Development of a prototype powered helmet respirator suitable for use in coalmines

A powered helmet respirator (PHR), the Racal “”Airstream AH6″”, has been available to mineworkers at selected collieries for a number of years. However, experience has suggested that the AH6 is too bulky, too heavy and has too low an airflow for use in collieries. It was also found that the AH6 severely interferes with communication, particularly when the wearer is using a tannoy or telephone. The Institute of Occupational Medicine (IOM) was therefore commissioned by the European Coal and Steel Community and the British Coal Corporation to develop a PHR optimised for use in coal mines.An IOM prototype PHR was developed, which was found in wearer trials to be significantly more acceptable than the AH6 (HOWIE et al, 1987). However a number of design characteristics of the prototype still caused wearer dissatisfaction; in particular the airflow rate was perceived as being too low, the prototype although lighter than the AH6 was still too heavy and the fans were too noisy.The Institute was consequently commissioned by the CEC to further develop the prototype PHR with the objective of increasing its acceptability under coal mine working conditions. This report describes the additional work carried out during this development.A second 1OM prototype PHR was produced in which the wearer’s perception of airflow was increased, weight was reduced by about 150 g, and balance andstability were improved by the use of a new cap lamp mounting bracket which brought the cap lamp 20 mm closer to the head. The headshell was slightly enlarged to accommodate a larger range of head sizes and the axis of the cap lamp was made the same as that on a conventional safety helmet. Noise levels from the fans were reduced by 4dB(A) and vision was improved by minimising reflections from the face and by reducing obscuration by foam seals between the visor and the face.Leakage tests were carried out with the PHR fitted to human subjects using the standard sodium chloride test aerosol specified for such tests in the draft European Standard for PHR, prEN 146, [CEN (1987)]. Tests were carried out with airflow velocities ranging between 0 and 7.5 m sec-1 impinging on the wearer’s body and the PHR, as against the aerosol being projected at the ‘vicinity of the wearer’s head’ at a velocity of 2 m sec-1 as specified in prEN 146. Leakage was found to be dependent upon the airflow and the wearer’s orientation to the direction of airflow, as well as upon the PHR design. These findings critically question the relevance of the standard PHR leakage test conditions in respect of workers who may be exposed to wind velocities in excess of 2 m sec-1. Total inward leakage results for the second prototype PHR in wind velocities 4.5 m sec-1 were below the 20 per cent target.Following development of the improved prototype PHR, several devices were produced and evaluated in ergonomic trials. These trials assessed the effects of the improved prototype on communication from PHR wearers to non-wearers and wearer acceptability under simulated mining conditions.The communication trials in the laboratory revealed that any visor which covers the wearer’s mouth will interfere with communication, although it was found that the prototype PHR should cause less interference than the AH6. However, in the wearer trials, communication was better for the AH6 than for the IOM prototype. This discrepancy between the laboratory and wearer trial results is probably in part due to the fact that the laboratory results relate to communication from a wearer to a non-wearer, whereas the wearer trials covered a combination of communication from wearers to wearers, wearers to non-wearers and non-wearers to wearers.The wearer acceptability trials indicated that in the assigned high priority areas of airflow, weight, balance, stability and shape/size the prototype was more acceptable than the AH6; in the areas of fan noise, effects on communications and vision, and visor mounting the AH6 was deemed to be the more acceptable. It is considered that the work involved in making any further improvements to the IOM’s prototype with regard to these aspects is fairly minor and would be best carried out during commercial development and manufacture. “”