8. Procedure
8.1 Compatibility testing of SCA shall be conducted using a ratio of 60 parts of coolant concentrate to 40 parts of a water-SCA mixture. The level of SCA in the total 60:40 mixture will be approximately twice the SCA manufacturer's recommended concentration.
8.2 Fill a 100-mL centrifuge tube to the 60-mL mark with coolant concentrate.
8.3 Determine the volume ofwater to be added based on the physical state and the recommended concentration of SCA to be evaluated. Add this volume of water to the centrifuge tube using a graduated cylinder. For example, if the SCA is a liquid to be added at the recommended concentration of 3 % by volume, twice the recommended concentration is 6 % or 6 mL. The volume of water to be added is 34 mL. This is 100 mL (volume of the centrifuge tube) less 60 mL (volume of coolant concentrate required) less 6 mL (volume of SCA required).
NOTE 1 - Using hard water will greatly influence the amount of solubles formed. Testing the purity of the water with a conductivity meter is recommended.
NOTE 2 - If the SCA is a solid, prepare a sufficient volume of a concentrated solution of the SCA in deionized water.
8.4 Pipette the required volume of SCA into the mixture of coolant concentrate and water. The sequence of mixing must be as follows: coolant concentrate, water, SCA solution. Cap with a solid rubber stopper and agitate thoroughly.
NOTE 3 - Glycol and water mixtures exhibit a volume contraction due to the partial molal volume effect. The final volume of the mixture should be less than 100 mL, as indicated in Fig. 1.
8.5 In a similar manner, add 60 mL of reference coolant concentrate, 34 mL of deionized water, and 6 mL of reference SCA solution to a second 100-mL centrifuge tube, and agitate thoroughly. The reference coolant must be used within 30 days of preparation. Discard and prepare a new reference if any insoluble material is observed.
8.6 Replace the rubber stoppers with clean air condensers prepared by inserting a 300-mm (12-in.) length of glass tubing through a properly sized one-hole stopper.
8.7 Insert a length of dry Nichrome or stainless steel wire into the condenser past the bottom of the condenser tube. Immerse the centrifuge tube to the level of the solution in a constant temperature bath at 88°C (190°F) for 24 h (see Fig. 1).
NOTE 4 - The purpose of the wire is to provide a means of directing condensate back to the centrifuge tube.
8.8 At the end of the heating period, remove the centrifuge tube and solution from the bath, and cool to room temperature (allow at least 1 h). Remove the air condenser stopper, and replace with a solid rubber stopper. Using a two-pan balance, balance the centrifuge tube, stopper, and sample against a centrifuge tube (with stopper) containing any suitable liquid.
NOTE 5 - The centrifuge tube may be balanced against another centrifuge tube containing another sample. A few drops of a solution of 60 % reagent grade ethylene glycol and 40 % deionized water may be added to one of the tubes to equalize the mass of the tubes.
8.9 Centrifuge for 30 min at 500 rcf. Calculate the speed of the centrifugal rotation in r/min as follows:
r/min = 299√rcf/r
where:
r = radius in centimetres of the circle of rotation described by the tips of the tubes when in a rotating position, and
rcf = 500.
The following may be used as a simple means of checking the calculation:
NOTE 6 - The following equation may be used when r is measured in inches:
r/min = 188√rcf/r
NOTE 7 - When operating a centrifuge, tubes must be balanced properly and used in pairs to distribute the centrifugal forces evenly.
8.10 At the end of the centrifuging, stop the centrifuge and immediately read the volume of solids in the bottom of the tube, estimating to the nearest part of a millilitre as appropriate.
8.11 Clean the centrifuge tubes using the following procedure:
8.11.1 Shake the tubes to loosen the insoluble materials, and discard liquid and insolubles into a suitable container. Fill the tubes partially with tap water, and repeat as necessary.
NOTE 8 - The insolubles may be loosened with the aid of an ultrasonic cleaner.
8.11.2 Rinse the inside of the tubes with 1:1 HCl solution, and discard washings.
8.11.3 Clean the tubes with a solution of detergent and water, brushing as needed.
8.11.4 Rinse the tubes three times with warm tap water.
8.11.5 Rinse once with acetone or isopropyl alcohol to remove water, and air dry.
9. Calculation or Interpretation of Results
9.1 The tendency to form insolubles for the SCA-coolant concentrate combination under evaluation is expressed in relation to the tendency to form insolubles for the reference SCA in a diluted reference coolant concentrate.
9.2 Calculate the ratio of the volumes of insolubles formed as follows:
ratio = A/B
where:
A = volume of insolubles formed in the test solution, and
B = volume of insolubles formed in the reference solution.