ASTM D3240 Test Method for Undissolved Water In Aviation Turbine Fuels
7. Calibration
7.1 The calibrating standard corresponding to a given undissolved water content is placed in the test pad window. Turn on the lamp and press the photocell comparator button. Zero the photocell comparator by adjusting the light modulating lever until there is a steady reading for 10 to 15 s. This may take a minute or longer. Always move the light modulating lever in the same direction when zeroing the photocell comparator in calibration or testing to eliminate errors caused by play in the lever. If the reading obtained does not agree with the calibration pad rating, adjust the photocell comparator by removing the plug screw on the side of the photocell comparator at the 45° bend. Insert a small screwdriver, and adjust as necessary. Repeat the above procedure until the rating obtained agrees with the calibration pad rating. The instrument should be calibrated prior to use each day, after each hour of use, and when the ultra violet light battery is replaced or recharged.
8. Rating Procedure
8.1 Remove the test pad from the sampling holder using tweezers and press between dry paper blotters or absorbent towels to remove excess fuel. To blot, press firmly (about 5 lb force) 3 or 4 times with heel of hand, moving the pad with tweezers each time.
8.1.1 For maximum accuracy the test pad should be read within 3 min after sampling is initiated. If the test pad is not read immediately after sampling, it must be placed in a desiccator to prevent moisture pickup from the air. However, ratings made on dried test pads, that is, not damp with fuel, should be avoided if possible since they will give high and erroneous results. Conversely, rating a pad which has not been well blotted will give a low reading as the excess fuel will absorb part of the UV light and decrease fluorescence.
8.2 Using tweezers, put the pad into the pad rating device. Turn on the lamp and press the photocell button. Continuously zero the photo-cell comparator by adjusting the light modulating lever until there is a steady reading for 10 to 15 s. Always move the light modulating lever in the same direction when zeroing the photocell comparator to eliminate errors caused by backlash. This may take a minute or longer. Turn off the instrument light immediately after use to conserve battery power.
8.2.1 The ultraviolet light must be on to get a reading. This can be checked by moving the light modulating lever from one extreme to another. The photo-cell comparator meter needle should swing violently when this is done.
8.3 Record the instrument reading (estimated to the nearest tenth) and the sample volume. Record the temperature of the fuel.
8.4 Visual observation of the used test pad is often of value. This is easily accomplished by removing the photocell comparator assembly and viewing the test pad. Free water in fuel is normally well dispersed and will form an even distribution of many, small points of yellow fluorescence on the test pad when illuminated by ultra violet light. Large water drops, however, will form a few, large spots of yellow fluorescence on the pad. These large spots may give erroneous readings; the result should be disregarded and a new sample tested.
9. Calculation
9.1 The instrument reads directly if the sample volume is 500 mL. Correct the readings for other sample volume by multiplying reading by 500/sample size in millilitres:
Free water, ppm = (meterreading, ppm)(500)/(sample volume, mL)
