604 
34 
PRESSURE APPARATUS 
Calibration pressures are developed inside a cylindrical steel 
pressure chamber* filled with a rust-inhibiting solution of potassium dichro- 
nate and sodium carbonate. O11 is not used, since upon repeated immersion in 
oil at high pressures the rubber insulation on the tourmaline gages would be 
damaged. The crystal end of the gage is sealed into the pressure vessel by 
neoprene washers sandwiched between split brass washers. The latter fit snug- 
ly around the copper-tube cable, and the neoprene washers are compressed by 4 
retaining screw through which the cable emerges from the chamber. The pres- 
sure is raised to some predetermined value which is read on a Bourdon-tube 
gage. Three such gages with ranges up to 2000, 5000, and 10,000 pounds per 
square inch have been used. These have been calibrated twice at the National 
Bureau of Standards. They can be read to within 1/4 to 1/2 of one per cent 
at the usual test pressures. 
In the calibration of the piezoelectric gages, a rapid change in 
pressure is required. Therefore, once the desired pressure has been attained, 
it is released by one of the following two methods. A release valve is sud- 
denly opened, permitting a return to atmospheric pressure in approximately 
0.05 second; or a thin brass diaphragm, sealed into the chamber on the oppo- 
site side from the gage, is punctured, releasing the pressure in 0.2 to 0.3 
millisecond. The diaphragm must be thick enough to withstand a given pres- 
sure, but thin enough to be readily punctured with, say, a thrust of a screw- 
driver. In practice, the diaphragms are made of brass, 1.12 inch in diameter 
and 0.003 inch to 0.008 inch thick, depending upon the pressure to which they 
are to be subjected. It requires but a few minutes to remove a punctured 
diaphragm and replace it with a new one. 
Unlike the transient character of a piezoelectric gage calibration, 
that of a glass resistance gage is static; i.e., the pressure remains at a 
predetermined value while the change in the gage resistance is measured. 
CIRCUITS FOR MEASURING THE OUTPUT OF THE GAGES 
For a piezoelectric gage the output to be measured is a quantity of 
charge, whereas for a resistance-type gage the output is a change in resis- 
tance. Accordingly, different circuits must be used in calibrating the two 
types. 
CALIBRATION OF THE PIEZOELECTRIC GAGES 
The charge AQ developed by a piezoelectric crystal under load is 
proportional to the change AF in the total force acting on the electrode 
* 
This apparatus was designed by Lt. Dennison Bancroft, USNR, of the David Taylor Model Basin staff. 
