps (P1 and P2 as
shown in fig below). This differential pressure generated is measured using “FisherRosemount make,
Model 3051TA2A2B21CQ4, smart differential pressure
transmitter. The output of this transmitter is 4 to 20mA DC linear output and is fed to
PLC for further computation of the flow. The flow ‘Q’ and ‘∆P’ has a square root
relationship, i.e. Q = K √∆P. (PT239 and PT265).
The accuracy for this V cone meter is ± 0.5% of flow rate and repeatability is of the
order of ±0.1%. The turn down ratio is 10:1. It may be noted that like other
differential pressure flow measuring instruments ( Orifice plates, Venturies etc), this
V-cone is also sized for the intended application.
The V-Cone meters has many advantages as compared to other traditional instruments
like Orifice plates and those are:
1)
V cone can measure almost all process fluids like liquids, slurries, gases
and steam.
2)
Short straight run requirements hence making it much effective from space
utilization point of view as well as from cost saving point of view for the
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3)
4)
entire plant. (Typically 0 to 3 diameters upstream and 0 to 1 diameters
downstream).
Low permanent pressure loss.
Durable against wear and sticking as the V-cone because of its design has
a unique self cleaning capability as the fluid runs of from the beta edge of
the cone.
Fig:9 :V-cone flow meter.
Flow Instrumentation for Water cooled cables: Flow through water cooled
High current power cables is monitored by Eletta (Sweden) make Differential
pressure type Indicator cum flow switch. This flowmeter is mounted on the return
line.
The Eletta Flow Monitor is based on the proven and dependable differential pressure
principle, using interchangeable orifice plates for different measuring ranges. The
Flow Monitors are working with differential pressure ranges, i.e. 50 – 200 mbar for
Type S2-FSS26 and the accuracy specified is ± 5% of FS. The Instrument consists of
two parts mainly i.e. the Pipe Section and the Control Unit. The Pipe Section is the
part that is mounted in the process pipe and the Control Unit is mounted directly
(standard) or remote on/to the Pipe Section. The Control Unit is gives the Flow
information and also contains the SPDT contacts. The Control Units S02 have a
local readout and is equipped with two independent adjustable alarms (micro
switches) which can be set for low and high flow alarm. The readout has a scale,
which shows the ordered flow range with a multiplier. The differential pressure
signal generated by the orifice plate is sensed by the diaphragm and in turn moves the
diaphragm linearly. This diaphragm movement is transmitted by linkages and gearing
mechanism to the indicator (pointer) inside the control unit.
……………………………………………………………………………………..
18
Chapter 5
Level Instrumentation
(LE100A/B, LE140, LE148):
Principle of Liquid helium level measurement:
This is based on “Hot wire”
method of detecting the depth of the liquid. The active element of the probe is a
Niobium-Titanium (NbTi) wire, whose superconductivity transition temperature is
above the boiling point of liquid helium. At the start of measurement the probe is
excited by a boost current, which is in excess of measuring current. This heats the top
of the element & causes the normal / superconducting interface to propagate down the
element. When this interface reaches the surface of the liquid, the much greater
cooling power available causes the propagation to cease. This is detected & the
booster current is reduced to normal measuring current. The voltage drop across the
probe is a function of the depth of immersion as the effective resistive length of this
NbTi wire probe changes with the depth of its immersion in liquid helium. With
suitable lookup table & electronics the actual height / depth of liquid is found.
LE100A/B, LE140, LE148 level elements are AMI (American Magnetics,Inc) make
liquid helium level sensors. The diameter of the NbTi probe is 6.35mm and the active
length for each sensor/probe depends on the application (refer Table#1). The overall
length is usually 1 inch longer than the active length (0.5 inch at top and 0.5 inch at
bottom).(6). The typical value of the boost current is of the order of 200mA and the
measuring/sensor current is of the order of 75mA. The maximum magnetic field for
which this sensor can function is 10 Tesla. Nominal sensor resistance is 4.5 ohms/cm
at 20K and 5.4ohms/cm at 300K. Normally this NbTi wire has a small heater attached
to initiate a resistive zone. The selection of the boost current and the measuring
current is very tricky affair as large helium losses can occur if high current is left on
continuously.
It may please be noted that the installation of this sensor probe should be avoided
where icing (frozen water ..) may occur since ice formations may cause erratic
operation as ice formation may stop the propagation of the normal (resistive) zone
before it actually reaches the liquid/gas interface. This may result in giving the
indication of a higher helium level than it actually exists.
………………………………………………………………………………………
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Chapter 6
13kA Current Lead Instrumentation
Fig:3: Instrumentation associated with Gaseous Helium cooling circuit for 13KA
and 600Amps Current Leads.
Connections of voltage taps and temperature sensors is done by Fisher connectors.
(Lead body isolation voltage to earth is 2KV in helium atmosphere). The room
temperature terminals are nickel plated for good electrical contact and the cold
terminals are silver plated for good electrical contacts.
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Chapter 7
Cryo O.K. For 1.9K, 4.2K and for HV @Cold:
All the cryo sensor signals for Cryo O.K. at 1.9K, 4.2K and HV@Cold are
highlighted in blue colour in the instruments table (Table#1).
Cryo O.K. at 1.9K: Following Process conditions must be fulfilled in order to have
Cryo O.K. at 1.9K:
Table # 3
SL.
Sensor
Parameter
Condition
No.
1
2
TT147
LT100A
3
4
TT130 A/B
LT140
5
FT136
Temp on line M2
Liquid Helium level in
CFB Helium vessel
13KA current lead
Liquid helium level in
X+Y line
He. Gas mass flow for
cooling the resistive
upper section of current
leads
<1.94K
> 40% level
< 300K
< 20%
>580mg/sec
Cryo O.K. at 4.2K: Following Process conditions must be fulfilled in order to have
Cryo O.K. at 4.2K:
Table # 4
SL.
Sensor
Parameter
Condition
No.
1
LT100A
2
3
TT130 A/B
LT148
4
FT136
Liquid Helium level in
CFB Helium vessel
13KA current lead
Liquid helium level in
line M2
He. Gas mass flow for
cooling the resistive
upper section of current
leads
> 40% level
< 300K
>76%
>580mg/sec
Cryo O.K. for HV@ Cold : Following Process conditions must be fulfilled in order
to have Cryo O.K. for HV@Cold:
Table # 5
SL.
Sensor
Parameter
Condition
No.
1
2
TT130 A/B
LT148
13KA current lead
< 155K
Liquid helium level in >76%
line M2
21
Conclusion: This write up is an effort to understand some of the instrumentation
aspects of the Cryogenic Instrumentation in SM18 Magnet test facility. Working
principles of various instruments were also discussed. The HF/LF data acquisition
system and the control philosophy is not discussed here.
References:
1. Cryogenic Infrastructure for Testing of LHC Series Superconducting Magnets
by J. Axensalva, V. Benda, L. Herblin, JP. Lamboy, A. Tovar-Gonzalez and
B. Vullierme
2. Cryogenic Engineering by Thomas M. Flynn.
3. Basic Cryogenic document- Cryogenics for LHC dipole by Uttam Bhunia,
VECC.
4. Signal Conditioning for cryogenic thermometry in LHC by J. Casas,
M.A.Rodriguez Ruiz…(CERN)
5. Installation guide for LHC cryogenic Thermometers prepared by Ch. Balle,
LHC/ACR.
6. Installation,Operation and maintenance Instructions for the AMI (American
Magnetics Inc) liquid helium level sensor.
7. CFB Operator guide by Antonio Tovar- Gonzalez, CERN
8. Review of Cernox Thin-Film Resistance temperature sensor by S. Scott Courts
and Philip R. (lake shore cryotonic Inc).
Acknowledgements: We wish to extend our gratitude and thanks to Dr. Vinod Chohan for his
encouragement in preparing this document. Thanks also to Shri Bruno Vullierme,
Shri Jean Paul Lamboy, Shri Christoph Balle, Giorgio D’Angelo & our other
colleagues who have shared valuable technical information and for their advice
and guidance.
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