Deep-SeaResearch, 1976. Vol. 23. pp. 989 to 993. Pergamon Press. Printed in GreatBritain. INSTRUMENTS AND METHODS Determination of the pressure-volume-temperature (PVT) surface of Isopar-M: a quantitative evaluation of its use to float deep-sea instruments A. A. YAYANOS* (Received 3 June 1975; in revised form 3 March 1976; accepted 5 March 1976) Abstract--A method is described for determining pressure-volume isotherms of liquids at high pressures. A volumometer without a ground glass joint i s used. The pressure-volume-temperature (PVT) surface of lsopar-M has been determined, and shows that this hydrocarbon liquid is suitable for floats to 1100 bars at 0°C. The so-called Tait equation has been used as the equation of state for the PVT values. INTRODUCTION ISOPAR-M, a liquid mixture of hydrocarbons, has been used to float deep-sea gear (e.g. SHUTTS, 1975; PHLEGER and SOUTAR, 1971). The principal advantages of this liquid are low cost and a high flash point. There are no data in the literature that can be used to assess the effect of temperature and pressure/;hat this substance would experience as a function of depth in the ocean. The work of CUTLER, McMICKLE, WEBB and SCHIESSLER (1958) describes the pressure-volume-tempera- ture (PVT) surface of high molecular weight hydrocarbons at temperatures greater than 37-8°C. This paper gives values on the PVT surface of Isopar-M from 0 to 25°C and 1 to 1100 bars. METHODS Isopar-M was purchased from the Humble Oil Co., Los Angeles, California. The manufacturer lists its density as 0.784gem -3 at 15"6°C. The coefficient of thermal expansion was found to be 90.8 × 10 -6 °C-1 over the range of 0 to 25°C. The volume of Isopar-M, 0 1 Vlso, in cm 3 g- was calculated as a function of temperature from the following equation, o = 1.2755 x exp[(T- 15.6) × 90-8 x 10- 5], (1) Viso where T is the temperature in °C. A pressure vessel was constructed from a tube of type 316 stainless steel (s.s.). It has a length of 182"8cm, an inner diameter of 2-54cm and an outer one of 5.08 cm. Each end has an 'O'-ring closure of the type described by GASCHE (1956). Each closure has a high-pressure connection. A block of 316s.s., 17-4 x 17.4 x 5.08cm, slips over the 5.08-cm o.d. pressure vessel and, as shown in Fig. 1, provides support for the windows. The top of the pressure vessel was provided with a device different from the one used by AMAGAT (1893) for raising and lowering the glass- ware inside of the vessel while under pressure. This device (Figs. i and 2) was made so that readily available valve packing could be used in the stuffing box around the rotating stem. The direction of rotation determines whether the string is wound or unwound. The advantages of this raising and lowering device are that its use does not change the pressure in the vessel, that it is easy to make, and that it can be secured to any standard high-pressure connection. The volumometer is shown in Fig. 3. It was made of Pyrex glass. The graduations were etched entirely around the stem to minimize parallax. The 1-mm opening at the tip of the stem is the only opening into the volumometer. The internal volume delimited by the uppermost graduation * Physiological Research Laboratory, Scripps Institution of Oceanography, University of California, La Jolla, California 92037, U.S.A.