WEIGHT EXPANSION MEASUREMENT SYSTEM
1.0 The weight expansion measurement system (WEMS) incorporates the methodology of volumetric expansion by measuring the weight (rather than the volume) of water displaced in a volumetric expansion test. The WEMS uses a siphon tube instead of an open top burette tube to measure the weight of the water displaced. The WEMS is based upon the scientific correlation between the volume of water and the weight of water: 1 cubic centimeter of water weighs 1 gram of water (at 4 degrees Celsius). Therefore the WEMS simply measures the weight of water displaced by using a weigh container on a digital weigh scale or load cell, and then determines the expansion of water displaced.
1.1 The weight expansion measurement system has certain practical advantages over the burette system. Most importantly it eliminates any possibility for operator interpretation error during burette meniscus expansion readings. Second, the expansion volumes can easily be determined with great precision and then communicated to operators or to computer-controlled automated testing systems. In addition, WEMS eliminates the need for fragile glass burette tubes and plumbing to multiple burette tubes.
1.2 However, there are several factors that must be addressed to ensure the accuracy of the WEMS: temperature, the buoyancy effect of the siphon tube, the effect of water column height, and the accuracy of the weight measurement device. All of these factors may affect the accuracy of the weight expansion readings unless adjustments are made to account for their impact on the readings.
1.3 The effects of water column height change and buoyancy of the siphon tube can be easily determined by pouring a known quantity of water into the expansion measurement container when the device is open to a test jacket. The weight measurement device can be easily tested and verified by using a calibrated reference weight to determine its accuracy. Water density changes due to temperature can be corrected mathematically using known water density values at various temperatures (See Appendix B).
1.4 In a hydrostatic testing system, the above factors can operate inversely such that the aggregate impact of all effects combined has a negligible impact on the accuracy of the expansion values derived. Various tests have been conducted to verify that such combined effects do have a very minute effect on expansion values at ambient temperatures around 22 Centigrade.
1.5 The accuracy of a weight measurement device is tested by placing a calibrated reference weight upon it to verify its accuracy. Next, 100 cubic centimeters of water at ambient temperature is weighed into a container using another calibrated digital scale with a resulting weight of 99.7 grams (see Appendix B for resulting weights at different temperatures). The 100 cubic centimeters of water are then poured into a 3 liter square expansion container that rests on the expansion measuring weigh scale. Expansion weight values are then recorded. The process is repeated in 100 cc increments with expansion weight measurements being recorded respectively at each 100 cc volume level.
The following expansion testing data and graphs clearly portray the minute and negligible impact resulting from the combined effects of temperature, buoyancy, and water column height.
Greetings from Galiso Incorporated
This letter pertains to the recent changes to the calibration values for your calibrated cylinders. Galiso technicians have found this to be necessary on many Recortest 4 installations and upgrades. The reasons that the cylinder values are no longer correct are thinning of the cylinder walls due to corrosion and or incorrect previous calibration.
Calibrating a cylinder on site with the installed / upgraded machine requires a check of each of the 4 calibration accuracy issues to establish that the cylinder calibration values that are done on site are accurate. When there is a discrepancy in prior calibration values to present ones, a check of each of these four calibration accuracy issues is important to prove the new calibration data is correct.
These issues are:
Accuracy of pressure measuring device . . . verified with master gauge.
Accuracy of expansion measuring device . . . verified with test weights
Temperature . . . measured to determine the weight of water during calibration
Buoyancy . . . checked with the a pour test and adjusted for temperature.
Regarding issue 1, the Recortest 4 uses a digital pressure transducer that is a high accuracy, traceable pressure standard suitable for calibration of cylinders. Because the .02% accuracy of this transducer is better than the .25% accuracy of the master gauge the master gauge is used to confirm that the transducer is functioning properly but we do not calibrate the digital transducer to the gauge. This we believe is a major factor in the need for us to re calibrate cylinders during Recortest 4 installations and upgrades. Prior to the installation / upgrade it was possible to adjust the pressure transducer to match the expansion values. We also verify that the pressure circuit comprised of the high pressure pump, valves and tubing, hoses, test head, and cylinder are not leaking.
Regarding issue 2, in most cases the load cell weigh bowl assembly is replaced by the digital Recortest 4 weigh bowl scale. The accuracy of this digital scale has proven to be very stable over time and eliminates the need for frequent calibration.
Regarding issue 3, When the ambient temperature is 4 degrees c / 39 degrees f, one cc of water weighs 1 gram but at 40c/104f room temperature, one cc of water weighs .992 grams which is a accuracy loss of negative .8%. By determining the ambient temperature we can correct its effect on the weight of water when we perform a pour test to verify the accuracy and leak integrity of the complete expansion circuit comprised of the weigh bowl, siphon tube, plumbing and valves, test jacket, test head and cylinder.
Regarding issue 4, We determine the effect of siphon tube buoyancy on the system that will be used to create calibration cylinder values. To do this we open the expansion circuit to the weigh bowl with the cylinder to be calibrated in the test jacket. Then we pour 100cc of water into a cup on a separate scale by using the weight of water at ambient temperature. For example if the ambient temperature is 20 degrees c / 68 degrees f, then we would pour 99.8 grams of water into the cup. Next we pour the water out of the cup into the weighbowl of the Recortest 4 that is connected to the test jacket with the calibrated cylinder. This test is performed several times to verify that the complete expansion circuit is accurate and stable.
We then run repeated tests at each calibration pressure to gather the expansion data for each pressure point. Finally we check the linearity of all calibration points and then create a certificate of expansion values for the calibrated cylinder. If you have any further questions about this work please feel free to contact our service team anytime.
Thankfully yours – The Galiso team
22 Ponderosa Ct.
Montrose, CO., 81401
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