IEC 60567 SAMPLING OF GASES AND OF OIL FOR ANALYSIS OF FREE AND DISSOLVED GASES
IEC 60567 OIL-FILLED ELECTRICAL EQUIPMENT - SAMPLING OF GASES AND OF OIL FOR ANALYSIS OF FREE AND DISSOLVED GASES - GUIDANCE
6 Preparation of gas-in-oil standards
As noted in Clause 1, the preferred method of assuring the performance of the entire system for gas extraction and analysis is to analyse oils containing known concentrations of gases (gas-in-oil standards).

Two methods of preparing gas-in-oil standards are described below, a general method and a simpler method. The first method has the benefit of producing much larger quantities of oil.

NOTE If gas-in-oil standards are commercially available, they can be used provided they fulfill quality assurance requirements.

6.1 First method: preparation of a large volume of gas-in-oil standard
6.1.1 Equipment
A suitable design of apparatus consists of (see Figures 7a, 7b, 7c):
- a magnetic stirrer (12);
- a 5 l oil vessel (13) equipped with three side-arm outlets, two of which are fitted with high-vacuum stop-cocks (1) and (2) and one with a rubber septum (16);
- a 6 l reservoir (14) equipped with a high-vacuum stopcock (3) connected with a 75 ml splash bulb (15) and a high-vacuum stopcock (4);
- a mercury displacement system (17) consisting of two 500 ml mercury glass bulbs connected with a high-vacuum oil-resistant flexible rubber tubing.

The gas-injection system (Figures 7e to 7g) consists of
- a set of gas tight syringes of appropriate volumes (18);
- a plastic three-way stopcock of standard type (7), modified to minimize dead volume by inserting pieces of stainless steel tubing (approximately 0,4 mm inner diameter and 1,5 mm outer diameter) into the inner branches and connected to a gas-injecting needle (10) (approximately 10 cm long, 0,25 mm outer diameter) to go through the rubber septum into the oil flask (16);
- gas cylinders equipped with a pressure reducer and a valve (20);

NOTE Gas cylinders containing each of the individual gases to be injected can be used. Ready-made mixtures of these gases can also be obtained from some gas suppliers.

- a length of oil-resistant flexible tubing connecting the gas cylinder to the three-way stopcock (7) and incorporating a hypodermic needle (approximately 0,3 mm inner diameter and 0,6 mm outer diameter). The latter acts as a leak to a water bubbler (21) which is used to check the absence of back diffusion.

NOTE Replace the length of tubing connecting the gas cylinder to the three-way stopcock after each gas change.

The oil sampling system (Figures 7h to 7k) consists of glass syringes (19) of suitable capacity equipped with two three-way plastic stopcocks (8)) and (9) and an oil-extracting needle (11) (approximately 0,6 mm inner diameter, 1,0 mm outer diameter and 120 mm long).

6.1.2 Procedure
NOTE All the circled numbers refer to Figures 7a to 7k. The positions A to D of the three-way stop-cocks (7, 8 and 9) are illustrated in Figure 7d.

6.1.2.1 Degassing the oil
a) Thoroughly clean all the glassware before assembling.
b) Secure the rubber septum and the flexible connections with pieces of twisted metal wire.
c) Clean, if necessary, approximately 1 l of mercury with pentane and filter through a finely pierced filter paper.
d) Adapt the oil reservoir (14), the splash bulb (15) and stop-cock (4) to the 5 l vessel (13). Connect the system to the vacuum pump (V) and evacuate the splash bulb (15) and the 5 l oil vessel (13) by opening stopcocks (2)and (4).
e) Fill the oil reservoir (14) with mineral insulating oil complying with IEC 60296 and allow oil to flow slowly through stopcock (3) into the 5 l vessel (13) until it is full. Then close stopcock (2) and remove the oil reservoir (14) and the splash bulb.

NOTE If the procedure is followed carefully (filling time about 4 h,) the oil in the vessel will be virtually gas-free.

f) Turn the oil vessel to bring it into the position shown in Figure 7c. Place a wet cloth over it to prevent its temperature rising and attach the mercury displacement system (17) to stopcock (1). Introduce 750 ml of mercury into the system with stopcock (6) closed.
g) Connect the system to vacuum (V), open stopcock (5) and evacuate the section between stopcocks (1) and (6). Open stopcock (6) to allow mercury to rise up to stopcocks (1) and (5), and then close stopcock (5). Raise the left bulb of the system (see Figure 7c) so that the mercury level is above the oil flask. Open stopcock (1). Remove the wet cloth and switch on the magnetic stirrer (12).

6.1.2.2 Preparation of gas-in-oil standards
See Figures 7c to 7g.
a) To inject the gases, attach the modified three-way stopcock (7), in position B, to the gas-injecting needle (10). Then push the needle through the rubber septum (16) into the oil. Turn the three-way stopcock to position D to purge the needle and stopcock with oil, then turn it to position B. Attach to stopcock (7) the barrel of a precision calibrated gas-tight syringe (18) of appropriate volume (precision within 1 %) and the gas cylinder connecting tubing (see Figure 7e). Allow a gentle flow of gas to flush the barrel, then slowly push the plunger several times into the barrel, finally down to the volume of gas to be injected, making sure there is continuous bubbling through the needle leak (21).
b) Switch stopcock (7) to the injection position A, lower the mercury level below the needle tip, and push the plunger to inject the gas volume into the oil. Switch stopcock (7) to position B and raise the left mercury bulb above the oil vessel.
c) Repeat the same procedure with each of the gases to be dissolved then remove the gas needle and stopcock (7).

When all the gases are dissolved, switch off the magnetic stirrer. Record the atmospheric pressure and temperature.

NOTE Instead of adding individual gases, it is more convenient to use a standard gas mixture, preferably containing all the gases listed in 8.1, in proportions representative of those found in the sampled oil.

6.1.2.3 Sampling of gas-in-oil standards
See Figures 7h to 7k.
a) To remove oil samples from the 5 l vessel (13), attach two three-way stopcocks (8) and (9) turned in position B, according to Figure 7b, to the liquid-withdrawing needle (11) and push the needle through the rubber septum (16). With the mercury level up, switch stopcocks (8) and (9) to the draw-off position D to purge them with oil (see Figure 7i).
b) Attach a syringe (19) and turn both stopcocks (8) and (9) to position A, and draw out a suitable volume of oil (see Figure 7j). Then turn stopcock (8) to position C and stopcock (9) to position B and remove the syringe together with stopcock (8) (see Figure 7k).

To remove more oil, additional mercury must be added to the mercury displacement system.

6.1.3 Calculation
Calculate the concentration of each gas "i" dissolved as follows:
Ci = Vi/V x10(6)
where
Ci is the concentration of gas "i", in 
Vi is the volume of gas "i" injected, corrected to 20 °C and 101,3 kPa in ml;
V is the exact volume of oil in the 5 l oil flask, in ml.

6.2 Second method: preparation of gas-in-oil standards in a syringe or a vial
See Figures 8a to 8g.

6.2.1 Equipment
Equipment consists of
- a set of gas-tight syringes of appropriate volume (2);
- a set of glass syringes of appropriate volume (1);
- a set of gas-injecting needles (4);
- three-way plastic stopcocks (3);
- gas cylinders equipped with a pressure reducer and a valve (20).

NOTE Gas cylinders containing each of the individual gases to be injected may be used. Ready-made mixtures of these gases can also be obtained from some gas suppliers.

6.2.2 Procedure
a) Bubble argon through 1 l of oil for 1 h under agitation, adjusting gas flow to maintain agitation of the oil in order to purge all the other dissolved gases. Degas this argon-saturated oil under a vacuum down to 1 Pa over an 8 um-filter in a 2 l vessel (residual contents of 500 ul/l O2 and 2000 ul/l N2 are acceptable). When the oil is completely degassed, break the vacuum with argon up to 20 kPa.
b) Weigh a 100 ml glass syringe containing 10 ml of glass beads. Connect the syringe to the bottom of the 2 l vessel and purge the syringe twice with 20 ml of oil. Make sure the cylinder and plunger are completely wetted with oil. Fill the syringe with 90 ml of degassed oil.
c) Connect a gas-tight syringe of the required volume with a needle long enough to introduce a gas mixture in the syringe cylinder part, for instance of 115 mm length, to a cylinder containing a standard gas mixture, and purge 4 times with the standard gas. Fill the syringe with a known volume of standard gas. Insert the needle through the tip of the oil syringe and transfer the volume of standard gas into the oil syringe. Reweigh to determine the actual oil volume. Wrap the oil syringe with aluminium foil to avoid degradation with daylight and attach it to a laboratory 3-D shaker. Shake for 1 h or until all gasses are dissolved.

NOTE 1 Any other procedure that assists dissolution of gases in oil (for example, mixing with magnetic stirrer or other) is suitable and can be used.

d) Record the atmospheric pressure and temperature, also refer to the note in 6.1.2.2. Calculate the dissolved gas concentration from oil volume, injected calibration gas volume, temperature and atmospheric pressure, as in 6.1.3. Express the quantities of gases injected in umoles or in ul converted to normal conditions (20 ºC; 101,3 kPa).
e) Repeat the procedure in order to obtain at least 3 gas-in-oil samples (in the case of headspace only) having concentrations that allow a calibration curve to be drawn covering the values expected in the unknown samples.

NOTE 2 If a total gas-saturated mixture is desired, pull dry CO 2 -free air or nitrogen into the syringe to make, with the gases already injected, a total of 10 %, to 8 % of the oil volume according to the gas chosen (see Figure 8f).

Turn the three-way (3) stopcock to position A and shake the syringe again until the air or nitrogen is dissolved. Cooling the syringe in a refrigerator accelerates the dissolution of the gas in the oil.

NOTE 3 A vial may be used instead of a syringe. Fill the vial with the same amount of degassed oil as that needed when the ordinary analysis is performed, with the exact volume of oil measured by weight. Introduce into this vial known quantities of a calibrated gas mixture, so that the concentrations in the oil are in the same range as in the unknown field sample.