616 
46 
From a measurement of the 
peak pressure inside the buffer the 
peak load can be deduced. This 
measurement has usually beem made 
with a Crosby high-pressure engine 
indicator. It was desired to cal- 
ibrate this instrument against a 
tourmaline gage. 
The gage was sealed into 
the buffer by a special adapter. 
(0) 50 milliseconds 
Figure 28 - Oscillogram Showing Pressure 
as a Function of Time Inside a 
Hydraulic Brake or Buffer An RC constant of 1 second was pro- 
This record was obtained with a TMB tourmaline gage. vided by shunting the gage with a 
The duration of this phenomenon is about 1000 times 01 f 
that of typical explosions described in this report. 0. -microfarad capacitor and using 
a cathode follower with an input 
impedance of 100 megohms. A trig- 
ger circuit was designed in which a capacitor discharge, actuated by mechan- 
ical contact of two wires, set off the sweep. Contact was made at a prede- 
termined time before impact when a bolt projecting from the moving car pushed 
one wire against the other. To record the pressure-time curve, a Du Mont 
Type 208 cathode-ray oscillograph was used together with the auxiliary equip- 
ment usually employed in recording transient pressures at the Taylor Model 
Basin. 
The car was released from various heights up to 20 feet, and a 
pressure-time curve was obtained for each impact. Figure 28 shows a typical 
record for a 15-foot drop. The initial, sharp pulse is due to a shock- 
pressure wave transmitted through the buffer. This is followed by the main- 
peak portion of the curve which shows the variation of pressure insPde-the 
cylinder as the piston moves through it. 
In an effort to check the validity of the pressure-time curve, the 
change in momentum mv of the car due to impact was compared with the total 
impulse [Fat deduced from the area under the pressure-time curve.* mis the 
mass of the car; v its velocity at the initial moment of impact; F = Pa, where 
P is the pressure recorded by the gage at time t, and a is the area of the 
piston. The integration, performed with a planimeter, is extended over a 
time from the initial moment of impact until the pressure returns to zero. 
The impulse measured in this way agreed with the calculated change in momen- 
tum to within 5 per cent. 
Thus, besides its usefulness in explosion research, the TMB tourma- 
line gage appears to be equally suitable for the measurement of much slower 
pressure changes. 
%* The recoil velocity of the car is negligible. 
