The present invention relates to an electronic measurement system for use at low temperatures and particularly at cryogenic temperatures.

Low temperature sensitive measurement apparatus is of potential importance in various technical fields, for example, in wind tunnel modelling. Wind tunnel modelling is important in the development of new aircraft where the development and production costs are typically very high. As the size of aircraft has increased it has become increasingly difficult to model them accurately because of difficulties in accounting for the Reynolds number. The Reynolds number is the product of velocity, density and length divided by viscosity. The latest jet planes, for example, the Boeing 7^7. have a Reynolds number of about 6θxl0 ⁶ . However, the maximum Reynolds number achievable in wind tunnels at ambient temperatures and wind velocities of about Mach 0.9 is only 5xl0 ⁶ . One way of reaching the Reynolds number is by increasing the model size and hence the wind tunnel size but this is very costly. Likewise increasing the wind velocity is also impractical. A further option is to decrease the temperature of wind tunnel operation. T1IJ.S has the dual benefit of increasing the density and reducing the viscosity. However to operate effectively cryogenic temperatures must be employed and typically about those of liquid nitrogen (77K) .

Conventional wind tunnels incorporate a device called a Scanivalve which is mounted inside the test model. It normally operates at ambient to low temperatures and consists of a cuboid casing with a number of

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pressure sensors positioned in one wall of the casing. These sensors are in communication with the inside of the casing through ducts. Connected to the casing is a source of calibration pressure. A piston possessing carefully machined conduits is located within the casing which is hydraulically moveable along one axis of the cuboid. Motion of the piston enables each pressure sensor to communicate either with a test point or the source of calibration pressure. 0 rings are provided at the duct entry to the inside of the casing to prevent leakage of pressure.

Use at very low temperatures causes the piston to become very difficult to move and the 0 rings fail to operate as they should leading to pressure leakage and large inaccuracies. The inventor experimented with alternative materials for the 0 rings but without success.

Another alternative is to arrange for electronics to be placed inside thermo-statically controlled enclosures that are thermally insulated against variable temperature difference up to 200K. This would enable the electronics to operate at ambient temperature if desired but there exists several disadvantages including, the limited volume available for multi-channel systems, long thermal equilibrium time constants, the generation of hot spots on the surface of the wind-tunnel model giving rise to poor aerodynamic flow quality, and the need for active temperature compensation of the sensors. Eminent research organisations using cryogenic wind-tunnels have been unsuccessful in attempting to solve these problems.

Outside the field of wind tunnels there are problems associated with

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large liquid containing tanks where accurate sensor measurement is required. For example in a tank which maybe 50 meters high there exists the opportunity for layers of cryogenic liquid, with differential density to be produced such that heat energy entering the system is not apportioned equally throughout the tank. This may give rise to the warming of a lower layer to the point when its density is less than that of upper layers which can result in what is called 'roll over' where the lower layer inverts with the upper layer. The accompanying rapid increase in evaporation may lead to severe damage to equipment. This problem is not at all easy to solve in that 'roll over' can occur in the large systems described with density variations as small as 0.1%. In a system as large as the one mentioned these small density variations represent a significant energy imbalance.

The present invention seeks to overcome these problems by providing a cryogenic sensor which can be both located and operated in a cryogenic environment and, in particular, to provide a cryogenic sensor capable of measuring small density variations.

According to one aspect of the invention there is provided a cryogenic sensor comprising; i at least one sensing element which produces an analogue signal which is a function of a physical property; ii electronic circuitry which converts the analogue signals into digital signals, the electronic circuitry being located in close proximity to the sensing elements, minimising the distance travelled by the analogue signals; and

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iii a means to transmit the digital signals from the electronic circuitry to a point of utilisation which is able to be located either inside or outside of the cryogenic environment; where the sensing elements, the electronic circuitry and the means to transmit the digital signals are located within the cryogenic environment and are operated at the cryogenic temperature characterised in that all of the semi-conductor electronic devices which operate within the cryogenic environment comprise FET devices only.

It has been found that FET semi-conductor devices are operable at far lower temperatures than those specified by the manufacturers so that the sensor apparatus is usable between 77 and 300K. For example the VLSI CMOS integrated circuits used have been found to operate down to 77K This compares to the manufacturers specified minimum operating temperature of 233K-

Possible means to transmit the digital signals include wires, co-axial cables and optic fibres.

Preferably, the electronic circuitry converts the analogue signals to digital signals using at least one self-calibrating analogue to digital converter integrated circuit.

Preferably the analogue to digital converters are delta-sigma analogue to digital converters.

Conveniently, 16 or 20 bit CMOS analogue to digital converters can be

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employed. These enable the direct conversion of analogue sensor signals to digital form, without separate pre-amplification or multiplexing of low level signals by components which are subject to temperature effects and noise injection.

The inventor has found, using a 16 bit analogue to digital converter, that for a sensing element producing an analogue signal with a voltage of 1 volt , a variation of ± 30μV can be measured giving a precision of better than 0.01%. A 20 bit analogue to digital converter is capable of converting with 0.001% precision.

The use of a self calibrating analogue to digital converter overcomes difficulties associated with re-calibration as a consequence of temperature variation. Typical 16 bit delta-sigma analogue to digital converters require re-calibration every 4 to 5K. Furthermore, using a self-calibrating 16 bit analogue to digital converters without separate pre-amplification or multiplexing enables analogue signals with minimum noise content to be converted into digital signals which require no further correction for temperature dependent effects.

When constructing the sensor apparatus of the invention it is important that the length of the wiring connecting the sensing elements and the analogue to digital converter is minimised so that any noise introduced to the analogue signal prior to its conversion is kept to a minimum.

Keeping the sensing elements and the analogue to digital converters at cryogenic temperatures significantly reduces the noise distortion of the analogue signal and therefore provides a much more accurate reading of

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the physical properties being measured than if one or both of these components were positioned outside of the cryogenic environment.

Ideally, the timing of the analogue to digital converters is controlled by a quartz crystal oscillator which is operated at the cryogenic temperature and located within the cryogenic environment as close as is practically possible to the digital to analogue converters.

Ideally, the electronic circuitry comprises at least one multiplexer. Each analogue signal output from each of the sensing elements can be passed directly to an individual analogue to digital converter. Therefore, it would be advantageous if at least one of the multiplexers is an analogue multiplexer which multiplexes some or all of the analogue signal produced by the sensing elements. Using analogue multiplexers enables several or all of the sensing elements to use one analogue to digital converter. This reduces the amount of electronic hardware. It also enables the number of connections which have to pass between the cryogenic environment and the warm environment to be kept to a minimum. Ideally, the analogue to digital converter integrated circuit employs an on-chip analogue multiplexer which has multiple inputs. 16 and 20 bit analogue to digital converter integrated circuits are available with on-chip analogue multiplexers capable of accepting four inputs. Also, at least one of the multiplexers can be digital multiplexers, multiplexing the some or all of the digital signals produced by the analogue to digital converter integrated circuits. If several analogue to digital converters are used, the digital signal outputs from these can be multiplexed to produce a single signal which can then be passed from the cryogenic environment to the warm environment.

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In order to provide greater control over the operation of the cryogenic sensor, control circuitry can be further added which is located in the cryogenic environment and operated at the cryogenic temperature. Preferably, the control circuitry comprises a processor. The processor can be selected from a microprocessor or alternatively can be a transputer preferably with bi-directional serial communication.

Preferably the sensing elements are operated sequentially. This can be controlled by the control circuitry. The multiplexers have several inputs to which the various sensing elements or possibly other types of device can be attached. The inventor has devised units with up to 48 inputs. Whilst it is possible to operate the inputs in parallel it has been found that it. is much more practical to operate them on a sequential basis. This reduces the introduction of noise through cross-talk between channels because only the channel being monitored would be active, the rest being switched off.

Additionally, biasing circuits, if required, can be located in the cryogenic environment and operated at the cryogenic temperatures. Amplifier circuits, also located in the cryogenic environment and operated at the cryogenic temperatures, can be used to amplify the analogue signals produced by the sensing elements.

Preferably, the sensing elements are formed from materials all having similar thermal expansion and contraction characteristics. Ideally, at least one of the sensing elements is a pressure sensing element and is advantageously a piezo-resistive silicon pressure sensor. Having the

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sensor formed from silicon overcomes the difficulties associated with signal corruption caused by differential expansion of materials.

Ideally, the cryogenic sensor is constructed as a hybrid circuit with the components mounted on a hybrid circuit board constructed with conduction lines as a thick film substrate. For compactness in construction the thick film hybrid circuit is produced in multi-layered form.

The components can be coated with PTFE or other similar polymer materials. This provides robustness and protect them from the ingress of moisture or the action of water.

Another aspect of the present invention is the method of measuring a physical property within a cryogenic environment comprising the steps of; i generating analogue signals which are functions of physical properties using sensing elements; ii converting the analogue signals into digital signals; iii transmitting the digital signals to a point of utilisation; where all three steps are performed using a cryogenic sensor and where all the electronic semi-conductor components of the cryogenic sensor which operate within the cryogenic environment comprise FET devices only characterised in that all of the steps are performed within the cryogenic environment.

Ideally, the further step of feeding the analogue signals produced by the sensing elements into an analogue multiplexer, the output analogue

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signal then being converted into a digital signal. This reduces the number of analogue of digital converter integrated circuits required and therefore reduces size and cost.

Preferably, the sensing elements are activated in a sequential manner. This reduces cross talk between the various analogue signals.

The invention will now be described by way of example and with reference to the accompanying drawing of which Fig. 1 shows a block diagram of the various components which are used for measuring cryogenic temperatures in a cryogenic environment.

Figure 1 shows the layout of circuit components in a cryogenic environment, 1.1, which are part of a system for measuring cryogenic temperatures and pressures in wind tunnels at temperatures as low as 77K. All of the circuit components which operate in the cryogenic environment are selected so that they comprise FET devices only, containing no bipolar type devices. The majority of the components used are CMOS type integrated circuits. The cryogenic sensor is constructed as a hybrid circuit with the components mounted on a hybrid circuit board and are covered with a PTFE coating to prevent the ingress of moisture. The hybrid circuit is designed so that the lengths of the wires carrying the analogue signals are minimised. The warm electronics (not shown) are situated in the warm environment, 1.2. The warm electronics, for this example, are the point of utilisation i.e. the point where the information obtained is processed. In other circumstances, the point of utilisation could be situated in the cryogenic environment, 1.1. Separating the warm environment, 1.2 and

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cryogenic environment, 1.1 is an interface, 1.3. The warm electronics provided via biasing circuits, 1.4, a signal to each of the temperature and pressure sensing elements, 1.5, 1.6, 1.7. The pressure sensing elements are piezo-resistive silicon pressure sensors. The sensing elements, 1.5. 1.6, 1.7. are activated sequentially to reduce cross talk between the analogue signals. The analogue output signals from each of the sensing elements, 1.5. 1.6, 1.7. are passed through an analogue multiplexer, 1.8, which then feeds the signal to an amplifier, 1.9. The multiplexer, 1.8, is controlled so that the signal from the sensing element which has been activated is switched to the output of the multiplexer, 1.8. The amplified signal at the output of the amplifier, 1.9. is then passed to an analogue to digital converter, 1.10. The analogue to digital converter, 1.10 is a delta-sigma type converter which is self-calibrating. The timing of the analogue to digital converter, 1.10, is controlled using a quartz crystal oscillator which is located as near to the analogue to digital converter, 1.10, as practically possible so as to minimise the distance which the clock signal has to travel. The digital output of the analogue to digital converter, 1.10, is fed to a communications circuit, 1.11, which communicates bilaterally with the warm electronics situated in the warm environment, 1.2. The information, once it has been sent to the warm electronics, is processed in an appropriate manner by a computer. Information could be sent from the warm environment, 1.2, via the communications circuit, 1.11, to the control circuit, 1.12 and vice versa. The digital output from the control circuit, 1.12, is fed to the multiplexer, 1.8, the analogue to digital converter, 1.10 and the communications circuit, 1.11.

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The use of a biasing circuits and a control circuits within the cryogenic environment is not critical but it is important that the sensing elements, the analogue to digital converters, the multiplexers and the amplifiers (if used) are within the cryogenic environment.

The invention is of particular use in cryogenic wind tunnels for aircraft design.

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