OVERVIEW OF DENSITY METERS USED AT SRP
FOR SAMPLE VALIDATION
Sherrod L. Maxwell, III
E. I. du Pont de Nemours & Co., Inc., Savannah River Plant
Aiken, SC
ABSTRACT
For the process control laboratory, the
verification of sample integrity prior to analysis is a critical prerequisite for reliable
analytical measurements. At the Savannah River
Plant (SRP), density check systems are employed
to verify that samples are representative of respective process tank solutions. A wide array
of Mettler density meters is used. With respect
to temperature control, water-bath thermostat ted,
electronically thermos tatted, and temperaturecorrected density meters have proven useful.
Also, density meters with measurements precise
to four, five, and six decimal places are required to meet overall sample validation needs.
Remote density cells are used to facilitate density measurements in radiobenches, glove boxes
and shielded cells. The selection of a density
meter system for a specific sample validation
application depends on numerous factors including: precision required; type of containment;
radiation levels; exposure to corrosive chemicals; and range of density measurements required.
Temperature-corrected density meters have proven
very useful for achieving measurements precise
to five decimal places under conditions where
the use of water baths is not safe, and the use
of sensitive electronic components is not practical. Computer software is used to correct for
ambient temperature changes in the density measuring cell. An automated six-place density
meter is now in use for validating dilute uranium
product samples. Customized software is used to
determine the error of density measurements as
well as sampling error.
INTRODUCTION
Sample validation, the verification of
sample integrity prior to analysis, is a critical prerequisite for reliable process control
and accountability measurements. Precise and accurate assays are of little value if the samples
analyzed are not representative of the designated
process or accountability tank solution. In
fact, such analytical results could cause unnecessary reprocessing and/or inventory differences.
During the past three years, a major effort
has been made at.SRP to determine the validity of
samples prior to assays by measuring their density, and comparing the average density of the
samples with the measured density of the process/
208
29808
accountability tank solution. This program has
been very beneficial in that time consuming assays on nonrepresentative samples have been minimized (or eliminated for key sample points).
Thus, in general, the reliability of analytical
results has improved.
Density measurements were selected for sample
validation because such measurements are rapid,
precise, and accurate. Experience has shown that
density measurements are an excellent indication
of sample validity. There are occasions when
densities can be misleading, due to a unique combination of poor mixing in a process tank and inadequate sampling; however, such occasions are
rare. At SRP, sample validation via density measurement has proven to be very reliable. As a
result, a wide array of density meter systems is
now in use to validate various samples.
THE NEED FOR SAMPLE VALIDATION
If the mixing in process accountability tanks
were always adequate to ensure a homogeneous
solution; if sampling equipment were always functioning properly; and if sampling procedures were
adequate and always followed, there would be no
real need for sample validation in the analytical
laboratory. But equipment does fail, procedures
are not always followed, and nonrepresentative
sampling does occur frequently.
Density measurements that were made recently
on physical inventory samples from a Purex process plutonium hold tank are summarized in Table
I* to illustrate sampling problems that occur,
and the importance of sample validation in providing reliable assays. Samples from this hold
tank and all canyon tanks (in contrast to finishing lines) at SRP have a tendency to be diluted with water if sampling is inadequate.
Water is used to prime the pumps that transfer
samples from the tanks to sample stations, and
water is also used to flush sampling lines after
sampling is complete. Initial replicate samples
(Table I) were obviously not identical. The plutonium concentration of the sample with the highest density was 2.010 g/L. Resamples were requested. Density values were somewhat higher,
and the density spread between replicate samples
*M. K. Holland, E. I. du Pont de Nemours & Co.,
Savannah River Plant. Unpublished Work.
was less than for the original samples; however,
the samples were still not identical, and were
not representative of the hold tank solution. A
second set of resamples was obtained, and density measurements on the replicates (Table I)
confirmed that the samples were identical, and
thus, probably representative of the hold tank.
The average plutonium assay value for the valid
set of samples was 2.467 g/L; thus the initial,
measured value of 2.010 g/L was biased low by
19%.
Sample validation is essential to reliable
assays for accountability and process control.
tween calculated and experimental biases*, as
shown in Table II. With density spreads of this
magnitude, there is little confidence that the
sample with the highest density is representative
of the process tank solution. It can also be deduced from Equation 1 that the lower the density
of the process solution, the smaller must be the
allowable spread between replicate samples to assure valid samples.
C
Where:
SELECTION OF DENSITY CHECK LIMITS FOR SAMPLE
VALIDATION
For a given sample point, two density check
limits must be established-- the maximum allowable spread between laboratory densities on replicate samples, and the maximum allowable spread
between the average sample density and the density of the process/accountability tank solution.
Factors to be considered in establishing density
check limits are: the importance of the sampling
point (i.e., input/output accountability tank or
physical inventory tank containing a large quantity of desired material); the nominal density
of the sample; the method of sampling and potential sampling errors; and the precision and accuracy of the process tank density measurement. At
SRP, density check limits have been established
as the Laboratories Department has expanded its
capabilities for providing precise, accurate
density measurements.
There is a direct correlation between the
range of measured densities on replicate samples,
and assay results. Equations listed below permit
the calculation of the plutonium (or other assay)
bias that should exist for any sample in a set of
replicate samples when the plutonium assay of
that sample is compared to the assay of another
sample in the set (assuming that the assays and
density measurements are without error).
Equation 1 is applicable to sampling subject to dilution with water; Equation 2 is applicable to
sampling subject to dilution with the previous
solution in the process tank. For both equations, it is assumed that the process tank solution is homogeneous, but improperly sampled.
D
Equation 1: % Pu Bias = - 100
Where:
x
l - 0.997
D]^ is the measured density of the comparison sample. Typically, this is the
sample in a set of replicate samples
that has the highest density value.
D2 is the measured density of the sample for which the % plutonium bias is
being calculated (typically the sample
with the lowest density in the set of
replicates).
0.997 is the density, g/mL, of water at
25°C.
For precise and accurate assays and density
measurements, there is excellent agreement be-
- C,
Equation 2: % Pu Bias = 100 x
DI is the measured density of the comparison sample.
D£ is the measured density of the sample for which the % Pu bias is being
calculated.
Dp is the average density obtained on
replicate samples from the previous
solution in the process tank.
Cp is the average plutonium assay value
on the previous solution in the process
tank.
GI is the plutonium assay value of the
comparison sample.
In Equation 2, if the previous solution in
the process tank is water, Dp becomes 0.997,
Cp becomes 0.0, and Equation 1 is obtained.
Both equations are useful in determining allowable density spreads for validation of samples.
In general at SRP, the density check limit
for replicate samples is more important in assuring valid samples than the limit for agreement
between lab density and process tank density
because the lab densities are more precise and
accurate than process tank densities.
Efforts are in progress at SRP to expand the use
of Ruska differential pressure instruments for
measuring liquid level and density of process
tank solutions, and to use a statistical average
of multiple measurements for more precise density values. As improved process density values
are available, density check limits will be narrowed. With respect to density check limits for
replicate samples, the smaller the density
spread, the less likely the set of replicate samples will be nonrepresentative of the process
tank solution.
DENSITY METERS - GENERAL ASPECTS
State-of-the-art density meters, produced by
Anton Paar of Austria and marketed by Mettler Instruments, have been found to provide the precision and versatility required to meet the variety of sample validation needs at SRP. Density
measurements are based on the precise measurement
of the period of oscillation of a vibrating U-shaped tube (Figure 1) that changes its frequency
when it is filled with different liquids or gas-
*M. K. Holland, E. I. du Pont de Nemours k Co.,
Savannah River Plant. Unpublished Work.
209
es. The density of the measured solution is related to the square of the oscillation period of
the vibrating U-tube via an instrumental constant that incorporates both the mass and constant volume of the U-shaped tube as well as the
"elasticity constant" of the oscillator system
itself.1
FIGURE 1.
Density Meter Measurement Cell
A wide array of Mettler density meters is
available. With respect to temperature control,
density meters can be grouped into three basic
categories: (1) meters that require thermostatting by an external water bath; (2) meters
in which the measuring cell is electronically
thermostatted; and (3) meters that utilize
electronic temperature compensation or temperature correction via software. Density meters
with measurements precise to four, five, and six
decimal places are available. Remote density
cells are available to facilitate density measurements in containment facilities. The selection of a density meter system for a specific
sample validation application depends on numerous factors including: precision required to
meet density check limits; type of containment;
radiation levels; exposure to corrosive chemicals ; and range of density measurements required.
Various density systems in place at SRP will be
discussed to illustrate selection, capability,
and application.
FOUR-PLACE DENSITY METERS FOR RADIOBENCH, GLOVE
BOX APPLICATIONS
In SRP process control labs, density checks
are performed in radiobenches using a Mettler DMA
46 electronically thermos tat ted, four-place density meter (Figure 2). Sample is introduced via
a semi-automated peristaltic pump, and the measuring cell is rinsed and dried using a fiveway valve controlled by a digital timer. Use of
a semi-automated peristaltic pump allows (1) a
reproducible, controlled fill speed for sample
solution, and (2) introduction of a premeasured
volume of sample. The DMA 46 was selected to
eliminate the need for a water bath that must be
contained, or monitored for activity in the bath
reservoir if not contained. The DMA 46 electron-
210
ically thermostat ted density meter does not require a water bath and is ideal for radiobench
applications. The four-place precision achieved
by the meter is adequate for most routine sample
validation checks. Because the radiation levels
in radiobenches at SRP are relatively low, there
is no appreciable damage to the meter's sensitive
electronics.
To validate routine, high activity 239pu
samples, a DMA 46 density meter was contained in
a glove box. Due to the presence of corrosive
chemicals (HN03 solution and vapors) in the
glove box, a plastic cover was provided to protect exposed calibration dials. Because the
lifetime of the DMA 46 meter is reduced in the
corrosive environment of the glove box, use of
the more durable DPR 402YE remote cell, in conjunction with the CPR-S control unit (described
below), is planned for the future validation of
routine 239pu glove box samples.
FIGURE 2.
Four-Place, Electronically
Thermostatted Density Meter
FIVE-PLACE DENSITY METER FOR
239
PU SAMPLES
23
8pU AND DILUTE
1
FIGURE 3.
Remote Density Cell
The DPR 402YE, DPR-S combination noted above
is already in use to validate high-activity
238pu samples in a glove box at SRP. Figure 3
shows Che DPR 402YE remote cell with its glass
oscillator U-tube. The aluminum cover plate was
replaced with a Lexan® (General Electric) plate
to allow a clear view of sample introduction.
The lower and upper, large glass tubes inside
the DPR 402YE contain the oscillator U-tube and
the remote temperature probe, respectively.
Figure 4 shows the DPR-S control unit with interfaced Epson HX-20 computer. The DPR-S displays
the period of oscillation and the temperature of
the measuring cell, and relays this information
to the computer via RS232C serial interface.
Software written at SRP corrects for temperature
FIGURE 4.
prior to uranium measurements and subsequent
material shipment to the Y-12 Plant at Oak Ridge.
Customized software, written at SRP, is used to
quantify the analytical error in the density
measurements and to estimate sampling error.2
If sampling error is detected, it can then be
quantified and included in the limit of error
estimate for this shipment. The automated
system consists of a DMA 60 display unit, two
DMA 602 remote cells, an SP-2 autosampler, a
high-stability HETO water bath, a Hart Scientific
digital thermometer with remote thermistors that
have NBS traceability, and an Epson HX-20 computer. Figure 5 shows the autosampler and the
two DMA 602 remote cells, one as a measuring cell
and one as a reference time base, contained in a
radiobench. Figure 6 shows the DMA 60 control
unit, the HETO proportional heating water bath
precise to +0.002°C, the Hart digital thermometer
accurate to +0.01°C, and the Epson computer. The
Epson computer controls calibration, data acquisition, and statistical testing for sampling
Control Unit, Computer for Remote Cell
fluctuations at the oscillator U-tube by correcting for expansion coefficients of the different sample solutions measured as well as for
the glass oscillator itself. The software
prompts the analyst through the calibration and
sample measurement procedure and prints density
measurements corrected to 25.0°C. The software
also provides adequate quality control to assure
proper calibration of the density meter.
9 ob
During the measurement of "°Pu solutions,
gas evolution in the density measurement cell
due to radiolysis tends to produce negatively
biased density values. Additional software is
used to perform measurements just before thermal
equilibrium is reached and before gas evolution
becomes significant. However, four-place precision is still achieved. When the same system
is used to validate dilute ^39pu samples where
gas evolution is not a problem, five-place precision is achieved. In this case, appropriate
software requires that tight stability limits be
met. The DPR-S control unit, because it is operated via computer software, has proven to be
highly versatile. Table III shows replicate measurements made on water, 238pu standards, and
duplicate ^39pu samples.
SIX-PLACE AUTOMATED DENSITY METER FOR DILUTE
URANIUM PRODUCT SAMPLES
An automated, six-place density meter with
thermostatted water bath is now in use to detect
sampling errors in dilute enriched uranium product samples. Sample integrity checks are made
FIGURE 5.
Auto-Sampler Remote Cells
FIGURE 6.
Support Equipment for Six-Place
Density Meter
error. A six-place density meter .is required to
estimate sampling error for the dilute uranium
product samples, because the typical solution
density is 1.01000 g/mL, and a 0.00001 g/mL
change in density correlates with 0.1% change in
211
uranium content. Table IV shows density data on
uranium samples using the automated system. No
sampling error was found for this set of samples
SHIELDED CELL DENSITY METERS
In the present process control lab, two
shielded analytical cells are used to handle
samples with high gamma radiation levels. A DMA
401 remote density cell with a glass oscillator
U-tube is in each shielded cell and each remote
cell is thermos tatted by a water bath contained
in a radiobench. Density values are displayed
to four decimal places by DMA 45 display units.
Figure 7 shows a DMA 401 remote eel1 and a DMA
45 display unit. On the remote cell, the aluminum cover was replaced by a clear Lexan® plate
to permit visual inspection of sample introduction. This remote cell with minimal electronics has been reliable in shielded cell applications for several years; however, the working
lifespan of a DMA 401 in shielded cells at SRP
averages only about nine months, due to electronic failures induced by the high radiation
field.
include five-place precision via signal averaging and stability limits, and computer control
FIGURE 8.
New Density Meter System for New
Shielded Analytical Cells
of the density meter operation, calibration, and
data acquisition.
FIGURE 7.
Present Density Meter for Shielded
Analytical Cell
A new process control laboratory is scheduled for startup in late 1986.
Each of four
shielded analytical cells will contain a density
meter remote cell, the DPR 412YS (Figure 8),
which has a steel oscillator U-tube and acidproof housing to provide improved durability.
In addition, electronics, similar to those inside the DMA 401, have been remoted at SRP to a
distance of ten feet to protect them from exposure to the high radiation in the shielded cells
HETO water baths that are commercially available
with sensitive electronics already remoted will
be placed inside two of the shielded cells to
thermostat the four remote density cells. Two
DMA 60 control units (Figure 8), each with the
capability to operate two separate remote cells
and to display eight-digit period measurements
for each, will be used to operate the four remote cells. Each DMA 60 will be interfaced to
two separate DEC Microvax II computers, each of
which will support other computerized operations
Expected benefits of the density meter system
212
ONLINE HEAVY WATER ASSAYS
In addition to sample validation, density
meters are proving useful for online assays of
heavy water (D20). Recent tests have shown
that online measurement of D20 is feasible
over a wide range of concentrations using the
DPR-S control unit and an online DPR 417YE remote cell. Software for the Epson HX-20 computer was written at SRP to correct for changes
in density due to temperature fluctuations, which
(1) affect each specific D20/H20 mixture differently , and (2) affect the steel measuring cell
itself. The software also converts online density measurements to a direct printout of mole %
D20. Measurements are precise to 0.1%, 95
C.L.**
SUMMARY
Use of a wide array of Mettler density
meters, with customized software as required,
ensures that representative samples are routinely obtained for process control and accountability assays. Nonrepresentative samples can
be detected, and the contribution of sampling
error to the overall error of the measurement
system can be estimated.
**M. S. Boerste and S. L. Maxwell, III. E. I.
du Pont de Nemours & Co., Savannah River
Plant. Unpublished Work.
TABLE I.
Purex Process Plutonium Hold Tank
Sample
Number
1
2
3
4
5
Density Range
Density, g/mL
Production
1.0403
1.06
1.1045
1.0927
1.1150
1.0672
0.0747
11
12
13
14
15
Density Range
1.1316
1.1316
1.1320
1.1325
1.1326
0.0010
TABLE II.
Lab
Pu, g/L, via
PU+3 Spectrophotometry
2.010
1.08
2.464
2.466
2.471
x = 2.467
Plutonium Biases
Sample
Number
Lab
Density
g/mL
1
2
3
4
5
1.0812
1.1163
1.0487
1.1236
1.0995
Pu, g/L
via Pu"1"-^
Spectrophotometry
1.172
1.708
0.710
1.828
1.445
% Pu Bias,
Calculation
from Densities
- 33.5
- 5.8
- 59.2
Reference
- 19.0
% Pu Bias,
Experimental
- 35.9
- 6.6
- 61.2
Reference
- 21.0
TABLE III. Density Via Temperature-Corrected
Meter System* in Glove Box
Density of
Water, g/mL
at 25°C
0.99709
0.99709
0.99710
0.99711
0.99713
0.99712
Avg =
Theo =
SD
0.99711
0.99707
.00002
Density, g/mL
of 238Pu
Standard
1.06195
1.06182
1.06179
1.06192
1.06190
Avg = 1.06188
SD =
.00007
Density, g/niL
of Dilute *39Pu
Sample
1.01198
1.01199
1.01197
1.01198
Avg = 1.01198
SD =
.00001
Density, g/mL
of Duplicate
239
Pu Sample
1.01194
1.01197
1.01196
1.01196
Avg = 1.01196
SD =
.00001
* System consists of DPR-S control u n i t , 402YE (glass) remote cell,
Epson HX-20 computer.
213
TABLE IV.
Density Measurements on Uranium Product Shipment
Trailer
Samples
ID #
U1A (1)
U1A (2)
Density
(g/mL)
1.009885
1.009888
U1A 1 (1)
U1A 1 (2)
1.009890
1.009890
1.009890
U1B (1)
U1B (2)
1.009885
1.009892
1.009888
U1B 1 (1)
U1B 1 (2)
1.009882
1.009896
1.009889
U1C (1)
U1C (2)
1.009887
1.009900
1.009894
U1C' (1)
U1C' (2)
1.009881
1.009899
1.009890
Avg. Density
(g/mL)
1.009886
Average Density = 1.009890
Std Dev = 3 x 10~6
Std Dev of Avg. = 9 x 10~7
REFERENCES
1. 0. Kratky, H. Leopold and H. Stabinger,
"Instruction Manual for DMA 46 Calculating
Density Meter," Anton Paar, Graz, Austria
2. E. P. Shine, "Density Meter Algorithm and
System for Estimating Sampling/Mixing
Uncertainity," Proceedings of the INMM 27th
Annual Meeting, New Orleans, Louisiana,
June 22-25, 1986.
The information contained in this article was
developed during the course of work under
Contract No. DE-AC09-76SR00001 with the U. S.
Department of Energy.
214