Scope: This Guideline provides a method to properly determine product density using glass thermo hydrometers.
Frequency As needed
DENSITY MEASUREMENT PROCEDURE BY USING GLASS THERMOMHYDROMETER
PREPARATIONS:
Density Thermo hydrometer specifications shall be in accordance with API MPMS, Chapter 9.3, “Thermohydrometer Test Method for Density and API Gravity of Crude Petroleum and Liquid Petroleum Products”. Recommended density thermohydrometers are:
Glass Thermohydrometers specified in ASTM E 100 measure product density in kilograms per cubic meter (kg/m3 ) and the product temperature in degrees Centigrade (°C). To obtain product density in kilograms per liter (kg/liter), divide the kg/m3 reading by 1000.
The accuracy of the hydrometer portion of the thermohydrometer used in this procedure shall have been certified within the previous year by a Saudi Aramco Laboratory or a recognized independent inspection agency, against a hydrometer, which is traceable to an NIST (NBS) or other recognized standard. Hydrometers should meet or exceed the accuracy specification for the particular hydrometer.
CALIBRATION EQUIPMENT:
A) Thermohydrometer
B) Glass Cylinder
PROCEDURE:
a) Check the hydrometer cylinder for cleanliness. All hydrometer cylinders shall be absolutely clean and dry. If required, obtain clean equipment or clean the existing equipment with a suitable
solvent, such as Stoddard Solvent or naphtha of similar volatility, and rinse with the liquid to be sampled.
b) Transfer the sample into the clean hydrometer cylinder with a minimum amount of splashing.
c) Place the cylinder containing the liquid in a vertical position, in a location free from air currents.
d) Select a thermohydrometer of suitable range.
e) Check the thermohydrometer for loose shot, detached gravity scale, detached thermometer scale, or a separated thermometer mercury column. If any of these conditions is found, obtain a defect-free thermohydrometer before proceeding with the test.
f) Ensure that the thermohydrometer is clean and dry using a lint free cleaning rag.
g) Remove any bubbles formed at the liquid surface in the hydrometer cylinder by touching them with a piece of clean paper.
h) Gently lower the thermohydrometer into the center of the hydrometer cylinder, taking care that the stem of the thermohydrometer is not wetted above the immersion level any more than necessary.
Note: Liquid on the thermohydrometer stem above the immersion level may lead to erroneous results.
i) Depress the thermohydrometer into the liquid about two scale divisions, impart a slight spin and release it so that it floats freely, away from the walls of the cylinder.
j) Allow sufficient time for the thermohydrometer to become stationary, all air bubbles to come to the surface, and the thermohydrometer temperature to stabilize. Normally, 3 to 5 minutes will be required.
Note: It may be difficult to ensure that the temperature of the thermohydrometer and liquid has stabilized. To provide this assurance, two successive determinations of density may be made with the same liquid and each determination corrected to 15 °C. The two corrected values should be within 0.5 kg/m3 to be acceptable. If repeatability cannot be obtained, the temperature may not have stabilized or loss of light hydrocarbons may be occurring.
k) With the thermohydrometer at rest and floating freely away from the wall of the hydrometer cylinder, determine the observed density using one of the following techniques, as applicable:
1) Transparent Liquid and Transparent Hydrometer Cylinder
i) Place the eye slightly below the level of the liquid surface.
ii) Slowly raise the eye level until the liquid surface changes from a distorted ellipse to a straight line.
iii) Read the thermohydrometer scale at the point where the line formed by the principal liquid surface cuts the scale. Estimate the thermohydrometer reading to the nearest 0.1 kg/m3.
2) Opaque Liquid and / or Opaque Hydrometer Cylinder (Refer to Figure P2-2, “Hydrometer Reading for Opaque Fluids”, on Page 8.)
i) With the eye level slightly above the plane of the liquid surface, estimate the thermohydrometer reading to the nearest 0.1 kg/m3 at the point where the liquid level is at its highest on the thermohydrometer stem.
ii) Estimate the height that the liquid meniscus rises above the main surface to the nearest 0.1 kg/m3.
Note: When determination of the actual meniscus correction is not practical, industry practice has been to add the following meniscus correction value:
iii) Determine the observed density by subtracting the estimate for the meniscus height obtained in Step 14.b.ii from the reading obtained in Step k.2.i.
a) Immediately after reading the density, partially withdraw the thermohydrometer from the liquid, keeping the bulb of the thermometer submerged, and read the temperature to the nearest 0.5°C.
Note: Complete withdrawal of the thermohydrometer from the liquid may distort the temperature measurement.
m) Report the observed hydrometer reading to the nearest 0.0001 kg/liter and the average test temperature to the nearest 0.5°C. To obtain product density in kilograms per liter (kg/liter), divide the kg/m3 reading by 1000.
Note: If required for manual proving or volume calculations etc.,
determine the density at 15°C from the observed hydrometer and temperature reading using ASTM D125080,
Table 53A (JP4 and crude oil) or 53B (gasoline, diesel, Jet A1, kerosene, bunker fuel oil). In order to do this, the hydrometer reading will have to be expressed in kg/m3.