1、Designation: D7961 15Standard Practice forCalibrating U-tube Density Cells over Large Ranges ofTemperature and Pressure1This standard is issued under the fixed designation D7961; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the
2、year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice outlines procedures for the calibration ofU-tube density cells. It is applicable to instruments
3、capable ofdetermining fluid density at temperatures in the range 10 Cto 200 C and pressures from just greater than the saturationpressure to 140 MPa. The practice refers to density cells asthey are utilized to make measurements of fluids primarily inthe compressed-liquid state. Examples of substance
4、s for whichthe density can be determined with a calibrated U-tube densitymeter include: crude oils, gasoline and gasoline-oxygenateblends, diesel and jet fuels, hydraulic fluids, and lubricatingoils.1.2 This practice specifies a procedure for the determinationof the expanded uncertainty of the densi
5、ty measurement.1.3 This practice pertains to fluids with viscosities 1 Pas(1000 centipoise) at ambient conditions.1.4 4 The values listed in SI units are regarded as thestandard, unless otherwise stated. The SI unit for mass densityis kilograms per cubic metre (kgm-3) and can be given asgrams per cu
6、bic centimetre (gcm-3).1.5 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior
7、 to use.2. Referenced Documents2.1 ASTM Standards:2D1193 Specification for Reagent WaterD4052 Test Method for Density, Relative Density, and APIGravity of Liquids by Digital Density MeterD5002 Test Method for Density and Relative Density ofCrude Oils by Digital Density AnalyzerD6792 Practice for Qua
8、lity System in Petroleum Productsand Lubricants Testing LaboratoriesD7483 Test Method for Determination of Dynamic Viscosityand Derived Kinematic Viscosity of Liquids by Oscillat-ing Piston ViscometerD7578 Guide for Calibration Requirements for ElementalAnalysis of Petroleum Products and LubricantsD
9、7740 Practice for Optimization, Calibration, and Valida-tion of Atomic Absorption Spectrometry for Metal Analy-sis of Petroleum Products and Lubricants3. Terminology3.1 Definitions:3.1.1 calibration, nset of operations that establishes therelationship between the reference density of standards and t
10、hecorresponding density reading of the instrument. D40523.1.2 certified reference material (CRM), nreference ma-terial one or more of whose property values are certified by atechnically valid procedure, accompanied by a traceable cer-tificate or other documentation that is issued by a certifyingbody
11、. D02.94 D6792, D02.03 D75783.1.3 density (), nmass per unit volume at a specifiedtemperature. D02.07 D74833.1.4 standard reference material (SRM), ntrademark forreference materials certified by NIST. D02.03 D77404. Summary of Practice4.1 This practice details the considerations and proceduresnecess
12、ary in order to complete a calibration of a U-tube densitymeter and an associated uncertainty analysis. The principalobjective of this practice is to provide the user with informationas to how different aspects of the calibration procedurecontribute to the overall uncertainty of related density mea-
13、surements obtained with a U-tube density meter.5. Significance and Use5.1 This practice covers a series of methods offered to aidusers in calibrating U-tube density meters to provide a measureof density and an associated expanded uncertainty. The refer-ence density, as obtained from either an equati
14、on of state(EOS) or CRM has an uncertainty that arises from the1This practice is under the jurisdiction of ASTM Committee D02 on PetroleumProducts, Liquid Fuels, and Lubricants and is the direct responsibility of Subcom-mittee D02.04 on Hydrocarbon Analysis.Current edition approved Dec. 1, 2015. Pub
15、lished February 2016. DOI: 10.1520/D7961-15.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.Copyright ASTM In
16、ternational, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States1uncertainty of the measurements of temperature, pressure, andalso the chemical purity of the substance studied (origin) or forthat matter of the certified reference material. This uncertaintyresults in a
17、n additional uncertainty for the density of thesesamples. Because the measurements made with U-tube densitymeters are not absolute, the uncertainty with which theinstrument calibration is determined is directly related to theuncertainty of the density obtained.6. Apparatus6.1 This practice is applic
18、able to U-tube density meterscapable of operating at temperatures of 10 C to 200 C andpressures to 140 MPa. Such instruments are commerciallyavailable and the measurement technique is well understood.Generally, the U-tube is electronically excited at a constantamplitude and a frequency meter is used
19、 to record thefrequency of the oscillation of the U-tube. This frequency isdependent upon the density of the fluid in the U-tube. As thetechnique is not a direct measurement of density the instrumentis calibrated with a fluid or fluids, the densities of which areknown accurately over a range of temp
20、erature and pressure. Acorrelation of density can then be formulated based upontemperature, pressure, and period of oscillation of the U-tubecollected during the calibration process.6.2 Additional equipment that is mandatory to carry out theprocedures described in this practice includes:6.2.1 Vacuum
21、 pump;6.2.2 Reagents to be used as calibration fluids;6.2.3 Reagents for cleaning the system; and6.2.4 Sample containers; stainless steel cylinders or glassflasks that can be evacuated.6.3 Additional equipment recommended for adherence tothis practice includes:6.3.1 High-accuracy temperature measure
22、ment device witha calibration traceable to a national metrology institute (NMI);6.3.2 High-accuracy pressure measurement device with acalibration traceable to a national metrology institute (NMI);and6.3.3 Computer and software for automated data acquisi-tion.7. Reagents7.1 The best calibration fluid
23、s are those referred to asCertified Reference Materials (CRM) which have correlationsfor density over the temperature and pressure range wheremeasurements will be made. If CRMs are not available, obtainfluids or gases in the highest purity available, preferably witha mole fraction greater than 0.999
24、 as cited by the manufactureranalysis.7.2 Water shall conform to Specification D1193 Type II orbetter. Recommended fluids, depending on the measuringrange (pressure, p, density, , and temperature, T) to be coveredin the calibration of the U-tube density meter include, but arenot limited to: water, m
25、ethane, ethane, propane, butane,nitrogen, methylbenzene decane, and dichlorotoluene. Equa-tions of state exist for all of these fluids and can be found inNIST Standard Reference Database 23 or in the references forthe specific fluid equations found in the reference section ofthis document. (1-9)37.3
26、 Recommended reagents for cleaning the instrumentinclude, but are not limited to methylbenzene, ethanol, acetone,white spirit, and quinoline.8. Calibration Procedure8.1 Choose one or more calibration fluids from those listedin Section 7. The fluids shall meet the following criteria:8.1.1 The fluid i
27、s readily available in high purity (molefractions greater than 0.999 as cited by manufacturer analysis).8.1.2 The fluid(s) selected for the calibration shall havepressure, density, temperature, (p, , T) surfaces which boundthose of the fluids to be studied. As several of the suggestedfluids have a s
28、omewhat limited density range in the tempera-ture and pressure boundaries of the U-tube instruments, it isoften useful to use two fluids in addition to vacuum for thecalibration in order for a larger p, , T surface to be covered bythe calibration equation. A good example of this is to usepropane and
29、 toluene. In contrast, selecting just one calibrationfluid which has a small p, , T surface (that is, water) greatlylimits the density range that can be determined from thecalibration equation.8.1.3 The fluid is well described by an equation of state, andthe uncertainty associated with density predi
30、ctions from thatequation is less than the desired overall uncertainty for densitymeasurements resulting from the calibration.8.2 Decide which calibration scheme is suitable for themeasurements.8.2.1 There are two schema that can be used to calibrate aU-tube density meter, and these are as follow:8.2
31、.1.1 Determine a calibration equation through the fit ofcalibration data measured over a range of temperatures andpressures; and8.2.1.2 Calibrate the instrument at a single point (deter-mined by a pressure and temperature) where measurementswill be performed. This method minimizes the influences ofp
32、ressure and temperature on the calibration that is performed atthe same temperature and pressure.8.3 Prepare the Sample(s):8.3.1 Degas samples prior to measurement. Those withboiling points below ambient temperature are ideally housed instainless steel cylinders and degassed through vacuum subli-mat
33、ion; at least three cycles of freezing with liquid nitrogen,evacuation, thawing, and then either heating or ultrasound.Liquid samples can be degassed by vacuum distillation into aglass flask (with a valve of PTFE (Teflon) or some other inertmaterial) or degassed in the glass flask by the previouslym
34、entioned procedure of freezing and evacuation.8.4 Clean the Instrument:8.4.1 Prior to beginning measurements, and after a samplehas been measured, the instrument is cleaned. This is done byfilling the measuring system with a cleaning agent. The best3The boldface numbers in parentheses refer to the l
35、ist of references at the end ofthis standard.D7961 152cleaning agents are those which will readily dissolve theresidue of the most recently measured sample. Examples ofcommon cleaning agents include methylbenzene, ethanol,acetone, white spirit, and quinoline. Take caution to use onlycleaning agents
36、that have normal boiling point temperaturesabove that of room temperature.8.4.2 When the system is thought to be clean, remove thecleaning agent by flushing the system with a more volatilesolvent. Then use a stream of dry air or dry gas to dry thesystem. Evacuate the system and set the temperature t
37、o one atwhich you have previously measured the period of oscillationunder vacuum. Compare past and present results. If the currentFIG. 1 Flow Chart of How to Choose a Calibration FluidD7961 153period of oscillation is greater than the past, further cleaningmay be necessary. The period of oscillation
38、 may shift particu-larly in a new instrument and especially as it is cycled overlarge ranges of temperature and pressure. Thus, an increase inthe period of oscillation is not necessarily indicative of theinstrument needing further cleaning however, this is recom-mended. If, after a second cleaning t
39、he period of oscillationremains high (greater than 0.02 s above the previous reading),it is likely the period has shifted and no further cleaning isnecessary. Once the instrument is clean, fill the system with thenext sample to be measured.8.5 Establish Convergence Criteria:8.5.1 Determine Instrumen
40、t Temperature Stability:8.5.1.1 The temperature and pressure stability (and as aresult the period of oscillation of the U-tube) is greatlydependent upon the mechanisms for temperature and pressurecontrol put in place by the user. As density is more strongly afunction of temperature than pressure, po
41、or temperature con-trol will have a more adverse effect on measurement repeat-ability and thus uncertainty. As such, temperature is commonlythe first of the convergence criteria that is met. Tests areconducted to determine the temperature stability of the instru-ment. These tests can be run with the
42、 system evacuated or filledwith a fluid. Set the system to a desired temperature. Once thegiven temperature setpoint has been reached, record at least 2 hof temperature data at regular intervals of 1 min or less. Use astatistical test such as the Mean Square Successive Difference(MSSD) technique (10
43、) to make certain there is no overallupward or downward trend in the data. If there is a significanttrend, thermal equilibrium most likely had not been reachedwhen the measurements were made, and this should be noted.It can take several additional hours after the setpoint has beenachieved for the in
44、strument to reach thermal equilibrium. Thisis particularly true at temperatures greater than 50 K aboveambient, or below ambient temperature. Calculate the standarddeviation of the temperatures measured. This is the stability ofthat temperature setpoint and can be used as the convergencecriteria for
45、 temperature. Repeat this procedure at 10 K to 20 Kintervals throughout the temperature range in which measure-ments will be conducted.8.5.1.2 During measurements, periodically calculate thestandard deviation of, at a minimum, the ten most recentlyrecorded temperature measurements. When the standard
46、 devia-tion is less than or equal to the convergence criteria, theinstrument can be considered to be at thermal equilibrium.8.5.2 Establish Pressure and Period of Oscillation Conver-gence Criteria:8.5.2.1 Convergence criteria for the pressure and period ofoscillation of the U-tube are determined in
47、much the same wayas that for the temperature. Data is recorded at regularintervals, that is, every 30 s for at least 2 h after the temperatureconvergence criteria has been met. Check to make certain thereis no overall upward or downward trend in that data. If there issuch a trend, equilibrium had mo
48、st likely not been reachedwhen the measurements were recorded. The standard deviationof at least ten consecutive measurements in that set can then beconsidered as the convergence criteria. During densitymeasurements, the system is considered to be at equilibriumwhen the standard deviations of the mo
49、st recent ten measure-ments of temperature, pressure, and period of oscillation of theU-tube, are all less than or equal to their convergence criteria.Data constituting the actual measurement is recorded for adesired amount of time. Examples of such measurementschema can be found in Outcalt and McLinden (11) andSegovia et al. (12).8.6 Conduct Calibration at a Single Point:8.6.1 This procedure is applicable for the operation of theU-tube density meter at a single temperature and pressure andis not to be used for a wider range of temperature and pressure.If
