NAT'L INST. OF STAND & TECH

NBS

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A111DS ^333^ 1 \

*<"*au o* * NBS TECHNICAL NOTE 1055

U.S. DEPARTMENT OF COMMERCE / National Bureau of Standards
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards' was established by an act of Congress on March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and technology

and facilitate their effective application for public benefit. To this end, the Bureau conducts

research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau's technical work is per- formed by the National Measurement Laboratory, the National Engineering Laboratory, and

the Institute for Computer Sciences and Technology.

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physical and chemical and materials measurement; coordinates the system with measurement

systems of other nations and furnishes essential services leading to accurate and uniform

physical and chemical measurement throughout the Nation's scientific community, industry,

and commerce; conducts materials research leading to improved methods of measurement,

standards, and data on the properties of materials needed by industry, commerce, educational

institutions, and Government; provides advisory and research services to other Government

agencies; develops, produces, and distributes Standard Reference Materials; and provides

calibration services. The Laboratory consists of the following centers:

Absolute Physical Quantities2 — Radiation Research — Chemical Physics —

Analytical Chemistry — Materials Science

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provides engineering measurement traceability services; develops test methods and proposes

engineering standards and code changes; develops and proposes new engineering practices;

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guidelines, and managing Federal participation in ADP voluntary standardization activities; provides scientific and technological advisory services and assistance to Federal agencies; and

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The Institute consists of the following centers:

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'Headquarters and Laboratories at Gaithersburg, MD, unless otherwise noted; mailing address Washington, DC 20234. 'Some divisions within the center are located at Boulder, CO 80303.
National Bufeao of Stand"*

SEP 3 1S82

Tests of Commercial Densimeters for

LNG Service

J. D. Siegwarth

J. F. LaBrecque

Thermophysical Properties Division

National Engineering Laboratory

National Bureau of Standards

Boulder, Colorado 80303

Sponsored by:

Gas Research Institute

8600 West Bryn Mawr Avenue

Chicago, Illinois 60631

**r«s<**

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued June 1982
i

National Bureau of Standards Technical Note 1055

Nat. Bur. Stand. (U.S.), Tech. Note 1055, 40 pages (June 1982)

CODEN: NBTNAE

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1982
CONTENTS

Page

1

.

INTRODUCTION 1

2. TEST METHOD 2

3. THE COMMERCIAL DENSIMETERS 4

4. DENSIMETER TEST RESULTS 5

4.1 Vibrating Cylinder Densimeter - First Kind 5

4.2 Vibrating Cylinder Densimeter - Second Kind 17

4.3 Vibrating Plate Densimeter 24

4.4 Displacement Densimeter 25

5

.

CONCLUSIONS 31

6. REFERENCES 33

m

Tests of Commercial Densimeters

for LNG Service

J. D. Siegwarth and J. F. LaBrecque

Densimeters for liquefied natural gas (LNG) from four

manufacturers were tested in liquid methane and an LNG-like

mixture of methane, propane, and nitrogen in the density

reference system (DRS). The calibration and performance of

one type tested for the first time are reported. The stability of the calibrations and performances of three densimeters of a type previously tested have been examined and are also reported here.

Key words: density; densimeter; liquefied natural gas; methane.

1. INTRODUCTION

Four different densimeter designs by four manufacturers were tested for use

in liquefied natural gas (LNG) in the density reference system [1,2] and the

results were presented in earlier reports [2,3]. The densimeters tested in this

first study were two vibrating types, a dielectric cell type and an Archimedes or displacement type. Though all proved capable of density measurement in cryogenic

liquids, all suffered from the same problem; they did not have a suitable

calibration for LNG service. Though the calibration of the displacement

densimeter tested was within specifications [2,3], subsequent tests showed the

calibration method applied to this densimeter inadequate.

To eliminate the calibration problem, NBS offered to provide transfer standards to the various manufacturers and users. The manufacturer or user would

choose the instrument he prefers for a standard and NBS would calibrate it against

the DRS densimeter. The DRS was rebuilt [4] to permit installation of different

densimeters, to improve its reliability and to decrease the time required to

acquire sufficient density data to calibrate the transfer standards. Later, the

densimeter in the DRS was also rebuilt. The complete rebuild has been described

in a new uncertainty statement [4].
2. TEST METHOD

The densimeters under test are compared to the DRS densimeter at a number of

temperatures between about 109 and 128 K along the saturation line in liquid

methane, occasionally in methane and nitrogen, and in LNG-like mixtures composed

of methane, propane, and nitrogen. The pressures have thus far been limited to

the range from 0.8 to 4 bars. The liquid temperature is changed by an electric

heater or a liquid-nitrogen cooling coil while the sample is stirred. Stirring

continues after the heating or cooling stops until the temperatures measured near

the top and bottom of the sample are stable and indicate the sample is nearly

isothermal. The temperature difference for most measurements was less than lOmK.

The stirrer is stopped and the instrument readings are recorded. These include

temperatures, pressure and the various densimeter outputs. The sample liquid is

stirred again and a second set of data is recorded. The sample temperature

usually increases perhaps by as much as one tenth degree between data sets

depending on the amount of stirring. Then the sample is heated or cooled to the

next chosen temperature. Normally, eight measurements at separate temperatures

are made. To obtain the mixture samples, the additional components were condensed into the methane sample.

The present DRS densimeter has an apparent uncertainty for one reading of

+0.055% at 425 kg/m^ density [3]. This is composed of a systematic uncertainty

of 0.022% and 3 times the estimated standard deviation of 0.011%. A schematic of

the rebuilt DRS is shown in figure 1 illustrating the relative positions of the

densimeters within the liquid sample. A more detailed description of the DRS is

contained in [3].

Because the two sets of data taken at one sample temperature setting are highly correlated, only one of the sets of data is used in the subsequent statistical analysis. Usually, no difference is observed in the statistical results

obtained from using only the first or only the second set of data in the analysis. This implies that sufficient time has passed to establish equilibrium con- ditions in the liquid before the data are recorded.
Electronic

Balance

Top Flange

Sample Inlet —*—

\

N 2 Vent—®

LN 2

Inlet

Reference Weight

Heat Exchanger

Vapor

Pressure Bulb

Sample Container

Silicon Crystal

Vapor

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N

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Radiation Shields

Dewar Wall

Cooling Coil

Radiation Shield

Cooling Coil

Sample

Cooling Coil

Liquid Sample

Densimeters

Under Test

Vacuum Jacket

Figure 1. Schematic diagram of the improved Density Reference System.

3
3. THE COMMERCIAL DENSIMETERS

Three of the densimeter types tested and reported earlier [2,3] have

undergone some additional testing. The three are the displacement densimeter,

the vibrating cylinder densimeter and the vibrating plate densimeter. A second

vibrating cylinder type has been tested for the first time.

3.1 Vibrating Element Densimeters

A vibrating element in these densimeters is locked in oscillation at one of

its lower resonant frequencies. When this harmonically vibrating element is

immersed in a fluid of variable density, the resonant frequency shifts with the

fluid density such that, in the ideal case,

p = A + Br2

where p is the fluid density, T is the period of vibration and A and B are con- stants. The constants A and B are derived from frequency measurements in fluids

of known density. Since temperature can affect the structural properties hence

the frequency of the vibrating element, such a densimeter should be calibrated

in the fluid or in fluids as near as possible in temperature and density to those

in which the instrument will be used.

3.2 Displacement or Archimedes Densimeter

This densimeter employs the Archimedes principle: an object immersed in a quiescent fluid experiences an upward force proportional to the mass of fluid it

displaces. If the mass and volume of the object are known, the density of the

fluid is determined by the relation

P

= P (M-Ma )/M . (2)

The density of the float p is given by M/V where M is the float mass in vacuum

and V is its volume. The quantity Ma is the apparent mass of the float when
completely immersed in the liquid whose density is being measured. These densimeters require calibration in fluids of known density to determine p and M for

the float since the volume and mass are not known. Also, the force balance

employed may not read mass directly. Some temperature effects exist also though

not included in the simple eq (2). The float density p is temperature

dependent as is Ma if the balance output is affected by temperature.

4. DENSIMETER TEST RESULTS

4.1 Vibrating Cylinder Densimeter -- First Kind

Three newer models of the vibrating cylinder densimeter reported in [3] were calibrated against the DRS densimeter for use as transfer standards. The

instruments were calibrated and returned to the manufacturer who uses them to

calibrate the densimeters he markets. These three densimeters are designated

below as numbers 1, 2, and 3. Numbers 1 and 2 were retested in the DRS about a year and a half later.

For the first tests, the data were acquired in four separate fillings of the

DRS. In the first filling, liquid methane was used as the sample liquid in which

the vibrating cylinder densimeter frequencies and the DRS densimeter densities

were recorded. Approximately 1% liquid nitrogen was then added and another

series of data taken. Finally, about 5% liquid propane was added and a third

series taken. Fillings 2, 3, and 4 were similar except that no separate series

of data were taken with a methane-nitrogen mixture. Densimeter No. 3 was not

tested in the first filling, but all three meters were inserted for subsequent

fillings.

The data for the first three fillings contained many values well outside the

bounds considered reasonable for normal deviation. All three densimeters showed

outlying data and not always on corresponding readings. This problem was apparently caused by bubbles trapped in the vicinity of the vibrating spool. When the

filter screens were removed from the densimeters for the fourth filling this

problem disappeared. The suspected outlying data points of the first three

fillings were not used in the data analysis.
The data were fit to eq (1) for each of the three densimeters under test for

the methane, methane plus nitrogen, and the LNG-like mixture. Figure 2 shows the

residuals from this fit for densimeter No. 1 versus temperature;, the pattern of

the points suggest at least a linear dependency on temperature. The methane and

the LNG-like mixture data cover essentially one temperature range but different

density ranges. A plot of the residuals versus density shows parallel lines

rather than a continuous line, indicating a predominant temperature effect rather

than a density effect. The behavior of the other two densimeters is similar.

The relationship that gives a better fit to the density data than eq (1) is:

density = A + B/f2 + CT (3)

where T is temperature in Kelvins, A, B, and C are constants, and f, the

frequency, is 1/t.

The residuals obtained from fitting the above relationship in f and T to the

data are shown versus chronological order in figures 3, 4, and 5 for densimeters

Nos. 1, 2, and 3, respectively. The different symbols represent different

fillings. Since the four sets of data taken for the four fillings represent

roughly replicated conditions, the mean level of the residuals from filling to

filling should be the same within the random uncertainty. To the extent this is

not the case, shifts in the mean level of the residuals may indicate shifts in

calibration. The shifts for meters No. 1 and No. 3 appear nonsystematic and may

be due to random effects. Meter No. 2, however, shows a decided drift downward

for the first three fillings. The filling-to-filling effects were removed before

estimating the values of A, B, and C in eq (3). The estimated coefficients for

eq (3) for the three densimeters are shown in table 1.
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Table 1. Coefficients for equation 3

A B C

Meter Constant SD Coef of r or 1/f2

SD Coef of T SD

5633.98 .85

5757.6 1.8

5635.9 1.8

No. 1 -189.46 .15

No. 2 -193.89 .32

No. 3 -192.88 .33

-.01737 .00071

-.0281 .0015

-.0150 .0016

These results apply to saturated methane or saturated methane + nitrogen

(1%) + propane (5%) from 109 K to 130 K.

These equations contain three uncertainties: the uncertainty in the

coefficients, the uncertainty because of instability in the frequency value, and

uncertainty because of shifting of the mean level of the meter. When possible,

these uncertainties are expressed in terms of standard deviations (SD). The

table below gives a summary of the estimated standard deviations. The shifting

for densimeters Nos. 1 and 3 is assumed random; however, for densimeter No. 2

such an assumption does not seem appropriate because for the first three fillings

the mean level of this meter drifts downward.

Table 2. Uncertainties in predicing density values

expressed in standard deviations (kg/m^)

SD 95% Upper SD for 95% Upper SD for Mean 95% Upper

Meter for Eg. Bound Measurement Bound Shift Bound

No. 1 .012 .014 .032 .038 .025 .062

No. 2 .025 .029 .067 .078 *

No. 3 .024 .030 .055 .069 .043 .102

*Meter drift nonrandom

The standard deviations in table 2 are estimates based on the data excluding

the outlying data points. The 95% upper bounds give a more conservative value

which takes into account the uncertainties in the data. The 95% upper bounds are

11
combined by squaring, adding, and taking the square root of the sum to obtain an overall standard deviation value. Three times this value is assumed to be the random

uncertainty for using the calibration equation. For densimeter No. 1 this is ±0.22

kg/m3 or ±0.05% based on 425 kg/m3 . For densimeter No. 3 this value is ±0.38

kg/m3 or ±0.09% based on 425 kg/m3 . No overall uncertainty has been calculated

for densimeter No. 2. An additional 0.022% should be added because of systematic

uncertainty in the DRS densimeter. The equations and uncertainties apply

specifically to measurement in the DRS. So, the equation for densimeter No. 1 predicts a value for density based on 1/f and T with an uncertainty of ±0.07%; and

the prediction for densimeter No. 3 has an uncertainty of ±0.11%.

In the fifth filling, the sample was liquid nitrogen from a storage tank.

The source liquid has an impurity level < 0.01% according to the supplier. Some

additional impurities may have been added by the time the sample has been placed

in the DRS. The accuracy of the DRS densimeter in LN2 has not been

established. No present evidence suggests, however, that DRS determined LN2

density is any less accurate than the methane density determination.

No prediction equation was estimated for liquid nitrogen, nor was the nitrogen data combined with the* previous data to estimate a new equation. However,

the above equations were used to predict the nitrogen densities based on the

observed 1/f and T from the nitrogen data. The extrapolated liquid nitrogen

densities disagree with measured densities by zero to -0.25% depending on the

densimeter and the temperature of the liquid nitrogen.

Both the shifts between fillings and the random scatter within fillings are smaller for these densimeters than the similar densimeter tested and reported earlier

[3]. No statistically significant difference is evident between the methane and LNG

results as was observed in the measurements reported in [3].

When densimeter numbers 1 and 2 were retested about a year and a half later,

the calibrations from the original tests were used to analyze the retest data.

The residuals for densimeter No. 1,

R = PDRS ~ (-189.46 + 5633. 98/f2 - .01737T),

12
are plotted against p^rs in figure 6. A different symbol is used for each

filling of the DRS. The residuals for the retest data are slightly higher than

those for the calibration data. They have a mean of 0.08 kg/m^. Three times

the upper bound of the standard deviation for mean shifts from table 1 is 0.19

kg/m3; so the 0.08 kg/m3 figure is well within this value. New calibration

coefficients based on the original calibration data and the retest data are:

Meter Constant SD Coef of 1/f? SD Coef of T SD

No. 1 -189.37 .11 5633.72 .66 -.01770 .00056

The new data showed negligible change in the coefficients.

The new upper bounds for the standard deviations (kg/m^) are:

Stand. Dev. 95% Upper SD for 95% Upper SD for 95% Upper

Meter for Eg. Bound Measurement Bound Mean Shift Bound

No. 1 .010 .012 .029 .034 .052 .109

The estimated SD for mean shift is about twice the value of the original

tests.

The residuals for densimeter No. 2,

R = PDRS " (-193.89 + 5737. 6/f 2 - .0281T), (5)

are plotted against pqrs in figure 7. These residuals show densimeter

No. 2 to be out of calibration. If the original calibration equation were used,

a -0.7 kg/m3 error at 425 kg/m3 and a 1.2 kg/m^ error at 460 kg/m3

results. Also the meter shows an increased sensitivity to the liquid in which

it is used. Refitting equation (3) to the retest data for densimeter No. 2 can produce the residuals shown in figure 8.

Meter No. 1 shows good stability and good accuracy. Meter No. 2 shows much

poorer stability and, consequently, poorer accuracy.

Based on the performance of meter No. 1, these results suggest that

selected meters can have good accuracy. Field testing is the next step.

Periodic recali brat ion is presently necessary to maintain accuracy at

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least until the stability of a number of meters has been established iji field use over long periods of time.

If the commercial densimeters are not to exceed the random uncertainties

reported here (for instance those of table 2), the filter screens must be removed

from the sensing unit. In subcooled liquid there may be no problem.

4.2 Vibrating Cylinder Densimeter - Second Kind

This densimeter has not been previously tested. The vibrating cylinder

consists of a tube forming the hole portion of a doughnut-shaped closed metal

case. This case, containing the driving and sensing elements, excludes the

liquid whose density is being measured from the outside wall of the vibrating

cylinder. This entire unit is immersed in the liquid, but can be supported by

bolting a flanged stem protruding from one side of the instrument to a support

stem passing through the liquid container wall. The electrical leads are contained within this stem. For the measurements reported here, the density

sensor was supported on its outer surface on a curved wood block. The axis of

the vibrating cylinder was horizontal with neither end near any solid object.

The manufacturer supplied a calibration for this instrument with correction

terms for the velocity of sound, pressure and temperature of the sample liquid.

The calibration equation provided is:

P b

= 2221.2363 + .010101521 x r 2 (kg/m 3

) (6)

where r is the period of oscillation in ps and p^ is the basic density. The

density, p^, corrected for the liquid velocity of sound is:

17
1 + 17273. 47A 2

,,.

Pi = Ph To ? 2

( 7 ) 1 D

1 + 3.38610 iU /r c/

where r is the same as above and C is the sound velocity of the liquid in m/s.

An appropriate expression for C is discussed below.

The density with the pressure effect correction is:

P 2

= P

X [1-6.77 x 10" 6

x AP] -0.0151 x aP (8)

where aP is the absolute operating pressure in bars less 1 bar. Finally, the

true density p is p 2 corrected for temperature:

p = P2

[1-1.38 x 10" 5 x AT] -0.08892 x AT (9)

where AT = (T - 23.14°C) and T is the liquid sample temperature in °C.

Since the densimeters were tested primarily in pure methane and an LNG like

mixture of methane, propane and nitrogen, appropriate velocities of sound for

both are needed. From the velocities of sound for methane given by Goodwin [5],

the approximate expression for C(T) is:

C(T) = -10.316 x T + 2490.4 (m/s) (10)

where T is temperature in Kelvins.

18
The LNG-like mixture consisted of methane with about 4 to 6% propane and 1 to 2% nitrogen. The concentrations vary both from filling to filling and with

temperature since nitrogen and methane preferentially leave the solution to fill

the vapor space as the liquid temperature is raised. Since velocities of

sound for liquid mixtures are not readily available, the sound velocity was estimated to be 0.95Cc + 0.05Cc3 at both 110 and 126 K, where Cq and Cc3 are the

methane and propane sound velocities respectively. Assuming a dependence linear

in T, the mixture sound velocity is:

C(T) = -10.44T + 2550 (m/s). (11)

The model used to analyze the density data is

Ap = p - pdrs

= p + a p DRS

+

0i + ei (12)

where p is the density determined by eq (9) for the vibrating cylinder

densimeter, pdrs> tne density determined by the DRS densimeter, y and a are the

constants of a linear fit, the oj are the offsets attributable to composition

differences in the case of the LNG like mixture and also to filling-to-filling

shifts in the densimeter calibrations, and the e-j are unspecified random errors.

This densimeter was tested in eight fillings of the DRS. For the first two

fillings, only the resonant frequency was recorded; for the remaining six, both

frequency and period were recorded on separate counters. The period reading had

a factor of 20 greater resolution than the frequency reading, an offset between

density determined by frequency and that determined from period was observed.

This offset corresponds to less than 0.1 Hz, the resolution of the frequency

counter for a 10 second counting interval, and the count always read low by

19
<0.1 Hz as would be expected for a resolution limitation. The period data were

used where available because of the increased resolution.

Figure 9 shows the Ap for methane versus DRS density (pdrs)* Symbols

1 through 8 represent the first through eighth fillings, respectively.

Offsets in the density data due to the frequency counter resolution is

apparent in the results of the first and second filling. Figure 10 shows Ap versus pdrs f° r tne LNG liquid mixture results. The pure methane velocity of

sound, eq (10) was used. Any offset in the density values for fillings 1 and 2

is completely masked by the much larger filling-to-filling shifts shown in figure

10. This tendency for the density data to lie on separate but parallel curves implies the presence of a liquid composition dependency in the densimeter.

Figures 9 and 10 both show a linear dependence of Ap on p and the slopes are similar. However, the Ap versus p curve for the liquid methane density is offset

from the average of the LNG density by approximately 12 kg/m3 at 430 kg/m3

when the liquid methane velocity of sound is used to calculate density. This

offset is reduced to about 8 kg/m3 by using eq (11) for C. The slope in the Ap versus p probably results from the increase in temperature, pressure, or both

that accompanies the decrease in density of the liquid sample. The offset

between the curve for the two different liquids must be caused by the composition

change, possibly through differences in velocity of sound.

Equation (12) was fit to the methane data excluding the first two fillings,

and the results are shown in the first line of table 5. We have assumed that the

ai are random shifts from filling-to-filling, and a standard deviation aa , has

been estimated for them. Combining the upper 99% confidence limit for aa

and the

upper 99% confidence limit for <%, we get (0.14 2 + 0.07 2

) 1 / 2 = 0.16 kg/m3 .

Three times this value represents the random uncertainty of a single density

measurement made using this densimeter in the DRS. Part of the random error is

due to the DRS densimeter, but this contribution is relatively small [3].

The equation was fit twice to the LNG data, excluding the first two fillings, once using the methane velocity of sound and once using the approximation

(eq (11) and fig. 10) for a 95% methane, -5% propane mixture; i.e., 95% velocity

20
O)c

ov«-

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«8^

in

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o m o o

. saq, oa, , e

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meter

and

the

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a

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E

Ul

to ^

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-Q E

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"«•

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en

21
eoE

O)

COz

HIa

<** V© 0>C O

•r- +J

<o ^>

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(U <u

x:

XJ •»-> C <D «o £

o» o »-> a> to E <u

o>o

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01

X

"O ••-E

oo

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•r— a» i—

x: i *->oz 1-

t/l

a> o

t- ••- 3 i. to <o

<o >

a>E L.O $**

u> a> c »->3 "O

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s4-»

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x:

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22
of methane + 5% velocity of propane. The results are given respectively, in

lines 2 and 3 of table 5. As would be expected, the estimated standard

deviations o^ and <re are about the same in both cases. Now estimating a standard

deviation for the a-j in this case is misleading. Most of the variation in the aj

results not from random shifts associated with densimeter, but from the varying

amounts of propane and nitrogen added to the methane. The scatter in the LNG

data from any one sample is comparable to the scatter seen in the pure methane

data. About six times ^ might roughly represent the sensitivity of the meter to

the 4% to 6% range of the propane content in the mixture. This amounts to 0.4%

spread for the average density value of the range 445 kg/m^ to 491 kg/m^.

Table 5. Results from filling data to equation

A P = u + a DRS + aj + e-jj

aa (kg/m

3

) a

e (kg/m 3

) y(kg/m3

) a(kg/m3

) Upper 99% Upper 99%

99% C.I.* 99% C.I.* Est C.B.+ Est C.B.+

Methane -16.25 ± 0.77 .0576 ± .0019 .045 .14 .052 .070

LNG -73.6 ± 3.7 .1634 ± .0080 .64 — .17 .23

(methane

velocity of

sound)

LNG -57.8 ± 3.4 .1346 ± .0073 .61 —- .15 .20

(LNG

velocity of

sound)

* Confidence interval

+ Confidence bound

The factory calibration relation for LNG was extrapolated from measurements in

other liquids. The temperature and velocity of sound corrections were calculated

from other liquids. The corrections determined in the factory calibration for

velocity of sound and temperature and/or pressure are obviously too small but can

23
be adjusted to greatly improve agreement between the measured and actual density.

The poor calibration of the densimeter disclosed by these tests once again

emphasizes the desirability of calibrating densimeters in the liquid in which it

will be used.

4.3 Vibrating Plate Densimeter

Since the original tests on the plate type densimeter, one additional unit

has been tested in the DRS. This unit, in three attempts to calibrate it,

suffered first from oil and water leaking into the stem and later from

unexplained shifts in calibration between fillings of the DRS and sometimes

during a filling. This unit was especially constructed to permit installation in

the DRS and the calibration shifts may be unique to this instrument. The first

vibrating plate densimeter tested behaved satisfactorily in cryogenic liquids.

The original unit tested showed only small variations from filling to filling and

functioned quite satisfactorily in liquid methane and LNG like mixtures.

Because problems arose during the tests of the one unit studied subsequent

to the first, we can add, at this writing, no additional information concerning

the accuracy, repeatability, and long term reliability of this vibrating plate

densimeter of a more recent design.

24
4.4 Displacement Densimeter

The calibration of the original densimeter tested agreed well with the

density reference system. This calibration was obtained after the densimeter had

been returned once, at least, to the manufacturer for recal ibration. Tests of

subsequent instruments showed calibration errors up to 1/2%.

An alternate method of calibration for these densimeters was suggested.

Liquid methane and LNG rather than liquid nitrogen and ambient temperature liquid

would be used. Since the DRS was the only available calibration device using

these liquids, the calibrations were done in it. Pure methane liquid and a mixture containing about 5% propane and to 2% pure nitrogen were the

cal ibration fluids.

In the first reported examination of a densimeter of this design, we noted a shift in the densities measured of about 0.1% relative to the DRS for the second

filling but observed no corresponding shifts in the densities measured by the

other densimeters present. During some later tests of other densimeters of the

same manufacture, such a shift was noted again. The densimeter design

subsequently was modified to eliminate this shift. We then tested three of the

modified units in the DRS. Since these modified units did not have a factory

calibration, they were calibrated in the DRS and tested for stability. They were

first calibrated then returned to their packing boxes. Transporting and handling were simulated by hauling the densimeters in an automobile distances of 300 to

800 km. The calibration constants derived from the initial data were entered

into the readout electronics and each of the densimeters were retested in the DRS

to determine whether calibration shifts had resulted from the simulated handling

and transport.

The following calibration equation was fit to the p, V, T data:

p = [a+b*V*(l+C*T)]/[l+d*T] (13)

where p is the density given by the DRS, V is the lift voltage of the

displacement densimeter under test, T is its temperature readout, and a, b, c, d are the calibration constants. Since this equation is not linear in its

coefficiencies, the data were fit by an iterative least squares method.

25
Table 6 gives the values of these coefficients, their estimated standard

deviations, and the residual standard deviations for each of the three

displacement densimeters based on the first set of tests.

The first of each of the two sets of readings taken at a particular

temperature setting was used in most instances.

As these were the first tests with the rebuilt DRS, we experienced some minor problems during the course of these tests. The stirrer failed during the

latter part of the first tests of No. 2, so equilibrium was obtained using only

the heater and cooling coils. Noisy readings from occasional internal

disturbances or exterior circuit noise was experienced. If a point looked

suspiciously high or low, the second set of readings was used. When both sets

shows the same behavior, the point was eliminated entirely. Since each

temperature setting is repeated twice in a test series, an outlying point is

usually obvious. For the initial test series on the three densimeters, only two

points were eliminated entirely and these were for densimeter No. 2.

As explained above, after the simulated hauling, a second set of data was taken. For meters No. 1 and No. 3, one outlying point each was removed from this

second set. Using the coefficients from the first series of tests, density

values were computed from the lift voltage and temperature for both the first and

second series of tests. The corresponding DRS values were substracted from the

values predicted by eq (13) and the results are shown in figures 11, 12, and 13

for densimeters No. 1, No. 2, and No. 3, respectively.

For densimeter No. 1, figure 11 shows an upward shift of about 0.025%. In the case of meter No. 2, figure 12 indicates a slope change as well as a shift.

Using the calibration equation from the first tests, the second test series

predicts 0.03% low for methane and about 0.01% low for the LNG-like mixture. The

least change from the first to the second test series occurred for densimeter

No. 3. Figure 13 shows the predicted methane values for the second test series

to be about 0.01% high for methane to about half that high for the LNG-like

mixture.

The estimated standard deviations for these meters ranged from 0.006% for

the first two to 0.01% for the third. However, the four outliers removed were in

error from about 0.05% to 0.2%.

The calibration procedure used produced a calibration that, when combined

with whatever effects handling might have had between the first and second tests,

agreed with the DRS to + 0.03% or better. The tests of these three indicate that

26
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28
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30
the calibration method is adequate and the shift in calibration appears to be

eliminated by the design changes incorporated into these densimeters. The

occasional larger shifts observed apparently were probably associated with the

equilibrium in the DRS. These were the first tests with the rebuilt DRS and some learning is required before the best results are obtained consistantly with a new apparatus. In any case, the largest shift is still small and repeated readings

possible in a stable temperature environment, such as a tank, would eliminate

occasional large deviations if associated entirely with the displacement

densimeter.

This densimeter now needs some testing in field environments to determine its

reliability and calibration stability over long periods of service.

5. CONCLUSIONS

For the vibrating cylinder sensimeter of the first kind, three essentially

identical units calibrated by NBS for transfer standards were tested. All three

units showed similar temperature dependence. Calibration equations for two of

the units were estimated to be good to ±0.1%. The third unit gave evidence of

drift and no prediction for its accuracy was provided. Two of the units were

retested approximately eighteen months later. One of the units showed good

agreement with the previous test results; the second unit was the one that had

showed evidence earlier of drift, and the results for it were relatively large

changes of calibration.

The vibrating cylinder densimeter of the second kind showed a marked

dependency on composition. Repeatability for this meter when used on methane was about 0.1%, but when used on the LNG-like mixtures, which differed in small

percentages of the amounts of propane and nitrogen, repeatability was about 0.4%

if composition was not taken into account.

As is explained in the text, the vibrating plate densimeter tested would not

maintain a calibration.

31
Three displacement type densimeters of the same design were tested that had no previous calibration. The meters were calibrated and showed good precision

(better than ±.05%); although for one of the meters two points were removed as outliers. The meters were removed from the DRS and subject to simulated hauling

as described in the text, and then retested. The retesting showed from 0.025%

high for one meter to 0.03% low for another. This difference includes both the

shifts of calibration from calculating and entering the calibration constants

into the electronics and any shifts resulting from handing the densimeter between

tests.

All the instruments tested over a span of time have shown improved ability

to measure LNG densities. More testing is necessary to establish accuracy and

reliability over long periods of use. Field installations are the place to do

that. Calibration systems are necessary for periodic calibration testing of

these densimeters.

The DRS will remain available for tests on a cost basis into the future. A

portable version of the DRS densimeter called the portable reference densimeter

(PRD) also is available for testing other densimeter calibration facilities

[6,7].

Results of comparison measurements between the DRS and the densimeter test

system at Gaz de France [9] via the PRD [6] show that even in the laboratory, the

density determined from compositional analysis and sampling is uncertain by about

+0.25%. The field measurement accuracy for liquid sampling and gas analysis is

much poorer than laboratory measurement based on the heating value studies by NBS

personnel [8]. Thus, +0.25% is probably a lower uncertainty limit for density

calculated from compositions obtained by liquid sampling and analysis. This

should be easily improved by direct density measurement with densimeters.

32
REFERENCES

[1] Siegwarth, J. D., B. A. Younglove, and J. F. LaBrecque, "Cryogenic

Fluids Density Reference System: Provisional Accuracy Statement",

National Bureau of Standards (U.S.)» Technical Note 698 (November

1977).

[2] Siegwarth, J. D., J. F. LaBrecque, and B. A. Younglove, "Test of

Densimeters for Use in Custody Transfer of LNG", Proceedings, 53rd

International School of Hydrocarbon Measurement, Norman, Oklahoma

(1978) pg 385.

[3] Siegwarth, J. D., B. A. Younglove, and J. F. LaBrecque, "An Evaluation

of Commercial Densimeters for Use in LNG", National Bureau of Standards

(U.S.), Technical Note 697 (October 1977).

[4] Siegwarth, J. D., and J. F. LaBrecque, "Cryogenic Fluids Density

Reference System: Provisional Accuracy Statement (1980)", National

Bureau of Standards (U.S.), Technical Note 1041 (1981).

[5] Goodwin, R. D., "The Thermophysical Properties of Methane, from 90 to

500K at Pressures to 700 Bar", National Bureau of Standards (U.S.),

Technical Note 653 (1974).

[6] Siegwarth, J. D., and J. F. LaBrecque, "A Portable Calibration

Densimeter for Use in Cryogenic Liquids", National Bureau of Standards

(U.S.), Technical Note 1035 (1981).

[7] Siegwarth, J. D., and J. A. Brennan, "LNG Instrumentation for Custody

Transfer", AGA Operating Section Proceedings; Proceedings of the

Transmission Conference, Atlanta, Georgia T-119 (1981).

[8] Parrish, W. R. , J. M. Arvidson, and J. F. LaBrecque, "Development and

Evaluation of an LNG Sampling Measurement System", National Bureau of

Standards Internal Report, NBSIR 78-887, (July 1978).

[9] Dewardt, F., and P. Mousset, "Contribution a

1 'Amel i oration de Comptage

du GNL - Evaluation de Densimetres et Echantillonneur pour GNL", Sixth

International Conference on Liquefied Natural Gas, Kyoto, Japan (1980),

Appendix, Paper 8.

33
NBS-114A (rev. 2-8C) U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO. NBS TN-1055

2. Performing Organ. Report No 3. Publication Date

June 1982

4. TITLE AND SUBTITLE

Tests of Commercial Densimeters for LNG Service

5. AUTHOR(S)

J. D. Siegwarth and J. F. LaBrecque

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8. Type of Report & Period Covered

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State. ZIP)

Gas Research Institute

10 West 35th Street

Chicago, Illinois 60616

10. SUPPLEMENTARY NOTES

J

Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here)

Densimeters for liquefied natural gas (LNG) from four manufacturers

were tested in liquid methane and an LNG-like mixture of methane, propane,

and nitrogen in the density reference system (DRS) . The calibration and

performance of one type tested for the first time are reported. The

stability of the calibrations and performances of three densimeters of a type previously tested have been examined and are also reported here.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

density; densimeter; liquefied natural gas; methane

13. AVAILABILITY

[xx] Unl i mi ted

| | For Official Distribution. Do Not Release to NTIS

^] Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402.

|XX] Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

40

15. Price

$7.50

USCOMM-DC 6043-P80

U.S. GOVERNMENT PRINTING OFFICE: 1982-579-150/297 REGION No. 8


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Federal Information Processing Standards Publications (FIPS

PUB)—Publications in this series collectively constitute the Federal Information Processing Standards Register. The Register

serves as the official source of information in the Federal Govern- ment regarding standards issued by NBS pursuant to the Federal Property and Administrative Services Act of 1949 as amended,

Public Law 89-306 (79 Stat. 1127), and as implemented by Executive Order 11717(38 FR 12315, dated May II, 1973) and Part 6 of Title 15 CFR (Code of Federal Regulations).

NBS Interagency Reports (NBSIR)—A special series of interim or

final reports on work performed by NBS for outside sponsors

(both government and non-government). In general, initial dis- tribution is handled by the sponsor; public distribution is by the National Technical Information Services, Springfield, VA 22161,

in paper copy or microfiche form.
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National Bureau of Standards

Washington. DC 20234

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