Subject :- Trench Shelter Systems - Trials at Chatham.
Reference S.8949 dated 15.9.38, and S.9009 dated 16.9.38
1. The trial referred to in the above quoted memos was carried out at Chatham on 22nd September, 1938.
2. Description of Shelter.
One block of the trench shelter described in the A.R.P. Memo. Attached to S.9009. was used for the trial - a plan is attached at Appendix I. The shelter differed in the following details from the plan -
(a) Lighting was provided by 13/30 watt. Electric Lamps.
(b) No emergency exit was built at the junction of sides A and B.
(c) No latrine alcove was built at the junction of sides A and B.
(d) Only partial revetment had been carried out, and the boarding in use for the purpose had been treated with creosote. The smell of creosote in the shelter was very pronounced.
(e) No floor boarding was provided.
(f) Chemical closets were not installed. The shelter was dug in sandy clay which was in dry condition.
3. Personnel and Available Air Space.
During the trial 153 men occupied the trench, which gave an allowance of approximately 44 cubic feet of air and a surface area of approximately 30 square feet per head.
4. Program of Trial.
The following program was carried out. Zero being at 10.02 hours B.S.T.
Zero - Air raid warning, troops rush to shelter.
Zero + 8 minutes - All exits closed.
Zero + 98 " - Troops put on respirators.
Zero + 128 " - All exits opened. Troops remaining in shelter.
Zero + 158 " - Shelter evacuated.
5. Air Sampling.
Samples of the air were taken at the points marked A., B., C and D on plan, for the estimation of oxygen and carbon dioxide content. The results are shown graphically in Appendix II.
It will be seen there is a gradual rise of CO2 to a maximum of 2.74% at the end of 2 hours and that there were only small variations between the results obtained at the different sampling points. At the end of two hours the Oxygen had fallen to 17.6%. As soon as the exits were opened there was a rapid improvement in the air conditions and samples taken 20 minutes afterwards showed 0.36% CO2, after 30 minutes the CO2 was 0.29% and the Oxygen 20.6%. Twenty minutes after the shelter was emptied the conditions approximated closely to the external air.
6. Temperatures and Relative Humidities.
At zero + 128 Minutes the outside temperatures were 69°F, Dry bulb and 62°F, Wet bulb.
Observations on the temperature in the shelter were taken at frequent intervals at the points A., B., C. and D., by means of a sling hygrometer. The results are recorded graphically at Appendices III and IV. It will be seen that soon after the shelter was occupied there was a rapid rise of about 10°, but during the remainder of the time further increase only amounted to about 3°. The maximum Dry bulb temperature recorded was 75°F. At zero + 120 minutes in position D. During the period of occupation the relative humidity after an initial rise, remained at about 79%. As soon as the exits were opened the temperature fell slowly and after 30 minutes had fallen about 3°.
After evacuation of the trench the temperature at the end of 30 minutes was about 3 - 4° above the initial temperature in the trench, but 2° lower than the external temperature. Only partial revetting had been carried out in Sides B and C. and the temperature at these points was about 2° less than in sides A and D. which were lined almost to the ground. The differences recorded between the temperatures in the two revetted sides and in the two un-revetted sides were insignificant and the average for the two groups has been taken.
The increased comfort in sides B and C, as compared with sides A and D, was, however, very noticeable, despite this relatively small difference in air temperature. It is thought that this may have been due to increased radiation losses from the body to the unrevetted walls, which were of course at a much lower surface temperature than the wood-lined ones. Such radiation effects are not adequately represented by measurements of wet and dry bulb temperatures alone.
1. It is to be considered that 3% CO2 should be taken as the maximum permissible in the shelter.
2. Under the circumstances of this trial this figure would have been reached in approximately 2 1/2 hours when all exits were closed. In this trial, however, the occupants of the shelter were fit young men completely at rest and under quiet conditions; further, no one vomited, and no one had occasion to use a latrine. The men had breakfasted at 7 a.m., three hours before the commencement of the trial.
Under War conditions when the occupants will be civilians of all ages and in all states of health, it may be anticipated that their output of carbon dioxide will be higher than that obtained in the present trial and the consumption of oxygen greater. It is considered, therefore, that the shelter should not be occupied for than two hours by 155 civilians in war-time.
3. With the exits open the rapid improvement in the conditions, in the atmospheric composition in the shelter and the downward tendency of the temperature, indicate that the shelter could probably be occupied indefinitely by the same number of people - i.e. 155.
4. After evacuation of the trench the atmospheric conditions rapidly improved, but the temperature only fell slowly. In half-an-hour, however, the trench could have been safely reoccupied for 2 hours. If it was reoccupied sooner, the period of occupation, with the exits closed, would have to be reduced accordingly.
These observations were carried out by Majors Walker and Hall of this department, and Mr. L.T.D. Williams of Porton.
(Sgd.) A. Fairley.
26th September, 1938.