The water solubility in oil is the maximum water content, in ppm, that this oil can dissolve at a given temperature. When the water content approaches or exceeds the solubility value, excess water separates from the oil, forming free water.
The Measurement Module of MO measures the relative saturation of water in oil (RS%), which is the ratio between the dissolved water content in the oil (ppm) and the water solubility in oil (ppm). The measurement of relative saturation is absolute and is not affected by temperature or oil type. Based on this measurement, the water content in the oil in ppm is calculated, along with conversions of relative saturation to the reference temperature and ambient temperature, using the water solubility as well. Hence, the determination of water solubility in oil is crucial for MO to allow these calculations.
The water solubility in oil varies with temperature, which is why the Measurement Module also measures this. It also depends on the oil's specific characteristics, represented by its solubility constants A and B.
The solubility constants A and B for water in oil mainly vary based on the type of oil, as illustrated by the typical values shown in the table below.
Oil type | Typical value of A | Typical value of B |
Mineral | 7,0895 | 1567 |
Silicon | 6,2906 | 1187 |
Envirotemp FR3 | 5,3318 | 687 |
These typical values can be used by the user for parameterization of the MO (see section 7.2.4 of the manual), achieving good results in most applications. However, there may be situations where, due to variations in other oil characteristics such as highly aged oil, acidity, or others, it may be necessary to determine these constants specifically for the oil under measurement to achieve greater accuracy. The following procedure can be followed for this purpose.
1. Install the MO in the oil for which the solubility constants need to be determined.
2. Vary the oil temperature to obtain MO readings at a minimum of two different temperatures, preferably three, with a temperature difference of at least 10 °C between them.
3. For each oil temperature, wait about 20 minutes to ensure measurement stabilization, then take an oil sample and note the relative saturation (RS%) and oil temperature measurements reported by the MO. For each temperature, a new oil sample should be taken, as variations in water content may occur due to absorption or release of water by the insulating paper or due to the dissolution of free water or vice versa.
4. Analyze the oil samples in the laboratory to determine their water content. Fill in the table below with the obtained results. The data presented below are an example of completion:
Measurements obtained from the MO | ||
Temperature (°C) | Relative Saturation (RS%) | Water content (ppm) obtained in the laboratory |
31,8 | 26,9 | 18,0 |
50,5 | 13,8 | 18,5 |
70,3 | 6,2 | 18,5 |
5. From the above data, fill in the following table:
Values of X X = 1 / (273,1 + Temperature ºC) | Values of Y Y = log (100 x pmm / RS%) |
0,0032798 | 1,82552 |
0,0030902 | 2,12729 |
0,0029121 | 2,47478 |
6. The values of X and Y in this table are related according to the equation of a straight line:
Y = -B . X + A
Where A and B are the water solubility constants in the oil that we want to find.
To obtain the values of these constants that best fit the measured data, the linear regression technique can be used. Linear regression can be easily performed using commercial software such as Microsoft Excel. The values of A and B can be calculated using the attached spreadsheet.
In the example above, the linear regression of the data results in:
Y = -1764 . X + 7,5999
From which we obtain:
A = 7,5999 e B = 1764
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