RVmagnetics Sensors Proven in Defence and Space Conditions tested in NPL UK
RVmagnetics’ Microwire sensors, embedded in glass fibre and carbon fibre composite samples, were subjected to liquid helium and liquid nitrogen environments, as well as exposure to neutron and gamma radiation. The results demonstrated stable and reliable sensor functionality across all conditions confirming the robustness, resilience, and adaptability of RVmagnetics’ technology for use in harsh environments, including defence and space applications.
TEVV Testing Objectives and Methodology
RVmagnetics has successfully completed a series of TEVV tests at the National Physical Laboratory (NPL) in London, marking another major milestone in broadening the application area of the company’s unique Microwire sensing technology for extreme environments.
This developmental test aimed to verify the functionality of RVmagnetics’ proprietary MicroWire sensors, miniaturized sensing elements consisting of a ferromagnetic alloy core and a glass coating under extreme environmental conditions. The objective was to verify the MicroWire’s ability to respond properly to the excitation signal in ionising radiation and cryogenic temperatures, thereby extending its potential for dual-use applications in harsh environments such as cryogenic, orbital and deep space applications and nuclear infrastructure monitoring.
Expected Sensor Behavior in Harsh Environments
The expected result was that the MicroWire sensor will maintain its bistable magnetic response and reliable switching field behaviour under both radiation and cryogenic conditions, enabling its use as a multi-physical sensing element in environments beyond standard laboratory conditions. The RVmagnetics focuses on improving the more matured technologies available in its portfolio – strain sensitive and temperature sensitive MicroWires that have proven their functionality in the harsh cryogenic conditions. From this point of view, one important fork of RVmagnetics activities began to focus on cryogenic pressurized vessels monitoring, such as liquid hydrogen, which temperature is around 20 Kelvin and liquid oxygen at around 90 Kelvin, since the MicroWires had been successfully tested to liquid Helium at 4 Kelvin. RVmagnetics certainly wants to accelerate TRL advancement toward industrial deployment in this area besides the areas that we are already involved in, including aviation, automotive and others. Some of the next steps with our customers and potential industrialization partners include detailed data analysis, optimization of composite embedding methods, and integration into early-warning and structural health-monitoring devices/systems.
Test Setup and Data Acquisition Process
The signal from the MicroWire in the form of induced voltage peaks is sensed by the sensing coils and it provides the information about the ambient magnetic field and the second measured quantity – mechanical stress or temperature – through its BH (magnetic hysteresis) loop. The MicroWires are exhibiting bistable magnetization – two stable saturated states – and the transition between them is triggered by the excitation coil’s magnetic field fed by a triangle-wave-shaped current signal. The transition occurs as a single Barkhausen jump at a certain value of the excitation magnetic field – this value is called the switching field (Fig. 1). The periodical change in magnetization of the microwire periodically induces voltage peaks in the sensing coil. The amplitude is not being measured, since the RVmagnetics system operates with the conversion of the ambient magnetic field and the switching field measurement into the time measurement.
From this point of view, the output signal resembles a pulse position modulation. The two time intervals t+ and t- measured on the ascending and descending slope of the excitation signal carry the necessary information – their difference is proportional to the value of the external ambient magnetic field, and their sum is proportional to the switching field. These intervals are measured by timers in a data acquisition system based on an STM32 microcontroller connected to a PC. The data from the NPL’s reference test equipment has provided information to cross reference the data measured by RVmagnetics.
Two experimental campaigns were conducted at NPL, one aimed on functionality of MicroWires in the ionising radiation environment and the second aimed on the cryogenic temperatures environment. The final types and amount of samples are summarized in Table 1.
Measurement and Systems Used
Only the MicroWire specimens (bare or embedded) and the excitation/sensing coils were exposed to the harsh environment. The data acquisition electronics outside the exposure chamber in ambient conditions, connected via shielded cabling. Three measurement systems were used, two four-channel type for radiation tests, one for neutron and gamma radiation and one single-channel system for cryogenic temperatures. In such a way the testing could be performed in parallel. All of the measurements were performed via ADC utilization method with 200 Hz excitation signal and using the 20-samples averaging. The DAQ system used at radiation tests used a sequential saving method. It saved 100 measurements in each minute. On the other side, the DAQ system used at temperature test used continual saving. The data acquisition rate has been 10 samples per second per channel, synchronously.
The reference NPL’s measurement equipment for temperature testing used during the tests included:
- Standard Platinum resistance Thermometer (SPRT) serial number 1929 with Calibration certificate reference 2025010220/1/PM02, dated 28 January 2025 - used for the measurement of temperature in liquid nitrogen.
- G144 Rhodium Iron (RhFe) thermometer with Calibration certificate reference 2022010291/1/PM04, dated 24 March 2022 - used for the measurement of temperature in liquid helium.
- Standard resistor serial number 240741 with Calibration certificate reference 2025060129/3, dated 15 July 2025,
- microK resistance bridge.
The equipment was primarily used for temperature reference after the system was immersed in liquid nitrogen/helium. After the resistance of the reference thermometer stabilized upon immersion, continuous measurement of MicroWire’s response was taken.
The NPL’s measurement equipment for gamma radiation testing used during the tests included:
- 60Co gamma ray beam,
- Secondary standard ionisation chamber (NPL 2611, serial number 207) with build-up cap. The equipment was used to irradiate the MicroWires. During the two measurement intervals (43h 39min and 88h 53min) the value of total dosage was 10722 Gy (3659 Gy and 7063 Gy).
The NPL’s measurement equipment for neutron radiation testing used during the tests included:
- 252Cf spontaneous fission source,
- Empty 30 cm x 30 cm x 15 cm ISO slab for MicroWire sensors and sensing units mounting.
Initially, the neutron source was at a distance that resulted in a 1 mSv dosage per hour. After the first 25 hours of a still stable MicroWires signal, the distance was adjusted to create a higher dosage of 25-32 mSv per hour, depending on sensor position. In a total time of 234 hours, the sensors received the dosage of 6600 mSv (sensor 1), 10841 mSv (sensor 2), 11003 mSv (sensor 3), and 6755 mSv (sensor 4).
Cryogenic Testing Results and Observations
The temperature tests were performed from 13.10.2025 to 17.10.2025, the radiation tests began 14.10.2025 and since the NPL participated on different tasks, the radiation tests were performed in available time slots keeping track of the dose the samples received up to 10.11.2025.
MicroWire Sensitivity to Liquid Nitrogen and Helium
Temperature measurements have been collected during cycles of immersion of the sample in liquid nitrogen/liquid helium, following withdrawal from the liquid and natural heating to ambient temperature. Figure 2 represents examples of temperature-dependent measurements, where the whole cycle is described as follows:
- A – The sample is passing through the neck of the dewar, where perpendicular magnetic fields disturb the signal from the MicroWire, creating visible fluctuations,
- B – The sample is being immersed in the liquid helium and responding to the temperature change,
- C – The sample is fully immersed in liquid helium, and continuous measurement of MicroWire’s stability is conducted,
- D – The sample is withdrawn from the liquid helium, and the response to the neck of the dewar is visible again,
- E – MicroWire’s temperature response to the heating to ambient temperature.
For the determination of the switching field’s change, it was necessary to recalculate all the measured values into the relative change of the switching field, as listed in Table 2, using the formula below:
Using the formula, it was possible to determine the percentage change in the switching field. In the case of the temperature-dependent MicroWire of composition Temperature_1, the relative change of switching field crossed the expected values of 50% to 192% in liquid nitrogen and 217% in liquid helium.
The other temperature-dependent composition, Temperature_2, exhibits an increase in switching field around 30%, while the stress-dependent MicroWires Strain_1 and Strain_2 show a change around 15%. Both types of carriers (carbon and glass-composite) appear to have a significant influence on the MicroWires’ switching fields, as all relative changes to the initial values are observed.
Sensor Performance in Radiation Environments
For radiation tests, both gamma and neutron radiation were used to irradiate different compositions and observe the differences in the MicroWires‘ switching field values. Even after exposure to ionizing radiation, the signal from the MicroWires remained similar to its initial values. The MicroWires used are specifically designed to be stress-dependent (Strain_1, Strain_2) or temperature-dependent (Temperature_1, Temperature_2). Radiation tests confirmed the robustness, as they continue to measure the response to environmental changes even after the tests were completed.
In the case of neutron radiation, the MicroWires also proved their robustness, surviving the extremely high dosage of radiation while still being able to measure. Same logic of element substitution can be applied also for neutron radiation.
Conclusion: MicroWire Sensor Robustness Confirmed
The performed TEVV tests confirmed that the RVmagnetics MicroWire maintain bistability and thus proper response and sensing capability in environments involving ionising radiation and cryogenic temperatures. Especially valuable are the results for the MicroWire samples embedded in the composite carriers, since the effects are very difficult to simulate and practical experiment results have brought more information about the mutual behaviour of the MicroWire and composite in tested harsh environments. The successful completion of the TEVV tests allows to speed up further improvement of the sensor’s applicability for defence, aerospace, and nuclear-infrastructure use cases. Cryogenic temperature testing demonstrates the reliability of MicroWire sensors without a carrier. At the same time, the properties of carbon/glass fiber composites directly or indirectly influence the sensitivity of a MicroWire’s response to continuous temperature changes. Moreover, stress-dependent MicroWires showed a fraction sensitivity of ~ 15%, compared to the temperature-dependent MicroWires, which showed a sensitivity of ~ 200%, exceeding the expectations made during the preparation phase. The MicroWire sensors proved to be a reliable physical quantity sensor in this kind of environment.
