GB2526084A - Determining a level of liquid cryogen in a cryogenic vessel - Google Patents

Abstract

The present invention relates to a method of determining a level (4) of liquid cryogen (3) in a cryogenic vessel (2). In order to provide a simple and reliable technique for determining a level (4) of liquid cryogen (3) in a cryogenic vessel (2) it is suggested to operate a heater (8) for a defined period of time to evaporate a certain amount of liquid cryogen (3), to determine the change (10) in pressure inside the cryogenic vessel (2), and to determine the level (4) of liquid cryogen (3) in the cryogenic vessel (2) based on a known correlation between the pressure change (10) and the cryogen level (4).

Description

DETERMINING A LEVEL OF LIQIJTL) CRYOGEN IN A CRYOGENTC VESSEL The present invention relates to a method of determining a level of liquid cryogen in a cryogenic vessel. Furthermore, the present invention relates to a system for determining a level of liquid cryogen in a cryogenic vessel.

In an MRI (magnetic resonance imaging) system, a cryostat may be employed, said cryostat comprising a cryogenic vessel holding a liquid cryogen, e.g. liquid helium, for cooling the superconducting magnet coils.

In such systems, it is necessary to regularly monitor the level of the liquid cryogen, while the system is still in an operational state. Ibis is necessary to detect leaks, indicated by an unusually high consumption of cryogen, and to ensure that the liguid cryogen is topped up at appropriate intervals. This is necessary to ensure that the magnet coils or other articles are cooled by the liquid cryogen. At low cryogen levels, parts of the magnet will be at a higher temperature than when the cryogen level is high. In the case of a superconducting magnet, this could lead to a magnet quench, which may be dangerous and damaging to the system. The magnetic field would also collapse as a result of the current in the coils reducing, due to energy loss, caused by the higher resistance of the coils in their normal state. However, the measurement process should not represent an undue heat input to the system. It is generally regarded as sufficient to measure the level of liquid cryogen once per day.

Exi stH ng methods for rneasuni ng cryogen level requi res fitting a cryogen level probe consisting of an assembly of wires (including superconducting wire) which current is passed through in order to heat the wire and measure voltage or resistance to establish how much of the probe is immersed in liquid helium. Existing cryogen level probes have been problematic in the past and can be difficult to replace. If level probes fail in field operation, the cryogen level cannot be determined, putting the system at risk.

It is therefore an object of the present invention to provide a simpie and reliable technique ffor determining a level of liquid cryogen in a cryogenic vessel.

This object is achieved according to the invention by a method of determining a level of liquid cryogen in a cryogenic vessel, comprising the steps of introducing a defined amount of heat energy into the cryogenic vessel to evaporate a certain amount of liquid cryogen, determining the change in pressure inside the cryogenic vessel, and determining the level of liquid cryogen in the cryogenic vessel based on a known correlation between the pressure change and the cryogen level.

The step of introducing a defined amount of heat energy into the cryogenic vessel may be performed by operating a heater for a defined period of time; or by ceasing refrigeration of the cryogen vessel for a certain period of time, a known parasitic heat influx to the cryogen vessel providing the required heat energy. These two methods may be combined.

The object of the present invention is also achieved by a system for determining a level of liquid cryogen in a cryogenic vessel, comprising a means for introducing heat energy for evaporating a certain amount of liquid cryogen, means for determining the change in pressure inside the cryogenic vessel, and means for determining the level of liquid cryogen in the cryogenic vessel based on a known correlation between the pressure change and the cryogen level.

The means for introducing heat energy may be a heater; or may be the structure of the cryogenic vessel itself, providing a path of conduction for a parasitic heat influx.

The object of the present invention is also achieved by a computer program to be executed in a computer, said program comprising computer instructions to determine a level of liquid cryogen in a cryogenic vessel based on a known correlation between a pressure change inside the crycgenic vessel and the crycgen level, which pressure change has been deternined after introducing a defined amount of heat energy into the cryogenic vessel to evaporate a certain amount of liquid cryogen, when the computer program is executed in the computer.

The introduction ot a detined amount of heat energy may be performed by operating a heater for a defined period of time A core idea of the invention is to provide a technique for determining a level of liquid cryogen in a cryogenic vessel, which does not require cryogen level probes. In fact, not a single probe has to be fitted to the cryogen vessel. Therefore, no additional heat load is added to the system by the presence of a probe.

In some embodiments, a heater is operated for a set period of time in order to introduce a defined amount of heat energy into the cryogenic vessel, and so to boil off a known mass of cryogen. Tn other embodiments, a parasitic heat load is permitted to persist for a set period of time, similarly to introduce a defined amount of heat energy into the cryogenic vessel, and so to boil off a known mass of cryogen.

This increase in the mass of cryogen gas within the cryogenic vessel results in a change in pressure inside the cryogenic vessel. A pressure rise is measured which is then converted to a cryogen level based on a known relationship between pressure change and cryogen level, and the increase in mass of the cryogen gas. The technique is based on the fact that a greater pressure H se wi 1 1 occur when the cryogenic vessel is fri 1 compared to when low liquid level is present, for a given mass of boiled-off cryogen, as the cryogen when liquid is incompressible.

Over existing design concepts the present invention shows the following advantages: reduction of system components, use of existing components, improved reliability and service life of the magnet, lower cost components. Further advantages of the present invention are the ability to determine cryogen level in a very cheap and reliable way, and the avcidance of additional heat load.

A major advantage is that the cryogen level can be determined in unfavourable situations, e.g. in the field, even with failed or questionable level probes.

These and other aspects of the invention will be further elaborated on the basis ot the tollowing embodiments which are defined in the dependent claims.

According to a preferred embodiment of the invention the heater is located inside the cryogenic vessel, in particular within the liquid cryogen. Preferably, such internal heater is located below the lowest allowable level of liquid cryogen. By this means, the inventive method can be carried out even if the amount of liquid cryogen in the vessel is at its minimum.

According to a preferred embodiment of the invention an existing internal heater is employed. Tn other words, no additional heater has to be fitted into the vessel. Preferably, an internal heater is used, which is adapted to be turned on and off by an electrical control system in response to variations in pressure within the cryogenic vessel in order to maintain a desired pressure level, and/or which is adapted to generate gas in order to ccol elements inside the cryogenic vessel.

According to a preferred embodiment of the invention a pressure sensor is used for measuring the pressure inside the cryogenic vessel. The pressure sensor is preferably located in the access neck of the turret, mounted on top of the cryostat. Preferably, an existing pressure sensor is employed.

According to a preferred embodiment of the invention the means for determining the pressure change comprising a data processing unit for comparing the result of a pressure measurement with the result of a previous pressure measurement. Preferably, the data processing unit is adapted for comparing the result of a latest pressure measurement, i.e. a pressure measurement after operating the heater, with the result of the most recent pressure measurement, i.e. a pressure measurement prior to operating the heater.

According to a preferred embodiment of the invention the pressure measurement prior to operating the heater is carried out immediately before turning on the heater; in other words, for each process ot determining the cryogen level two pressure measurements are carried out, nameiy a first pressure measurement is carried out prior to operating the heater, and a second pressure measurement is carried out after operating of the heater. According to another preferred embodiment of the invention, the pressure measurement prior to operating the heater is the pressure measurement which has been carried out on a previous occasion of cryogen level determination; in other words, in this embodiment for each process of determining the cryogen levei only one pressure measurement is carried out, which is a pressure measurement after operating the heater.

According to a preferred embodiment of the invention the level of liquid cryogen in the cryogenic vessel is determined by means of a data processing unit based on a known correlation between the pressure change and the cryogen level, said correlation being known from experimental data and/or from calculations. In other words, the relationship between pressure change and cryogen level has previously been identified by experimental and/or calculative studies. Using the results of these studies a general correlation rule has been established or a reference table has been generated, which is stored in a memory unit, accessible by the data processing unit.

Preferably, once a correlation has been determined for one type of cryostat, this correlation can be employed for any number of cryostats of this particular type in order to determine the cryogen level.

These and other aspects of the invention will be described in detail hereinafter, by way of example, with reference to the following embodiments and the accompanying drawings; in which: Fig. 1 shows a schematic illustration of a cryostat, Fig. 2 shows a block diagram of the system, Fig. 3 shows pressurisation curves at different cryogen levels.

Fig. 1 shows a cryostat 1 such as may be employed for holding magnet coils for an MRT (magnetic resonance imaging) system. A cryogenic vessel 2 holds a liquid cryogen 3, e. g. liquid helium, at a high cryogen level 4. A low cryogen level is indicated using a dotted line 5. The space 6 in the cryogenic vessel above the level 4 of the liquid cryogen 3 may be filled with evaporated cryogen. The cryogenic vessel 2 is contained in a vacuum jacket (not shown) . One or more heat shields (not shown) may be provided in the vacuum space between the cryogenic vessel 2 and the vacuum jacket. A turret with an access neck 7 is provided at the top of the cryostat 1, allowing access to the cryogenic vessel 2 from the exterior. This is used to fill the cryogenic vessel 2, to provide access for current leads and other connections to superconductive coils housed within the cryogenic vessel 2. A refrigerator is typically provided, for example mounted in the access neck 7, to counteract a parasitic heat influx through the structure of the cryogen vessel 2 to the cryogen 3.

An internal heater 8 may be located inside the cryogenic vessel 2, near the bottom 9 of the vessel 2, below the low level 5 of liquid cryogen 3. In a certain embodiment of the invention, heater 8 is a 40 W resistance heater operated by supply of a current therethrough.

The heater 8 is adapted to be turned on and off to provide a defined heating power for a defined period of time by an electrical control system 11, said control system 11 being part of a central control unit 12 which is located outside the cryogenic vessel 2.

By operating the heater 8, a certain mass of liquid cryogen 3 is evaporated. If the heater 8 is always operated in the same way, for example the heater 8 is energised for 2.5 minutes at 40 Watts; always the identical mass cf gaseous cryogen is produced. Because the liquid cryogen 3 is incompressible, the gas pressure in the space 6 above the level 4 of the liquid cryogen 3 rises. Therefore, the rise of gas pressure inside the cryogenic vessel 2 depends on the cryogen level 4. The higher the cryogen level 4, the greater the pressure rise 10. In return, by measuring the pressure inside the cryogenic vessel 2, the cryogen level 4 can be determined.

Alternatively, or in addition, the refrigerator (not shown) is adapted to be turned off and on for a defined period of time by an electrical control system, which control system may be part of a central control unit 12 which is located outside the cryogenic vessel 2. By ceasing operation of the refrigerator, a certain mass of liquid cryogen 3 is evaporated due to parasitic heat influx through the structure of the cryogenic vessel. If the refrigerator is always operated in the same way, for example the refrigerator is turned off 15 minutes at 40 Watt, always the identical mass of gaseous cryogen is produced. Because the liquid cryogen 3 is incompressible, the gas pressure in the space 6 above the level 4 of the liquid cryogen 3 rises. Therefore, the rise of gas pressure inside the cryogenic vessel 2 depends on the cryogen level 4. The higher the cryogen level 4, the greater the pressure rise 10. Tn return, by measuring the pressure inside the cryogenic vessel 2, the cryogen level 4 can be determined.

The period of time the heater 8 is operated, and/or the refrigerator is turned off, is preferably set in a way that the cryostat 1 does not lose cryogen. In other words, a venting pressure shall not be exceeded.

For simplicity, the remainder of this description will refer to embodiments wherein a heater 8 is used to introduce the required heat energy into the cryogen vessel, and control of the refrigerator is not used. However, as will be clear to those skilled in the art, similar considerations will apply to embodiments wherein the heater is replaced or supplemented by turning off the refrigerator.

A pressure sensor 13 for measuring the pressure inside the closed cryogenic vessel 2 is located in the access neck 7. Any suitable pressure sensor 13 may be employed. The pressure sensor 13 is connected to the central control unit 12 in order to provide measuring data for further processing. Twc pressure measurements are carried out using the pressure senscr 13. A first measurement is carried out immediately betore turning on the heater 8, and a second measurement is carried out after the heating period, i.e. after the heater 8 has been turned off again. For example, the operation of the heater 8 brings the pressure from 15.3 to 15.7 psia in 2.5 minutes.

A data processing unit 14 is located cutside the cryogenic vessel 2, as part of a central control unit 12. The data processing unit 14 is adapted for determining a pressure rise 10 by ccmparing the result of the second measurement with the result of the first measurement. For example, the data processing unit 14 determines a pressure rise 10 of 0.4 psia within the set period of time. The data processing unit 14 is further adapted for determining the level 4 of liguid cryogen 3 in the cryogenic vessel 2 based on a known correlation between the pressure change and the cryogen level. For this purpose, the data processing unit 14 has access to a memory unit 15, in which all relevant data is stored. The memory unit 15 is also part of a central control unit 12.

From experimental and/or calculative studies the relationship between pressure change and cryogen level is known for the type of cryostat used, thereby taking into account the particular cryogen volume, the volume of the cryogenic vessel 2, among others. This relationship can he generally given as delta p = f(L), wherein p denominates the gas pressure, L denominates the cryogen level, and f denominates the functional correlation between both.

Examples of experimental data which are used to define such a relationship are shown in Fig. 3.

For the particular cryostat 1, as illustrated, the operation of the heater 8 leads to a pressure rise 10 from 15.3 to 15.7 psia in approximately 3.5 minutes in case of a cryogen level of 30% (curve 16) . The operation of the heater leads to a pressure rise from 15.3 to 15.7 psia in approximately 3 minutes in case of a cryogen level of 50% (curve 17) -The operation of the heater leads to a pressure rise 10 from 15.3 to 15.7 psia in approximately 2.5 minutes in case of a cryogen level of 95% (curve 18) . After 2.5 minutes of heater operation the pressure rises to 15.55 psia in case of a cryogen level of 30%, to 15.65 psia in case of a cryogen level of 50%, and to 15.7 psia in case of a cryogen level of 95%.

In the present embodiment, the heater 8 is operated 2.5 minutes, raising the pressure by 0.4 psia to 15.7 psia. Accordingly, taking into account this rate of pressure increase, the data processing unit 14 determines a cryogen level 4 of 95%. The data processing unit 14 is connected to a signalling device 19, e.g. to a monitor or the like, in order to indicate the cryogen level 4 to a user of the cryostat 1.

The inventive concept is immediately applicable to MRI systems in the field, even with failed helium level probes, e.g. in an emergency case. Note that for carrying out the method, the magnet has to be recondensing and at thermal equilibrium, which is nearly always a case with magnets in the field. Tn order to receive a correct response, the method shall not be carried out in cases of different gas distributions inside the cryogenic vessel 2, e.g. in case of a freshly filled cryostat, or in case of ramping.

The described technique is particularly suitable for determining cryogen level in the field. The accuracy of level determination is not very high, compared with high end level probes. However, with the invention a very cheap, simple and reliable technique of level testing is provided, which allows the user to detect whether the cryogenic vessel contains a sufficient quantity of cryogen to allow operation of the superconducting magnet.

The invention is preferably used in combination with conventional cryogen level determination systems, e.g. cryogen level probes, in order to provide an alternative determining technique as fallback system.

All appliances are adapted to carry out the method according to the present invention. All devices, e. g. the heater 8, the pressure sensor 13, the data processing unit 14 and the central control unit 12 with the electrical control system II, are constructed and/or programmed in a way that the procedures for obtaining data and for data processing run in accordance with the method of the invention.

The data processing unit 14 is adapted for performing all tasks of calculating and computing the measured data as well as determining and assessing results. This is achieved according to the invention by means of computer software comprising computer instructions adapted for carrying out the steps of the inventive method, when the software is executed in the data processing unit 14. The data processing unit 14 itself may comprise functional modules or units, which are implemented in form of hardware, software or in form of a combination of both.

The technical effects necessary according to the invention can thus be realized on the basis of the instructions of the computer program in accordance with the invention. Such a computer program can be stored on a carrier such as a CD-ROM or DVD or it can be available over the internet or another computer network. Prior to executing the computer program is loaded into the computer by reading the computer program from the carrier, for example by means of a CD-ROM or DVD player, or from the internet, and storing ft ftthememory of the computer. The computer ftcludes nter sift a central processor unit (CPU) , a bus system, memory means, e.g. RPMorRCMetc., storagemeans, e.g. floppydiskorharddiskunits etc. and input/output units. Alternatively, the inventive method could be implemented in hardware, e. g. using one or more integrated circuits.

It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrative ii:i embodiments, and that the present invention may be errbodied in other specific forms without departing from the spirit or essential attributes thereof.

Reference numerals 1 cryostat 2 cryogenic vessel 3 liquid cryogen 4 high cryogen level low high cryogen level 6 space 7 access neck 8 heater 9 bottom pressure rise 11 electric control system 12 central control unit 13 pressure sensor 14 data processing unit memory unit 16 pressure curve at 30% level 17 pressure curve at 50% level 18 pressure curve at 95% level 19 signalling device