ASTM D2624 method for electrical conductivity of aviation and distillate fuels
PORTABLE METER METHOD
6. Apparatus
6.1 Conductivity Cell and Current-Measuring Apparatus - Because hydrocarbon conductivities are extremely low compared to aqueous solutions, special equipment that is capable of giving an almost instantaneous response with application of voltage is needed.

6.2 Thermometer, having a suitable range for measuring fuel temperature in the field. A thermometer holder should be available so that the temperature can be directly determined for fuel in bulk storage, rail tank cars, and trucks.

NOTE 1 - The Emcee Model 1153 and D-2 Inc. Model JF-1A-HH measures and stores the sample temperature during the test cycle.

6.3 Measuring Vessel - Any suitable vessel capable of holding sufficient fuel to cover the electrodes of the conductivity cell.

7. Reagents and Materials
7.1 Cleaning Solvents - Use isopropyl alcohol (Warning - Flammable) if water is suspected followed by analytical grade toluene (Warning - Flammable. Vapor harmful).

7.1.1 A mixture of 50 % volume analytical grade isopropanol and 50 % volume analytical grade heptane (Warning - Flammable. Vapor harmful) is a satisfactory substitute for toluene.

8. Sampling
8.1 Fuel conductivity measurements should be made in situ or at the point of sampling to avoid changes during sample shipment. If it is necessary to take samples for subsequent analysis, the following precautions should be taken:
8.1.1 If the cell is in contact with water and the instrument is switched on, an immediate offscale reading will be obtained. If the cell has been in contact with water, it shall be thoroughly rinsed with cleaning solvent, preferably isopropyl alcohol, and dried with a stream of air. In hot, humid conditions, condensation on the cell can occur, which can cause abnormally high zero, calibration and sample readings. This can be avoided by storing the cell at a temperature 2 to 5°C in excess of the maximum ambient temperature where this is practicable.

8.2 The sample size should be as large as practicable (see 6.3).

8.3 The conductivity of fuels containing static dissipator additives is affected by sunlight and other strong light sources. Samples in clear glass containers can experience significant conductivity loss within 5 min of sunlight exposure. See Practice D4306 for further discussion.

NOTE 2 - Test method results are known to be sensitive to trace contamination from sampling containers. For recommended sampling containers refer to Practice D4306.

8.4 Prior to taking the samples, all sample containers, including caps, shall be rinsed at least three times with the fuel under test. Used containers should be thoroughly cleaned with cleaning solvent, if necessary, in accordance with D4306, paragraph 6.6, and air dried.

8.5 Conductivity measurements should be made as soon as possible after sampling and preferably within 24 h.

9. Cleaning Procedures
9.1 If the cell is in contact with water and the instrument is switched on, an immediate offscale reading will be obtained. If the cell has been in contact with water, it shall be thoroughly rinsed with cleaning solvent, preferably isopropyl alcohol, and dried with a stream of air. The meter may display a non-zero reading caused by condensation forming on the cell when the meter is taken from a cool, dry environment and subjected to hot, humid conditions. This condition can be avoided by storing the cell at a temperature 2 to 5°C in excess of the ambient temperature, when practicable.

9.2 In normal use, the probe on handheld instruments should be cleaned with toluene or a mixture of heptane and isopropanol and air-dried after use, to ensure that ionic materials absorbed on the probe during previous tests will not contaminate the sample and give an erroneous result.

10. Calibration
10.1 The calibration procedure will be dependent upon the equipment used. The procedures for the instruments listed in Footnote 3 are described in Annex A1-Annex A7.

11. Procedure
11.1 The specific instrument calibration procedures detailed in Annex A1-Annex A5 are an essential part of the following generalized procedures. The appropriate calibration steps for the instrument used should be followed prior to commencing the subsequent procedures.

11.2 In Situ Field Measurement on Tanks, Tank Cars, Tank Trucks, etc. - For field measurements the conductivity meters referred to in Footnote 3 are considered suitable. The use of these meters in hazardous locations may be restricted by the regulatory agency having jurisdiction. The EMCEE 1152 and Malik MLA 900 have an extension cable or can be equipped with one to lower the cell into the tank. High impedance hand held meters are susceptible to electrical transients caused by extension cable flexing during measurements. Failure to hold the apparatus steady during measurement can result in significantly poorer precision than shown in Table 1. The following instructions apply to the meters referenced in Footnote 3.

11.2.1 Check meter calibration as detailed in Annex A1, Annex A2, Annex A4, Annex A5, or Annex A7, depending on the meter used. Bond the meter to the tank and lower the conductivity cell into the tank to the desired level taking care to avoid partial immersion or contact with tank water bottoms, if present. Move the conductivity cell in an up-and-down motion to remove previous fuel residues. (Warning - To prevent static discharge between a charged fuel and a conductive probe inserted into a tank, the appropriate safety precautions of bonding and waiting for charge dissipation should be observed. For example, the American Petroleum Institute in RP 2003 recommends that a 30-min interval be allowed after pumping into a storage tank before an operator mounts a tank to insert a sampling device. This will also ensure that the fuel is electrically at rest.)

11.2.2 After flushing the cell, hold it steady and after activating the instrument record the highest reading after initial stabilization. This should occur within 3 s. On instruments with more than one scale range, select the scale that gives the greatest sensitivity for the conductivity value being determined. Ensure that the appropriate scale multiplying factor (or scale range) is used. Record the fuel temperature.

NOTE 3 - The Emcee Model 1153 automatically measures and records the reading at 3 s. The D-2 Model JF-1A-HH Samples 10 times upon activation, allow the center bar indicator on the display to come to center which indicates the current reading has repeated, once repeated press the sample button again to display the conductivity, temperature data and store the data to the instruments memory.

11.3 Laboratory and Field Measurements on Sampled Fuels:
11.3.1 Preparation of Containers (Metal or Glass) - Prior to taking samples, take extreme care to ensure that all containers and measuring vessels have been thoroughly cleaned. It is preferable that containers are laboratory cleaned prior to shipment to the field for sampling (see Section 8).

11.3.2 Measurement - Rinse the conductivity cell thoroughly with the fuel under test to remove fuel residues remaining on the cell from previous tests. Transfer the fuel to the measuring vessel and record the conductivity of the fuel using the procedure applicable to the particular apparatus. If one of the conductivity meters referenced in Footnote 3 is used, follow these instructions: Rinse the cell concurrently with the rinsing of the measuring vessel. Then transfer the sample to be tested to the clean, rinsed measuring vessel. Check meter calibration as detailed in Annex A1, Annex A2, Annex A5, or Annex A7, depending on the meter used. Fully immerse the conductivity cell into the test fuel and measure the conductivity following the procedure in 11.2.2 and the appropriate Annex. Record the fuel temperature.

NOTE 4 - In order to avoid erroneous readings, it is important to ensure that the bottom of the conductivity cell does not touch the sample container. This is applicable to all containers, whatever the material of construction.

NOTE 5 - When using an analog meter, measurements exceeding the range of the meter are obvious. With the Emcee Model 1152 Digital Meter and the Maihak MLA 900 Meter, measurements exceeding the range of the meter are indicated by a single digit "1" in the left side of the display where 1000s are shown. The D-2 Model JF-1A reports to the display the text, "Reading Out of Range". A qualitative conductivity estimate (for which precision has not been established) can be made by inserting the probe in the sample to the first set of holes closest to the tip, which are at the mid point of the sensing portion of the probe. Since the displayed conductivity is inversely proportional to the depth of immersion, the value displayed, if any, should be doubled. Conductivities less than 1 pS/m up to 20 000 pS/m can be determined using Test Method D4308. When using the Emcee Model 1153 Digital Meter, measurements exceeding the range of the meter "OVER" will be displayed.

12. Report
12.1 Report the electrical conductivity of the fuel and the fuel temperature at which measurement was made. If the electrical conductivity reads zero on the meter, report less than 1 pS/m.

NOTE 6 - It is recognized that the electrical conductivity of a fuel varies significantly with temperature and that the relationship differs for various types of aviation and distillate fuel. If it is necessary to correct conductivity readings to a particular temperature, each laboratory would have to establish this relationship for the fuels and temperature range of interest. Refer to Appendix X2 for additional information of the effect temperature has on the electrical conductivity of fuels.

13. Precision and Bias
13.1 The precision of this test method as determined by statistical analysis of test results obtained by operator-instrument pairs at a common test site is as follows. The precision data generated for Table 1 did not include any gasolines or solvents. The precision data given in Table 1 are presented in Fig. 1 for ease of use.

NOTE 7 - A precision program is being considered to develop a single precision statement for all portable meters.

13.1.1 Repeatability - The difference between successive measured conductivity values obtained by the same operator with the same apparatus under constant operating conditions on identical test material at the same fuel temperature would, in the long run, in the normal and correct operation of the test method, exceed the values in Table 1 only in one case in twenty.

13.1.2 Reproducibility - The difference between two single and independent measurements of conductivity obtained by different operators working at the same location (13.2) on identical test material at the same fuel temperature would, in the long run, in the normal and correct operation of the test method, exceed the values in Table 1 only in one case in twenty.

13.2 In 1987, a test program was carried out to investigate reproducibility of results when samples are shipped between laboratories. (See Appendix X1.) While repeatability values were similar to those in Table 1, it was concluded that adequate reproducibility values were not obtained due to changes in conductivity of samples during shipment and storage. In the event of dispute or concern regarding shipped sample conductivity, it is recommended that operators come to the bulk fuel storage site to measure conductivity on bulk fuel or on freshly obtained samples according to cited procedures. This assures that a sample identical to the bulk supply is tested by either or both parties and the precision data shown in Table 1 shall apply.

13.3 The Maihak MLA 900 Emcee Model 1153, and meters provide a sample temperature measurement. Precision of the Maihak MLA 900 is shown in Table 2. Precision of the D-2 Inc. Model JF-1A-HH is shown in Table 3.

13.4 Bias - Since there is no accepted reference material or test method for determining the bias of the procedure in Test Methods D2624 for measuring electrical conductivity, bias cannot be determined.