THE SVTP INSTRUMENT AND SOME APPLICATIONS TO OCEANOGRAPHY 
by J.R. LOVETT 
Oceanic Research Division 
Research Department 
U.S. Naval Ordnance Test Station 
China Lake, California 
ABSTRACT 
In the SVTP instrument, sound velocity is 
measured by a modified NBS "sing-around"” veloci- 
meter, temperature by a Wien-bridge oscillator, 
and pressure by a Vibrotron. 
The sound-velocity section may exhibit shifts 
in calibration of about 2.25 m/sec unless 
adjusted correctly and then calibrations are 
stable within 0.15 m/sec. The temperature oscil- 
lator is accurate to +0.01°C for periods of sev- 
eral months. The Vibrotron has a short-term 
repeatability of 0.25% of bandwidth. 
Data processing systems consist of counters, 
discriminators driving X-Y plotters, and tape 
recorders with data later digitized or displayed 
on X-Y plots. 
Some applications in oceanography are in 
fathometry and internal wave studies. 
DESCRIPTION OF THE INSTRUMENT 
A sound-velocity, temperature, and pressure 
(SVTP) instrument was built at the U. S. Naval 
Ordnance Test Station (NOTS) to give continuous, 
concurrent measurement and data transmission of 
these three oceanic parameters at any depth 
(Fig. 1). 
This completely transistorized instrument 
uses potted plug-in modules. The modules use 
the new welded-cordwood stacking construction 
for greatest reliability in the smallest space 
(Fig. 2). 
The sound-velocity section of the instrument 
is the velocimeter developed by Greenspan and 
Tschiegg of the National Bureau of Standards and 
modified to operate in IRIG telemetry band 8 
(3,000 eps center frequency). This is achieved 
by decreasing the free-running frequency and in- 
creasing the path length to 24.7 cm. Care must 
be taken in setting the reflective path length, 
or the instrument may exhibit shifts in calibra- 
tion of about 2.25 m/sec due to the input trig- 
gering on a precursor of the 3-megacycle pulse 
(Fig. 3). The connection between the amplifier 
and the triggering circuit is broken thus dis- 
abling the sing-around circuit. Leads are 
brought out to adjacent pins on the plug. Then 
the output of the amplifier is observed while 
the reflectors are adjusted so that the pulse 
168 
has a good clean rise and there are no multiple 
reflections. When the desired waveform is ob- 
tained, the two pins are connected in order to 
trigger the blocking oscillator. There is a 
provision made to add negative feedback if the 
amplifier stages are oversensitive. The sound- 
velocity section is accurate to 0.30 m/sec and 
has a repeatability of 0.15 m/sec. 
Temperature is measured by a thermistor- 
controlled Wien-bridge oscillator developed at 
NOTS. This oscillator works in the temperature 
range 0-30°C and in the frequency band 5,000- 
8,000 cps, thus giving a sensitivity of 0.01°C 
per cycle per second. By using aged thermistors, 
capacitors stable to less than 10 ppm/°C, and 
other techniques, the accuracy is +0.01°C. Maxi- 
mum variation from a best straight line 5-20°C 
of +0.02°C is achieved by a 3-point match of the 
oscillator and thermistor curves. 
The Vibrotron is used for pressure measure- 
ments in the SVTP instrument; however, there is 
some uncertainty as to the future availability of 
this transducer. Pressure is sensed by deforma- 
tion of a diaphragm that in turn produces a 
change in tension, and, hence, in frequency 
(IRIG band 12) of a vibrating wire. ‘A simple 
oscillator is used to sustain the forced vibra- 
tions. Repeatability, short term, is +0.25% 
linearity is within 43% of a straight line 
between end points; and temperature sensitivity 
is less than +0.1% of bandwidth per °C change of 
zero frequency. 
A summing cable-driving amplifier combines 
the sound-velocity, temperature, and pressure 
signals so that they may be transmitted as a 
mixed frequency signal over a single-conductor 
cable. Six thousand feet of semi-buoyant poly- 
ethelene covered cable with a breaking strength 
of 900 pounds has been used. When in sea water 
it acts like a 50-ohm coaxial cable, and if 
driven with a matched amplifier, the voltage 
loss at 10 kilocycles is 2 db/1,000 ft. When on 
the winch, capacitance of the cable is negligible 
while the inductance is about 310 millihenries. 
At 10,000 eps, this inductance gives an impedance 
of about 20,000 ohms. Therefore, to avoid an 
intolerable voltage loss, a high input impedance 
amplifier must be used to drive the filters which 
separate the signals. 
Nickel-cadmium batteries are internally 
