Scope: This Guideline provides the process required to properly determine product density using glass hydrometer
Frequency: As needed
DENSITY MEASUREMENT BY USING GLASS HYDROMETER
PREPARATIONS:
Hydrometer specifications shall be in accordance with API MPMS, Chapter 9.1, “Hydrometer Test Method for Density, Relative Density (Specific Gravity), or API Gravity of Crude Petroleum and Liquid Petroleum Products”. Recommended density hydrometers are:
A Saudi Aramco Laboratory or a recognized independent inspection agency, against a hydrometer, shall have certified the accuracy of the hydrometer used in this procedure within the previous year, which is traceable to an NIST (NBS) or other recognized standard. Hydrometers should meet or exceed the accuracy specification for the particular hydrometer.
Recommended density test thermometers are:
CALIBRATION EQUIPMENT:
A) Hydrometer
B) Glass cylinder
C) Thermometer
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 hydrometer and a thermometer of suitable range.
e) Check the hydrometer for loose shot or a detached gravity scale, and the thermometer for a separated mercury column. If any of these conditions is found, obtain a defect free
hydrometer or thermometer, as applicable, before proceeding with the test.
f) Ensure that the hydrometer is clean and dry using a lint free cleaning rag.
g) Remove any bubbles formed at the liquid surface by touching them with a piece of clean paper.
h) Gently lower the hydrometer into the center of the hydrometer cylinder, taking care that the stem of the hydrometer is not wetted above the immersion level any more than necessary.
Note: Liquid on the hydrometer stem above the immersion level may lead to erroneous results.
i) Insert the thermometer into the cylinder and continuously stir the sample, taking care that the mercury thread is kept fully immersed and that the stem of the hydrometer is not wetted above the immersion level.
j) As soon as the temperature reading on the thermometer has stabilized, partially remove the thermometer, keeping the mercury thread fully immersed, read and record the temperature to the nearest 0.25°C.
k) Remove the thermometer.
l) Depress the hydrometer 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.
m) Allow sufficient time for the hydrometer to become stationary, all air bubbles to come to the surface and the hydrometer temperature to stabilize. Normally, 3 to 5 minutes will be required.
Note: It may be difficult to ensure that the temperature of the hydrometer 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 0.0005 kg/liter to be acceptable. If repeatability cannot be obtained, the temperature may not have stabilized or loss of light hydrocarbons may be occurring.
n) With the hydrometer 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 hydrometer scale at the point where the line formed by the principal liquid surface cuts the scale. Estimate the hydrometer reading to the nearest 0.0001 kg/liter.
2) Opaque Liquid and / or Opaque Hydrometer Cylinder
i) With the eye level slightly above the plane of the liquid surface, estimate the hydrometer reading to the nearest 0.0001 kg/liter at the point where the liquid level is at its highest on the hydrometer stem.
ii) Estimate the height that the liquid meniscus rises above the main surface to the nearest 0.0001 kg/liter.
Note: When determination of the actual meniscus correction is not practical, the following meniscus correction values may be used:
iii) Determine the observed density by subtracting the estimate for the meniscus height obtained in Step n.2.ii from the reading obtained in Step n.2.i.
o) Immediately after reading the density, insert the thermometer and cautiously stir the sample, keeping the mercury thread fully immersed.
p) As soon as the temperature reading on the thermometer has stabilized, partially remove the thermometer, keeping the mercury thread fully immersed, and read and record the temperature to the nearest 0.25°C.
q) If the temperature read in Step n differs from the temperature read in Step j by more than 0.5°C, repeat the procedure starting at Step k, until the pre-test and post-test temperatures agree within 0.5°C.
r) Calculate the average observed test temperature to the nearest 0.5°C from the values observed immediately before and after the final hydrometer reading.
s) Report the final observed hydrometer reading to the nearest 0.0001 kg/liter and the average test temperature to the nearest 0.5°C.
Note: If required for manual proving or volume calculations etc., determine the density at 15°C from the final observed hydrometer reading and the average test temperature using ASTM D1250-80, 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 multiplied by 1000, to convert it from kg/liter to kg/m3.