Inherent work suit buoyancy distribution:effects on lifejacket self-righting performance

Introduction: Accidental immersion in cold water is an occupational risk. Work suits and life jackets (LJ) should work effectively in combination to keep the airway clear of the water (freeboard) and enable self-righting. We hypothesized that inherent buoyancy, in the suit or LJ, would be beneficial for enabling freeboard, but its distribution may influence LJ self-righting. Methods: Six participants consented to complete nine immersions. Suits and LJ tested were: flotation suit (FLOAT; 85 N inherent buoyancy); oilskins 1 (OS-1) and 2 (OS-2), both with no inherent buoyancy; LJs (inherent buoyancy/buoyancy after inflation/total buoyancy), LJ-1 50/150/200 N, LJ-2 0/290/290 N, LJ-3 80/190/270 N. Once dressed, the subject entered an immersion pool where uninflated freeboard, self-righting performance, and inflated freeboard were measured. Data were compared using Friedman’s test to the 0.05 alpha level. Results: All suits and LJs enabled uninflated and inflated freeboard, but differences were seen between the suits and LJs. Self-righting was achieved on 43 of 54 occasions, irrespective of suit or LJ. On all occasions that self-righting was not achieved, this occurred in an LJ that included inherent buoyancy (11/54 occasions). Of these 11 failures, 8 occurred (73% of occasions) when the FLOAT suit was being worn. Discussion: LJs that included inherent buoyancy, that are certified as effective on their own, worked less effectively from the perspective of self-righting in combination with a work suit that also included inherent buoyancy. Equipment that is approved for use in the workplace should be tested in combination to ensure adequate performance in an emergency scenario

Frequency stabilised laser diodes and their use in length metrology

This thesis is concerned with the use of indexed-guided GaA1As laser diodes in length metrology. Since these lasers have important differences from more commonly use HeNe lasers in interferometry, the thesis begins with a review and investigation of these differences. A key problem is the relatively large linewidth (typically 30 MHz). This is reduced, by using the optical feedback from a resonant confocal etalon, to less than -10kHz. Potential spectroscopic frequency references are discussed and the choice made of the rubidium (Rb) D lines at 780nm and 795 nm, using either Doppler-free features or the linear absorptions. The optically narrowed laser is then stabilised to the various hyperfine components. By measuring the reproducibility of the difference frequency between two systems, a relative frequency reproducibility of $\approx$ 1x10$^{-10}$ is demonstrated. Laser frequency stabilities are measured for a number of different cases, for example with the lasers free-running and offset locked. for two optically narrowed RB-stabilised laesrs at $\tau$=10 s, the relative frequency stability is $\approx$ fx10$^{-12}$. The hyperfine intervals of the two Rb lines are also measured for both isotopes ($^{85}$Rb and $^{87}$Rb), representing the most complete study of Rb-stabilised diode lasers to date. From these results, values for the hyperfine constants and isotope shifts are obtained. Finally, the absolute frequencies of both RB-stabilised laser diodes are measured by interferometric comparison with an iodine stabilised HeNe laser at 633 nm with a relative accuracy of $\pm$ 1x10$^{-9}$. To demonstrate the use of a frequency tunable laser diode in distance measurement, a swept wavelength measurement system was developed with a demonstrated accuracy of a few parts per million. The thesis ends with a discussion of longitudinal mode control in laser diodes and some potential future uses in length and optical frequency metrology

“Float First:” Trapped Air Between Clothing Layers Significantly Improves Buoyancy on Water After Immersion

Approximately 450,000 people drown annually worldwide. The capacity of immersed adults and children to float in clothing is less well understood, but it is possible that air trapped between clothing layers increases buoyancy. Study 1 (n = 24) quantified this buoyancy and the consequence of any buoyancy by measurement of airway freeboard (mouth to water level distance). Study 2 examined the capability of children (n = 29) to float with freeboard used as the outcome measure and is expressed as a percentage of occasions that freeboard was achieved. Buoyancy (Newtons; N) was provided for winter clothing as 105 [+ 12], Autumn/Spring 87 [+ 13], Summer 68 [+ 11]N. Average freeboard was 63 (+ 2) % for winter clothing, 62 (+ 2) % for autumn/spring clothing, 66[+ 2] % for summer clothing, and 15[+ 1] % for the control condition. Children were more buoyant, 95 [+ 17] % freeboard), irrespective of gender, than adults. “Float first” should be advocated

“Float First:” Trapped Air Between Clothing Layers Significantly Improves Buoyancy on Water After Immersion

Approximately 450,000 people drown annually worldwide. The capacity of immersed adults and children to float in clothing is less well understood, but it is possible that air trapped between clothing layers increases buoyancy. Study 1 (n = 24) quantified this buoyancy and the consequence of any buoyancy by measurement of airway freeboard (mouth to water level distance). Study 2 examined the capability of children (n = 29) to float with freeboard used as the outcome measure and is expressed as a percentage of occasions that freeboard was achieved. Buoyancy (Newtons; N) was provided for winter clothing as 105 [+ 12], Autumn/Spring 87 [+ 13], Summer 68 [+ 11]N. Average freeboard was 63 (+ 2) % for winter clothing, 62 (+ 2) % for autumn/spring clothing, 66[+ 2] % for summer clothing, and 15[+ 1] % for the control condition. Children were more buoyant, 95 [+ 17] % freeboard), irrespective of gender, than adults. “Float first” should be advocated

An ion trap design for a space-deployable strontium-ion optical clock

Optical atomic clocks demonstrate a better stability and lower systematic uncertainty than the highest performance microwave atomic clocks. However, the best performing optical clocks have a large footprint in a laboratory environment and require specialist skills to maintain continuous operation. Growing and evolving needs across several sectors are increasing the demand for compact robust and portable devices at this capability level. In this paper we discuss the design of a physics package for a compact laser-cooled 88Sr+ optical clock that would, with further development, be suitable for space deployment. We review the design parameters to target a relative frequency uncertainty at the low parts in 10^18 with this system. We then explain the results of finite element modelling to simulate the response of the ion trap and vacuum chamber to vibration, shock and thermal conditions expected during launch and space deployment. Additionally, an electrostatic model has been developed to investigate the relationship between the ion trap geometrical tolerances and the trapping efficiency. We present the results from these analyses that have led to the design of a more robust prototype ready for experimental testing.Comment: 21 Pages, 20 Figures, Approved for publication in "Proceedings of the Royal Society A

Guidelines for developing optical clocks with 10−1810^{-18} fractional frequency uncertainty

There has been tremendous progress in the performance of optical frequency standards since the first proposals to carry out precision spectroscopy on trapped, single ions in the 1970s. The estimated fractional frequency uncertainty of today's leading optical standards is currently in the $10^{-18}$ range, approximately two orders of magnitude better than that of the best caesium primary frequency standards. This exceptional accuracy and stability is resulting in a growing number of research groups developing optical clocks. While good review papers covering the topic already exist, more practical guidelines are needed as a complement. The purpose of this document is therefore to provide technical guidance for researchers starting in the field of optical clocks. The target audience includes national metrology institutes (NMIs) wanting to set up optical clocks (or subsystems thereof) and PhD students and postdocs entering the field. Another potential audience is academic groups with experience in atomic physics and atom or ion trapping, but with less experience of time and frequency metrology and optical clock requirements. These guidelines have arisen from the scope of the EMPIR project "Optical clocks with $1 imes 10^{-18}$ uncertainty" (OC18). Therefore, the examples are from European laboratories even though similar work is carried out all over the world. The goal of OC18 was to push the development of optical clocks by improving each of the necessary subsystems: ultrastable lasers, neutral-atom and single-ion traps, and interrogation techniques. This document shares the knowledge acquired by the OC18 project consortium and gives practical guidance on each of these aspects

Guidelines for developing optical clocks with 10-18 fractional frequency uncertainty

There has been tremendous progress in the performance of optical frequency standards since the first proposals to carry out precision spectroscopy on trapped, single ions in the 1970s. The estimated fractional frequency uncertainty of today's leading optical standards is currently in the 10−18 range, approximately two orders of magnitude better than that of the best caesium primary frequency standards. This exceptional accuracy and stability is resulting in a growing number of research groups developing optical clocks. While good review papers covering the topic already exist, more practical guidelines are needed as a complement. The purpose of this document is therefore to provide technical guidance for researchers starting in the field of optical clocks. The target audience includes national metrology institutes (NMIs) wanting to set up optical clocks (or subsystems thereof) and PhD students and postdocs entering the field. Another potential audience is academic groups with experience in atomic physics and atom or ion trapping, but with less experience of time and frequency metrology and optical clock requirements. These guidelines have arisen from the scope of the EMPIR project "Optical clocks with 1×10−18 uncertainty" (OC18). Therefore, the examples are from European laboratories even though similar work is carried out all over the world. The goal of OC18 was to push the development of optical clocks by improving each of the necessary subsystems: ultrastable lasers, neutral-atom and single-ion traps, and interrogation techniques. This document shares the knowledge acquired by the OC18 project consortium and gives practical guidance on each of these aspects.EU/Horizon2020/EMPIR/E

"Float First:" Trapped air between clothing layers significantly improves buoyancy after immersion

Approximately 450,000 people drown annually worldwide. The capacity of immersed adults and children to float in clothing is less well understood, but it is possible that air trapped between clothing layers increases buoyancy. These studies aimed to quantify buoyancy and the practical implications thereof. Study 1 (n= 24) quantified this buoyancy and the consequence of any buoyancy by measurement of airway freeboard (mouth to water level distance). Study 2 examined the capability of children (n = 29) to float with freeboard used as the outcome measure and is expressed as a percentage of occasions that freeboard was achieved. Buoyancy (measured in newtons; N) was provided for winter clothing as 105(± 12)N, for autumn/spring clothing as 87(± 13)N, and for summer clothing as 68(± 11)N. In all cases, buoyancy was greater than for the control condition of 61(± 11)N. Aver­age freeboard was 63(± 2) % for winter clothing, 62(± 2) % for autumn/spring clothing, 66(± 2)% for summer clothing, and 15(± 1)% for the control condition. Children were more buoyant, 95(± 17)% freeboard, irrespective of gender, than adults. "Float first" is advocated as a primary survival mechanism

Performance of emergency underwater breathing systems in cool (25°C) and cold (12°C) water

Introduction: The shortfall between breath-hold time on cold-water immersion and the time required to make an underwater escape from a helicopter provides the rationale for emergency underwater breathing systems (EUBS) for passengers flying over cold water. This study compared three types of EUBS: a compressed gas system (CG); a rebreather system (RB); and a hybrid system (H). Methods: Each EUBS was examined during water deployment (Wdep) and over 90 s in cool (25°C) and cold water (12°C) immersion to the neck (Imm) and submersion (Subm). Subjects wore standardized clothing, including dry suit. Measures included: Wdep time, stay time (Imm and Subm), dyspnea rating, O2 and CO2 remaining in rebreather bags [H and RB (partial pressure mmHg)], and gas volume used (CG). Results: Mean data show Wdep was slowest in the H (17.7 s) compared to the RB (10.0 s) and CG (8.1 s). Stay time was greatest in the H (90.0 s) compared to the RB (68.3 s) and CG (87.0 s); stay time in CG was also greater than RB. Dyspnea ratings were greater in RB trials (6.5 cm) compared to the CG (2.4 cm) and H (1.9 cm). Across devices, stay time in cold water was shorter during submersion than immersion (85.9 s vs. 70.1 s). During submersion stay time was shorter in cold compared to cool water (12°C: 62.8 s; 25°C: 77.5 s). Discussion: The data suggest that the CG and H devices outperformed the RB device, but the H device required longer to deploy