611 
4y 
insulation is coated on, and the value obtained after the gage has been coated 
with rubber. This question is being investigated.* 
It is also planned to study the effect, if any, on KA when the gage 
is subjected to a series of explosions. In this connection it should be ob- 
served that the oscillographic calibration technique has a great advantage 
over the other methods, in that it can be applied to a gage after repeated 
explosions. It is found, for example, that the impedance of a gage after 
several explosions usually drops well below the 30,000 megohms required for 
calibration with the microcoulometer. Hence recalibration by this method is 
impossible. On the other hand, the oscillographic method does not demand an 
impedance of more than about 10 megohms, and the gage impedance seldom drops 
below several hundred megohms. When it does, the gage becomes useless in any 
event, for the RC of the gage circuit is then too small. 
CALIBRATION OF THE GLASS RESISTANCE GAGE 
The glass resistance gage is calibrated with a Wheatstone bridge. 
The gage is sealed into the pressure chamber in the same way as are the piezo- 
electric gages, except that a brass washer soldered to the cable above the 
glass element facilitates the closure. 
A series of pressure increments AP up to 6000 pounds per square 
inch are then applied to the gage, and the change AR, in its resistance is 
measured. Displacements on the galvanometer scale, which are proportional to 
the resistance changes AR,, are plotted against AP. From the slope of the re- 
sulting line the calibration constant of the gage 1/R, (AR,/AP) is obtained. 
Values of this constant are of the order of 1.5 x TOPs Soma pax equene noe 
Since R, is about 100 ohms, AR, is of the order of 0.015 ohm for a change in 
pressure of 1000 pounds per square inch. 
PART 5. GAGE PERFORMANCE IN TESTS 
UNDERWATER EXPLOSION TESTING 
It was thought desirable to test the piezoelectric gages under a 
variety of circumstances, using larger charges than could be fired at the 
Taylor Model Basin at the time this work was done. Tests were therefore con- 
ducted by Taylor Model Basin personnel at the Underwater Explosives Research 
Laboratory, Woods Hole. Considerable work on explosion pressures had been 
done by investigators at that laboratory with tourmaline gages manufactured 
by the Stanolind Company, and some of these gages were available for direct 
comparison with TMB tourmaline gages. Therefore it was decided to concentrate 
* Dr. A. Borden reports that preliminary experiments indicate a difference of less than 1 per cent 
between the bare and the coated crystals. 
