Implementation of an Ultrasonic Instrument for Simultaneous Mixture and Flow Analysis of Binary

Gas Systems M. Alhroob, R. Bates, M. Battistin, S. Berry, A. Bitadze, P. Bonneau, N. Bousson, G. Boyd, G. Bozza,

O. Crespo-Lopez, C. Degeorge, C. Deterre, B. DiGirolamo, M. Doubek, G. Favre, J. Godlewski, G. Hallewell, A. Hasib, S. Katunin, N. Langevin,, D. Lombard, M. Mathieu, S. McMahon, K. Nagai, A. O’Rourke, B. Pearson,

D. Robinson, C. Rossi, A. Rozanov, M. Strauss, V. Vacek, R. Vaglio, J. Young, and L. Zwalinski.

Abstract–Precision ultrasonic measurements in binary gas systems provide continuous real-time monitoring of mixture composition and flow. Using custom microcontroller-based electronics, we have developed an ultrasonic instrument, with numerous potential applications, capable of making continuous high-precision sound velocity measurements. The instrument measures sound transit times along two opposite directions aligned parallel to - or obliquely crossing - the gas flow. The difference between the two measured times yields the gas flow rate while their average gives the sound velocity, which can be

Manuscript received X XX, 2015; accepted X XX, 2015. M. Alhroob, G. Boyd, A. Hasib, B. Pearson, M. Strauss and J. Young are

with the Department of Physics and Astronomy, University of Oklahoma, Norman, OK 73019, USA (e-mail: m.alhroob@cern.ch george.boyd@cern.ch, a.hasib@cern.ch, b.pearson@cern.ch, strauss@ou.edu, jyoung@ou.edu ).

R. Bates and A. Bitadze are with the School of Physics and Astronomy, University of Glasgow, G12 8QQ, UK (e-mail: r.bates@physics.gla.ac.uk, a.bitadze@physics.gla.ac.uk).

M. Battistin, S. Berry, P. Bonneau, G. Bozza, O. Crespo-Lopez, B. DiGirolamo, G.Favre, J. Godlewski, D. Lombard, R. Vaglio and L. Zwalinski are with CERN, 1211 Geneva 23, Switzerland (e-mail: michele.battistin@cern.ch, stephane.berry@cern.ch, pierre.bonneau@cern.ch, gennaro.bozza@cern.ch, olivier.crespo.lopez@cern.ch, beniamino.di.girolamo @cern.ch, gilles.favre@cern.ch, jan.godlewski@cern.ch, didier.lombard@cern.ch, romain.vaglio@cern.ch, lukasz.zwalinski@cern.ch).

N. Bousson, G. Hallewell, M. Mathieu and A. Rozanov are with the Centre de Physique des Particules de Marseille, 163 Avenue de Luminy 13288 Marseille Cedex 09, France (e-mail: bousson@cppm.in2p3.fr, gregh@cppm.in2p3.fr, michel.mathieu@cern.ch, rozanov@cppm.in2p3.fr).

C. Deterre and A. O’Rourke are with Deutsches Elektronen-Synchrotron, Notkestrasse 85, D-22607 Hamburg, Germany (email:cecile.deterre@desy.de, abigail.orourke@desy.de).

M. Doubek and V. Vacek are with the Czech Technical University, Technick 4, 166 07 Prague 6, Czech Republic (e-mail: martin.doubek@cern.ch, vaclav.vacek@cern.ch).

C. Degeorge is with the Physics Department, Indiana University, Bloomington, IN 47405, USA (e-mail: c.degeorge@cern.ch).

S. Katunin is with the B.P. Konstantinov Petersburg Nuclear Physics Institute (PNPI), 188300 St. Petersburg, Russia (e-mail: sergey.katunin@cern.ch).

N. Langevin is with the Institut Universitaire de Technologie of Marseille, University of Aix-Marseille, 142 Traverse Charles Susini, 13013 Marseille France (e-mail: langevin@cppm.in2p3.fr).

S. McMahon is with the Rutherford Appleton Laboratory - Science & Technology Facilities Council, Harwell Science and Innovation Campus, Didcot OX11 OQX, UK (e-mail: stephen.mcmahon@cern.ch).

K. Nagai is with the Department of Physics, Oxford University, Oxford OX1 3RH, UK .(e-mail:koichi.nagai@cern.ch).�

D. Robinson is with the Department of Physics and Astronomy, University of Cambridge, UK (email: robinson@hep.phy.cam.ac.uk).

C. Rossi is with INFN - Genova, Via Dodecaneso 33, 16146 Genova, Italy (e-mail:cecilia.rossi@cern.ch).

compared with a sound velocity vs. molar composition look-up table for the binary mixture at a given temperature and pressure. The look-up table may be generated from prior measurements in known mixtures of the two components, from theoretical calculations, or from a combination of the two.

We describe the instrument and its performance within numerous applications in the ATLAS experiment at the CERN Large Hadron Collider (LHC). The instrument can be of interest in other areas where continuous in-situ binary gas analysis and flowmetry are required.

I. INTRODUCTION

We describe a group of microcontroller-based ultrasonic instruments for continuous real-time flowmetry and composition analysis of binary gas mixtures. The instruments exploit the phenomenon whereby the sound velocity, in a binary gas mixture at a given temperature and pressure, is simply a function of the molar concentration of the two components of differing molecular weight.

Five of these instruments (referred to hereafter as sonar instruments) are used within the ATLAS Detector Control System (DCS) for flowmetry, coolant composition determination, and leak detection in the evaporative cooling systems of the ATLAS silicon tracker [1]. The broad extent of their implementation is shown in Fig. 1.

The silicon tracker contains three sub-detectors located close to the LHC beams which must be kept below 0 °C to maximize their lifetime under the intense radiation from beam collisions. The evaporative coolants currently used are octafluoropropane (C3F8), in both the Pixel detector and the silicon-microstrip Semi-Conductor Tracker (SCT), and carbon dioxide (CO2), in the new innermost layer of the Pixel detector: the Insertable B-Layer (IBL). These coolants were chosen for their dielectric nature and radiation resistance as well as for their non-flammable, non-toxic, and non-ozone-depleting characteristics. In the future, C3F8 may be blended with up to 25% hexafluoroethane (C2F6) to allow Pixel and SCT operation at even lower temperatures if necessary [2].

Three of the five sonar instruments are located in an underground technical area (USA15) where they - with the aid of selection valves and vacuum pumps - aspirate and analyze gas from the nitrogen-purged (anti-humidity) environmental gas enclosures surrounding the aforementioned sub-detectors (one instrument per sub-detector), as shown in Fig. 1.

Fig. 1. The five sonar instruments incorporated into the ATLAS silicon tracker evaporative cooling system: one degassing sonar on top of the thermosiphon

condenser; one angled flowmeter sonar in the vapor return line of the thermosiphon; three aspirating sonars, one for each sub-detector of the ATLAS silicon tracker (Pixel, IBL, and SCT).

Two of the three aspirating sonar instruments monitor the nitrogen (N2) from the environmental enclosures of the SCT and Pixel sub-detectors for trace quantities of C3F8, while the third monitors the N2 from the volume surrounding the IBL for traces of CO2.

Two other sonar instruments (also shown in Fig. 1) are installed in the upgraded external cooling plant, which will operate as a thermosiphon (Section III-B, [3]), circulating C3F8 through the 204 cooling channels of the Pixel and SCT detectors. One of these (the angled flowmeter sonar) will measure C3F8 vapor flows up to 0.4 m3·s-1 returning to the thermosiphon condenser, while the other (the degassing sonar) will be used to detect air ingress into the condenser.

II. SYSTEM OVERVIEW

A. Operation The principle of operation of the sonar instruments is

shown in Fig. 2. The amplification and biasing scheme is adapted to the SensComp Model 600 instrument-grade 50 kHz capacitive-foil ultrasonic transducer [4] operated as a transceiver with a DC bias voltage of around +300 V.

Fig. 2. Principle of operation of one channel of the ultrasonic instrument,

which allows simultaneous flowmetry and binary gas analysis.

When transmitting, the transducer foil is excited by down-going (300 V→0 V) square-wave pulses generated from low-voltage precursors in a dsPIC33F microcontroller, which also reverses the transmission direction. The receiving chain incorporates two amplification stages followed by a comparator, also implemented in the microcontroller [5]. Transmission synchronously starts a 40 MHz transit time clock, which is stopped by the first pulse crossing the user definable comparator threshold.

The difference between transit times (TD and TU in Fig. 2) in the opposing directions is used to compute the gas flow rate, while their average is combined with temperature and pressure measurements to compute the binary gas composition by interpolating between stored sound velocity vs. concentration curves. These analog temperature and pressure measurements are made with an analog-to-digital

convertor implemented in an Analog Devices microcontroller (ADμC).

The bi-directional transit times, as well as the vapor temperature and pressure measurements, are buffered, averaged, time-stamped, and pipelined by a FIFO (First In, First Out) memory implemented in the dsPIC33F microcontroller for transmission to a SCADA (Supervisory Control And Data Acquisition) computer. Presently, the mixture calculations are made using this computer, but in future versions of the instrument these calculations may be made in an on-board microcontroller to allow full standalone operation. The instrument presently measures sound velocity with an uncertainty of less than 0.025 m·s-1 over acoustic paths in the range of 0.5 to 1 m, providing for precise measurements of leak concentrations.

B. Architecture Fig. 3 illustrates the communication architecture of the

system where, in addition to the sonar instruments, electronic modules have been built for the control of vacuum pumps and selection valves for gas aspiration from the silicon tracker volumes. Also, since the five sonar instruments are widely dispersed – in height alone by over 90 meters – a protocol for long distance communication with the SCADA computer is required. The MBED LPC1768 platform [6] is installed and communication uses TCP/IP Modbus over Ethernet.

Fig. 3. System communication architecture diagram.

The SCADA computer is a Dell PowerEdge R610 running Siemens SIMATIC WinCC [7] under Linux. In addition, the microcontroller sends 4-20 mA analog current signals proportional to transit times to the thermosiphon PLC-based (Programmable Logic Controller) hardwired control system.

C. Software and Analysis The readout and control of the sonar instruments, as well as

their selection valve and vacuum pump drivers, is based on a project developed using WinCC that incorporates a graphical user interface and calculates binary gas compositions and flow. The software also archives data to the ATLAS DCS database and generates alarms that are propagated through the

Gas

flow

Driver

Receiver

HV Bias generator

μ-controller

Threshold generator Communica on

interface

Acous

c transdu

cers

Up/Down Direc onal Switch

Threshold

Transit me TD

Transmi ng transducer input signals

Bias

Voltage

D U

Sound signals on receiving transducer

Transit me TU

t [s]

U [V

]

t [s]

U [V

]

Pixel sonar tube

IBL sonar tube

SCT sonar tube

ATLAS alarm finite state machine handler for expert intervention.

For our analysis of the binary gas compositions we use the general formalism for the speed of sound in a gas mixture,

, (1)

where R is the molar gas constant (8.3145 J·mol-1·K-1), T is the absolute temperature in Kelvin, Mm is the combined molar mass of the mixture, is the adiabatic index for the mixture, and is the magnitude of the sound velocity. Mm and are given respectively by

and

where is the molar fraction of component i in the mixture while and are the component’s molar specific heats at constant pressure and volume, respectively.

The and values for the individual mixture components are calculated by the NIST REFPROP package [8] for a phase-space of possible operating temperatures and pressures. The results of some of these calculations are shown in Figs. 4 and 5 for the mixing of CO2 and N2.

Fig. 4. Molar specific heat at constant pressure vs. temperature for N2 (left

vertical axis) and CO2 (right vertical axis) at a range of pressures.

Fig. 5. Molar specific heat at constant volume vs. temperature for N2 (left vertical axis) and CO2 (right vertical axis) at a range of pressures.

Using (1), these values are used to calculate the sound

velocity vs. concentration curves, some of which are shown in Figs. 6 and 7. The 15 °C curves are highlighted because that is the most probable operating temperature for at least three of the five instruments.

Fig. 6. Speed of sound predictions in N2 with up to 0.1% molar

concentration of CO2 at various possible operating temperatures and near atmospheric pressure. The zoomed region shows the effect that ±100 mbar of pressure has on the speed of sound predictions at a single temperature.

The relative precision in our mixture determination

depends on the precision in our sound velocity measurements and on the molecular weight difference between the two gas components. Individual contributions (shown in parentheses below) to the present overall ±0.025 m·s-1 sound velocity measurement uncertainty, δc, come from:

• ±0.1 C temperature uncertainty in the sonar tube (equivalent to ±0.022 m·s-1)

• ±1 mbar pressure uncertainty in the tube (±0.003 m·s-1) • ±0.1 mm transducer inter-foil measurement uncertainty

following calibration (±0.018 m·s-1) • ±25 ns electronic transit time measurement uncertainty

(±0.0005 m·s-1). Finally, the relative precision in the mixture determination, δf, at any concentration of the two components is given by δf = δc/|m| (2)

36.6

36.7

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29.168

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13 15 17 19 21 23 25

C P CO2 (J·mol

-1·K

-1)

C P N

2 (J·mol

-1·K

-1)

Temperature (°C)

N2 @ 1.05 Bar

N2 @ 1.00 Bar

N2 @ 0.95 Bar

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CO2 @ 1.00 Bar

CO2 @ 0.95 Bar

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13 15 17 19 21 23 25

C V CO

2 (J·mol

-1·K

-1)

C V N

2 (J·mol

-1·K

-1)

Temperature (°C)

N2 @ 1.05 Bar

N2 @ 1.00 Bar

N2 @ 0.95 Bar

CO2 @ 1.05 Bar

CO2 @ 1.00 Bar

CO2 @ 0.95 Bar

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eed of

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·s-1)

Frac onal Molar Concentra on of CO2 in N2

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13 °C

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15 °C @ 1.1 Bar

15 °C @ 1.0 Bar

15 °C @ 0.9 Bar

where m (m·s-1·[%leak-concentration]-1) is the local slope of the sound velocity/concentration curve.

In the case of C3F8 coolant leaks into the N2 envelopes of the ATLAS SCT and Pixel detectors, the average slope is -12.55 m·s-1·[%C3F8]-1 (Fig. 7) for C3F8 concentrations in the range of 0 to 0.1%. Using (2), this slope yields a mixture precision of ±0.002 %, which improves upon a previous long-term study – using an earlier version of the electronics and an older instrument design – of the C3F8 leak rate into the Pixel detector envelope reported in [9] and [10].

Fig. 7. Speed of sound predictions in N2 with up to 0.1% molar

concentration of C3F8 at various possible operating temperatures and near atmospheric pressure. The zoomed region shows the effect that ±100 mbar of pressure has on the speed of sound predictions at a single temperature.

For the IBL monitoring, the smaller difference in molecular

weight between CO2 and N2 results in a shallower slope in the sound velocity/concentration curves and therefore an increased uncertainty in the mixture determinations. For example, in the 0 to 0.1% molar concentration range, of most interest in leak detection, the slope is only -1.12 m·s-

1·[%CO2]-1 (Fig. 6). Using (2), this slope results in a mixture precision of ±0.022 %.

III. INSTRUMENT IMPLEMENTATIONS As seen in the previous section, the instruments are

particularly well adapted to measurements of mixtures with low concentrations of a heavy vapor in a lighter carrier, corresponding to many leak detection applications.

A. ATLAS Silicon Tracker Volume Aspiration As shown in Fig. 1, the SCT sub-detector is monitored

through four aspiration lines (two from the “barrel” region and two from the endcaps), while the Pixel detector and IBL each have one aspiration line. The instruments monitoring the N2 gas envelopes of these three sub-detectors composing the ATLAS silicon tracker are conveniently mounted together with their electronics in an underground service cavern. These instruments, along with their mounting and coolant manifolds, can bee seen in Fig. 8 prior to their installation in the cavern.

Fig. 8. Pre-installation assembly of the three aspirating sonar instruments

insulated and mounted on a supporting tray with their coolant manifolds. The instruments have a cylindrical gas cavity inside of a

copper tube that is concentric with an outer stainless steel tube forming an insulating (or temperature controlling) water jacket (Fig. 9). A key feature of the design is ease of maintenance. Since the instruments operate continuously for several months at least, the ultrasonic transducers can become fatigued (one manifestation of which is missing the first pulse on the receiving-end and reporting too slow a transit time) and need replacing. In the earlier designs of the tube [9], [10] this task was more difficult and time consuming.

Fig. 9. Design detail of an aspirating sonar instrument.

End flanges (Fig. 10) house the ultrasonic transducers and

NTC (Negative Temperature Coefficient) temperature sensors, together with the gas and electrical connections. These include a hermetic tri-axial BNC connector for

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Transducer face

End-cap

Insula ng transducer moun ng

Inner copper tube

Outer stainless steel tube

Chilled water annulus

Thermistor well

Extended thermistor mount

transducer signals, a HD15 multi-pin temperature sensor signals, and a gas inlThrough the use of an O-ring mounted in pistand a threaded-cap retaining-ring, each fremoved, re-installed, and even interchanged desired.

Fig. 10. End flanges for the aspirating sonar instrume The end flange illustrated in Fig. 9 is act

flange with an extended temperature probe rgas cavity, whereas current standard flangetwo NTCs mounted inside of the flanRecessed slots in the flange (Fig. 10) allowextended temperature probes at various lendetailed temperature profile in the gas ccurrently underway to determine whether needed, and to decide their best locations.

Following the LHC restart in April 2015, and barrel gas envelopes have been continone at a time, through the SCT sonar witcycle. Fig. 11 shows a 10-day subset of this provides a closer look at the data over a single

Fig. 11. Percent molar concentration of C3F8 in theSCT endcaps and barrel over a 10-day period.

The coolant concentration in the SCT bar

consistent with measurements made before

connector for let/outlet fitting. ton configuration flange is easily between tubes if

ents.

tually a modified reaching into the es have only the nges themselves. w for up to three ngths for a more avity. Tests are such probes are

the SCT endcap nuously sampled, thin a four-hour data and Fig. 12

e day.

e N2 envelopes of the

rrel envelopes is the 2013 LHC

shutdown [10], while the lower cendcaps is due to the greater envelopes. A six-minute delay is apoints following each sample crefreshing time of gas through thesonar tube.

Fig. 12. Percent molar concentration ofSCT endcaps and barrel over a 1-day period.

B. ATLAS Thermosiphon MonitoriThe two other versions of the

integrated into the monitor and coATLAS C3F8 thermosiphon recicommissioned), are described in the

1) Degassing Sonar In the new cooling system the ne

recirculate C3F8 coolant in the ATdetectors is hydrostatically generatdifference between the elevated cosubterranean cavern. In order forcondenser, the condenser must opthan the silicon tracker cooling charound 1.7 bar abs). The condenser will typically opercondensation pressure of 300 point of the system it is the locatioto collect. A sonar device mouncondenser (Fig. 13) monitors the cochanges in sound velocity. Air coupper reservoir, which is arouncondenser. An encroachment of thesonar tube decreases the transit time

The degassing sonar is equippecontrolled heater jacket to maintarange 15 → 25 °C, counteracting costainless steel piping and valves link

Sound velocity will be contcompared to velocity vs. concentrmixtures in a similar way to the N2analyses in Section II-B. When ththat expected in C3F8 vapor by more

concentration in the SCT leak-tightness of these

applied when plotting data change to allow for the e ~1.5 litter volume of the

f C3F8 in the N2 envelopes of the .

ng e sonar instrument, being ontrol systems of the new irculator (currently being e following sections.

ecessary liquid pressure to TLAS Pixel and SCT sub-ted from the ~92 m height ondenser and the ATLAS r vapor to return to the

perate at a lower pressure hannels (which operate at

rate at -60 C for a C3F8 . As the lowest pressure

n ingressed air is expected nted vertically above the oncentration of air through ollects preferentially in its nd 2 m higher than the e air down into the vertical e in the acoustic path. d with a thermostatically-ain its temperature in the old conduction through the king it to the condenser. tinuously monitored and ration curves for C3F8/air 2/C3F8 and N2/CO2 mixture he transit time falls below e than a pre-defined

Fig. 13. Left: the degassing sonar vertically mounted above the thermosiphon condenser, before the fitting of its temperature control jacket and insulation.

Right: a schematic of the degassing sonar.

threshold, the analyzer tube and collection tank will be isolated from the condenser vapor volume and evacuated to eliminate the gas considered too rich in incondensable air.

The precision of binary mixture analysis is less demanding in this application than in monitoring the silicon tracker environmental volumes. An air concentration of a few percent will probably be used to trigger the venting: triggering at too low a level would waste expensive C3F8 vapor, while triggering at too high a level could allow the condenser pressure to rise to a level that would unacceptably reduce the C3F8 vapor return flow.

2) Angled Flowmeter Sonar The final sonar instrument is an angled flowmeter that will

measure the C3F8 vapor flow returning to the thermosiphon condenser from the silicon tracker (Figs. 1 and 14).

This custom sonar instrument uses the same ultrasonic transducers and electronics as the previously described instruments. The flowmeter must operate at the required high volume flow of around 0.4 m3·s-1 in C3F8; equivalent to a linear flow velocity of around 22 m·s-1, or Mach 0.2. Extensive computational fluid dynamics simulations were made [9] to determine the optimum transducer placement to minimize pressure drop and turbulence in the acoustic path.

The acoustic path crosses the 135 mm diameter flow tube at 45°. The transducers are adjustable in angle via three point mountings on flanges located outboard of isolating ball valves (Fig. 14), which permit replacement or adjustment of the transducers without interrupting the gas flow. When open, the ball valve orifices exceed the diameter of the transducers.

The total acoustic path length, L, between the membranes of the two transducers includes two zones of static gas and an

Fig. 14. Angled flowmeter sonar: photograph and parameters

Dmain = 135 mm

LEA

LEB

Interflange distance

806 mm

LTA

LTB

LF

= 45

LT

Gas Port

“active” zone of length traversing the flowing gas. Although sound transit times only vary in proportion to the flow velocity in the active zone, the lengths of the static-gas zones must be determined following transducer installation/replacement.

The acoustic path length was initially calculated using the combination of direct time-of-flight measurements between the transducers in static air and the theoretical sound velocity at the measured temperature and pressure from the NIST-REFPROP package [8].

A previously reported verification of the instrument against an anemometer in a Venturi-boosted air flow [10] showed a precision of ±2.3% in the maximum available air velocity of ~10 m·s-1 through the main tube. Following these tests, the transducers were temporarily removed from the instrument for it to be welded into the thermosiphon vapor piping (Fig. 14). Measurements with C3F8 are soon anticipated during thermosiphon commissioning.

The flowmeter was built with transducer maintenance and replacement as priorities since both can result in changes to the acoustic path length that can be problematic to re-measure when the process gas flow cannot be stopped for static distance measurements. The instrument electronics provide for echometry measurements of distance by both transducers to help overcome this problem. Gas ports in the transducer flanges (Fig. 14) allow the admission and evacuation of calibration gas in the end-zones outboard of the valves.

The acoustic path length, L, can be defined (Fig. 14) as L = LF – (LTA+LTB) where LF is the measured distance between the flanges of the acoustic tube and LTA and LTB are the forward penetrations of the two transducer membranes in their mountings.

Initial calibrations in static air [10] included measurements of LEA and LEB by echometry. The distances between the flanges and the equators of the closed ball valves are (LTA+LEA) and (LTB+LEB). These summed distances are invariant to subsequent remounting of the transducers.

Following replacement of the transducers the new distances to the ball valves, LEA´ and LEB´, are measured by echometry with the transducer volume filled with static calibration gas at known temperature and pressure. Such measurements are based on hundreds of transit time measurements and can measure transducer positions relative to the mounting flanges, LTA´ and LTB´, to higher precision than an optical measurement. Thus, the new acoustic path length, LNEW, can be determined without disturbing the flowing gas by LNEW = LF - ((LTA + LEA - LEA´) + (LTB + LEB - LEB´)).

Following these echometry measurements, the calibration gas is evacuated from the outboard transducer volumes and the ball valves are opened to re-establish the full acoustic path.

The principle of operation of the angled flowmeter with an acoustic path inclined at an angle α to the flow tube of diameter Dmain with zones of static gas of total length L´ in the acoustic path L = (Dmain/sin(α) + L´) has been previously detailed in [10]. LEA and LEB are components of L´.

The measured flow velocity, , in the main tube is related to the transit times and , measured in opposite directions over the full acoustic path, by

(4)

where is the sound velocity in the process gas, and can be expressed in terms of the same variables [10].

Sound velocity and flow rate will be continuously monitored using the ultrasonic flowmeter. Previous studies [9], [10] lead us to expect that binary gas composition analysis to a precision of ±0.3%, in blends of C2F6/C3F8, should be possible in this instrument. Thus, should fluorocarbon blends be circulated in the thermosiphon system to allow lower operating temperatures of the silicon modules of the pixel and SCT detectors, the combined analysis capability of the flowmeter will be indispensable.

IV. CONCLUSION We have developed ultrasonic instruments combining real-

time flowmetry and binary gas analysis based on bidirectional acoustic measurements. Five such instruments are presently integrated into the cooling systems of the ATLAS silicon tracker and the ATLAS DCS.

Three instruments are currently in use to detect low levels of C3F8 and CO2 vapor leaking into the N2 environmental gas surrounding the sub-detectors of the ATLAS silicon tracker. Very recent analysis of gas aspirated from the SCT endcaps and barrel shows expected precision and good agreement with previous studies. A previously reported long duration continuous study of more than a year [10] has demonstrated a sensitivity to mixture changes better than 5x10-5 to C3F8 leaks into N2.

Two further instruments are being commissioned into the new thermosiphon recirculator. One will monitor and trigger the venting of accumulated ingressed air in the thermosiphon condenser. The other is configured as an ultrasonic flowmeter for the returning vapor flow and will operate in flows up to 0.4 m3·s-1. If required, this instrument will also allow simultaneous molar composition monitoring of C2F6/C3F8 blends, should lower temperature operation of the ATLAS silicon tracker be desired in the future.

The instruments discussed in this work also have many potential applications where simultaneous binary gas analysis and flow measurement are required.

REFERENCES [1] D. Attree et al, The evaporative cooling system for the ATLAS inner

detector, Journal of Instrumentation, 3(2008) P07003 [2] R. Bates et al, "The cooling capabilities of C2F6/C3F8 saturated

fluorocarbon blends for the ATLAS silicon tracker," 2015 JINST, DOI:10.1088/1748-0221/10/03/P03027 http://iopscience.iop.org/1748-0221/10/03/P03027/

[3] J. Botehlo-Direito et al, General description of the full scale thermosiphon cooling system for ATLAS SCT and Pixel detectors, CERN EN-CV Report Document 1083852 ver.1 (27 November 2010) https://edms.cern.ch/document/1083852/

[4] SensComp, Inc. Livonia, MI 48150, USA http://www.senscomp.com/ultrasonic-sensors/

[5] M. Alhroob et al, "Development of a custom on-line ultrasonic vapour analyzer and flow meter for the ATLAS inner detector, with application to Cherenkov and gaseous charged particle detectors," 2015 JINST, DOI:10.1088/1748-0221/10/03/C03045, http://iopscience.iop.org/1748-0221/10/03/C03045/

[6] MBED LPC1768 platform : http://mbed.org/platforms/mbed-LPC1768 [7] Siemens SIMATIC WinCC SCADA system:

http://w3.siemens.com/mcms/human-machine-interface/en/visualization-software/scada/Pages/Default.aspx

[8] E. Lemmon, M. Huber and M. McLinden, REFPROP Standard reference database 23, version 9.0 U.S. National Institute of Standards and Technology (2010)

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[10] R. Bates et al., “A custom online ultrasonic gas mixture analyzer with simultaneous flowmetry, developed for the upgraded evaporative cooling system of the ATLAS silicon tracker”, in proc. of 3rd International conference on Advancements in Nuclear Instrumentation, Measurement Methods and their Applications (ANIMMA) Marseille, France, 23-27 June, 2013, IEEE Trans. Nucl. Sci. 61 (2014) 2059, DOI:10.1109/TNS.2014.2326961

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