A secondary working standard of flow calibration has been developed to be used in place of a primary weight-time standard, and which can thereby effect a 75 percent reduction in calibration time while maintaining acceptable accuracies. The secondary standard uses six previously calibrated turbine-type flowmeters built into two manifold systems containing automatically switched flow valves. The pair of systems is capable of covering the flow range of 0.0004 to 19 l/s (0.007 to 300 gpm) with the uncertainty in volume flow rate not exceeding + or - 0.25 percent over the range of 0.06 to 19 l/s and not exceeding + or - 0.5 percent over the range 0.0004 to 0.06 1/s. Data reduction and plotting of results are by computer

Hobart, H. F.

English

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NASA TECHNICAL	 NASA TM X-71876
MEMORANDUM
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ti"-4
(NASA-TM-X-71876) AN AUTOMATED SECONDARY
	 W76-18404
<	 STANDARD FOR CALIBRATING LIQUID FLOWMETERS
(NASA)	 9 p HC $3.50	 CSCL 14B
ZUnclas
G3/35 14336
AN AUTOMATED SECONDARY STANDARD FOR
CALIBRATING LIQUID FLOWMETERS
by Howard F. Hobart
Lewis Research Center
Cleveland, Ohio 44135
TECHNICAL PAPER to be presented at Twenty-second
International Instrument Symposium sponsored
by the Instrument Society of America
San Diego, California, May 25 L 27, 1976
0
aao
4
AN AUTOMATED SECONDARY STANDARD FOR CALIBRATING LIQUID FLOWMETERS
by Howard F. Hobart
NASA-Lewis Research Center
Cleveland, Ohio
ABSTRACT
A secondary working standard of flow calibration
has been developed to be used in place of a primary
weight-time standard, and which can thereby effect
a 75 percent reduction in calibratior time while
maintaining acceptable accuracies. The secondary
standard uses six previously calibrated turbine-
type flowmeters built into two manifold systems
containing automatically switched flow valves. The
pair of systems is capable of covering the flow
range of 0.0004 to 19 1/s (0.007 to 300 gpm) with
the uncertainty in volume flow rate not exceeding
±0.25 percent over the range 0.06 to 19 1/9 and
not exceeding ±0.5 percent over the range 0.0004
to 0.06 1/s. Data reduction and plotting of re-
sults are by computer.
INTRODUCTION
Large numbers of flowmeters are in use at various
facilities throughout the Lewis Research Center.
For the measurement of liquid flow rates, the
turbine-type flowmeter is the most widely used in-
strument; it is followed in frequency of use by
head-type and variable-area-type flowmeters.
Aside from their obvious function of measuring fuel
flow rate, these flowmeters are used in such di-
verse applications as monitoring oil flow rate to
bearings, measuring coolant flow rate in heat
transfer systems, measurement of cryogenics, etc.
Because of the large number of meters in use at
Lewis, there has been a need for a secondary stan-
dard calibration system capable of performing
rapid, accurate calibrations.
For many years the primary reference standard has
been the weight-time dynamic calibrator. The high
accuracy attainable with this device is obtained at
the cost of a relatively long time required to per-
form a calibration. Some alternatives to the
weight-time calibration method have appeared in the
literature in recent years. Reference 1 mentions
the use of a reference flowmeter for comparison
calibrations and reference 2 describes various cal-
ibration techniques including the calibration of a
turbine flowmeter with a second calibrated turbine
flowmeter. This report describes a secondary flow
standard based on "master" turbine-type flowmeters
built into a manifold system with automatically
switched valves. The system covers the flow range
0.0004 to 19 1/s (0.007 to 300 gpm) with acceptable
limits of accuracy and vastly reduced calibration
times. The primary weight-time system is used to
calibrate the secondary standard system occasion-
ally, but infrequently.
This report also describes the automatic data re-
duction and plotting of calibration results by com-
puter.
APPARATUS AND PROCEDURE
Manifold System
There are two similar manifold systems differing
only in physical size. Each system consists of
three flowmeters and two electrically operated
valves connected in a series-parallel arrangement.
Connections to either manifold are made with flexi-
ble hoses having quick disconnect end fittingb.
Figure 1 is a schematic drawing of one of the mani-
fold systems.
At very low flow rates both valves are closed and
meters are connected in series with the output of
the smallest meter displayed on the readout. As
the flow increases to the point of maximum flow for
the smallest size meter, the smaller valve opens to
shunt the smallest meter and the output of the next
larger size meter is displayed, and so on. Because
the impedance of a meter is greater than that of
the ball-type valves, overranging of any meter is
avoided. Figure 2 is a photograph of both mani-
folds showing all six master flowmeters mounted on
a portable cart.
Designed for use with water as the measured fluid,
all wetted materials are either brass or stainless
steel to prevent corrosion. Each meter is located
downstream of ten diameters of straight tubing, or
a straightening vane, or both, to minimize swirl.
Furthermore, the meters are calibrated in situ to
eliminate effects of changes in piping configura-
tion. Table 1 lists characteristics of the six
meters used in the two manifolds. There is a back-
up meter for each size in case of failure.
EUI•
 7H]a,
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It is apparent that the success of the manifold
system depends on not having any valve leakage.
The type of ball valve used has a brass body with a
chrome-plated ball and teflon seals. The valves
are not subjected to temperatures other than ambi-
ent, and pressure diffe-entials are low. One
year's experience shows no evidence that the valves
leak.
Flow Controller and Readout
The flow controller shown schematically in figure
is the brain of the system. This instrument can
accommodate the output of three flowmeters. De-
pending on the flow rate, internal logic circuitry
adjusts the air-operated electrically controlled
ball valves to select the proper size of meter and
also to display the output of the selected meter.
There are provisions to adjust the up and down
switchover frequencies at limits to provide an
overlap between meters, to prevent hunting. Addi-
tional features include the choice of automatic or
manual control and the availability of thumb-
switchea on each channel to adjust the time base
for displaying the output in engineering units, if
desired.
The counters used to measure outputs of both the
test and master meters are designed to measure low
frequency by utilizing precise period-measuring
techniques and then to calculate and display the
frequency in hertz.
Primary Standard
Turbine flowmeter manifold systems are commercially
available; however, one of the unique features of
the system described herein is its use in conjunc-
tion with a primary-standard dynamic weight-time
calibrator. This primary standard has a dual flow
range which corresponds to the ranges of the high-
and low-flow manifolds. There is sufficient over-
lap in ranges between the two manifold systems so
that only one of them is needed to cover the entire
flow range of any size of test meter. Figure 3
shows the connections between the primary and sec-
ondary standards and the meter being tested. For
the sake of simplicity, only one range cf both the
primary and secondary systems is shown in figure 3.
The primary standard has four functions: (1) to
produce the flow; (2) to provide clean, filtered
water at constant temperature; (3) to control the
flow; and (4) to permit calibration of the master
flowmeters, in situ, at any time. The probable
error (p.e.) of the primary standard is 0.10 per-
cent. The primary standard has the ability to pro-
duce flow rates up to 19 1/s (300 gpm) and to main-
tain a preset temperature to within '0.5 0 C. The
flow :ate is infinitely adjustable over the range,
and once set at a particular rate, will remain con-
stant to within 0.1% with minimum pulsations.
CALIBRATION PROCEDURE
Master Flowmeter Preparation
All six master meters were first calibrated over
the flow range of each meter using the primary
standard. Calibration results are plotted as meter
sensitivity, K, in cycles per unit volume against
meter output, F. in hertz. The calibration fluid
was water at 240 C. The meters were calibrated two
additional time to check repeatability, which was
approximately ±0.15 percent with the exception of
the smallest meter, whose repeatability was t0.4
percent. An average curve was drawn through the
three sets of data points and from this curve a
table of F vs. K for each meter was formed and
stored in a computer. The interval between succes-
sive F-values in the table varied with the slope of
the curve so that a straight line interpolation
between any two adjacent points in the table did
not deviate from any average curve by more than
±0.25 percent and in most cases by less than ±0.1
percent. The number of points varied from 5 to 15.
Figure 4 shows the difference in calibration curve
shape between the largest and smallest size meters.
ordinates have been normalized to provide a clearer
comparison between the meters. Measuring flow
rates below about 0.006 1/s (0.1 gpm) with turbine-
type flowmeters becomes increasingly difficult, for
the ratio of driving torque on the flowmeter rotor
to retarding forces (magnetic, viscous, or fric-
tional drag) is reduced, and as a result, linearity
and repeatability are degraded.
Two important precautions involving the handling of
the master meters are carefully observed. First,
at the completion of any daily operation where the
meters are wetted, they are removed from the mani-
fold, flushed and immersed in alcohol, and then
dried with a hot air gun at a temperature of about
700 C. Second, because the meter end connections
are stainless steel AN-flare-type tube fittings,
soft copper conical seals are used to provide leak-
free joints and also to avoid having to exert ex-
cessive wrench torque, which might cause thread
galling or meter body deformation and consequent
calibration shifts.
Test Meter
The meter to be calibrated is installed between the
primary standard and the manifold system as shown
in figure 3. The usual precautions of having ade-
quate lengths of streight pipe upstream and down-
stream and/or the use of straightening vanes are
observed. These precautions are particularly impor-
tant where the test meter range is at the low end
of the manifold-system range, because a reducer and
an expander must be used to connect the test meter
to the calibration system.
Experience has shown that new or rebuilt turbine-
type flowmeters require a run-in period of at least
15 minutes at full flow prior to taking data, in
order to stabilize their calibrations. The majotity
of test meters are turbine-type. However the secon-
dary standard 1s also very convenient for testing or
setting flow switches, determining the C of valves,
and calibrating head-type and variable-area flow-
meters.
Data Collection
During the run-in period of a turbine-type meter,
the temperature of the fluid is brought to 240 C
(the same temperature at which the master -.eters
were calibrated) and the output of the teat meter
is examined on an oscilloscope to verify proper
functioning. The flow rate is adjusted to the full
scale of the test meter and held long enough to
record the frequency F of the test meter, the fre-
quency F of the mastef meter and also to identify
which master is in use, N . Between 12 and 15 flow
settings, equally spaced sn a logarithmic scale,
cover the range of the test meter. Since the final
calibration curve is presented on a rectilinear
plot, this results in a closer spacing of data at
the low end of the range, that helps to define the
knee or drop-off of the curve.
The flow rate generated by the primary standard is
very constant. Flowmeters in good working condi-
tion will produce outputs that vary less than
1 percent during the duration of a given setting.
At the conclusion of a calibration test, the piping
of the manifold is blown dry with clean, dry com-
pressed instrument air. If the meter tested was a
turbine-type flowmeter, the same cleaning and dry-
ing procedure is used for this meter as described
for the master flowmeters.
DATA REDUCTION
Figure 5 is a block diagram of the flow of data
from the flowmeters to the end product, which is a
printed table of results and a plot of K against
output frequency.
The program developed for use with the secondary
standard flow system has three modes. The first,
"UPDATE," is for the purpose of storing the pre-
viously determined table of K vs. F for each of the
master meters. Each table is identified by the
number of the master, N , and is used to determine
the volume flow rate theough the system. If a mas-
ter meter is recalibrated and a shift in calibra-
tion necessitates a new table, the old data are
automatically erased as the new data are inserted.
The "DISPLAY" mode produces a hard copy of any
stored table requested. The most common =.e of
this mode is checking for typing errors after in-
serting a new table of values.
The "DATA" mode is used to calculate the value of
the test-meter factor K t , at each frequency Ft.
The terminal prompts the operator for the values of
and F . With the flowmeters connected in
series, volumi flowrate is the same through each
meter, and since the master meter was calibrated at
the same temperature as the test meter, the volume
flow rate is computed from the stored tables as
V - Fm/Km and K t is simply: K t - Km x (Ft/Fm).
Heading information such as meter manufacturer,
size, model number, serial number, etc. are in-
cluded with each set of data for identification and
these headings are reproduced on the final results.
Figure 6 is a typical table produced by the compu-
ter and figure 7 is a typical graph plotted from
values in the table. The plotting subroutine exam-
ines the ranges of both K and F, and determines the
grid spacin4 such that a ratio of about I percent
per inch is used for the ordinate, and increments
of 100 or 200 hertz per division are used for the
abscissa.
An alternative method of plotting which might have
merit would be to normalize the K factor by divid-
ing each value of K by Kfa, the full scale value of
K. The value of Kfa would have to be printed on
the graph. This method of presentation makes it
easy to determine at a glance over what frequency
range the factor K remains constant to any prede-
termined limit. In either case, only the coordi-
nates of the calibration points are plotted, the
reasons being that some of the curves have complex
shapes and some engineering judgment is needed in
drawing the best curve. Also, it is easier to elim-
inate a bad data point, if necessary.
CONCLUDING REMARKS
In order to reduce manpower spent on routine flow-
meter calibrations, an automated secondary standard
has been developed. Approximately one hour is re-
quired to perform a calibration on the primary stan-
dard. This time is reduced to about fifteen minutes
when using the secondary standard. At the present
time the major portion of liquid flow calibrations
are performed on the secondary standard, the excep-
tion being special cases that require either cali-
bration at some temperature other than ambient, or
calibration inaccuracies not exceeding ±0.15 per-
cent.
Accuracies of meter calibrations performed with the
secondary standard approach 99.75 percent. This
accuracy statement is based upon knowing the magni-
tude of errors associated with each of the measure-
ments being made. The master meters have a p.e. of
t0.2 percent, F t and Fm have a p.e. of ±0.10 percent,'
and errors due to density changes are eliminated
by maintaining a constant temperature for all cali-
brations of both master and test meters.
Initially the master meters were checked against the
primary standard every few weeks to assure reproduci-
bility. This time interval has been extended to
over a month because of the good repeatability indi-
cated by these checks. During the past 12 months,
experience with the six master meters has shown that
only one of the meters suffered a calibration shift
of over 1/4 percent. The bearings were replaced on
this meter and this problem was resolved. This good
record of reliability can probably be attributed to
the careful handling given the masters. Although an
extra set of meters was purchased initially as
spares, none of the masters has needed replacement
after about one year of service.
REFERENCES
(1) 1972. "Installation, and Calibration of Turbine
Flowmeters," Instrument Society of America Spec.
ISA-RP31.1.
(2) Olsen, L. 0. 1974. "Introduction to Liquid Flow-
meters," National Bureau of Standards Rept.
NBS-TN-831.
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TABLE 1. - MASTER FLOWMETER CHARACTERISTICS
Manifold Meter
no.
Flow range Frequency,
Hz
Linearity,a
at ±0.5 percent
Repeatability,b
average deviation,
liters/sec gpm (full scale) percent
High range 1 1.6 to 20 25 to 300 1800 18:1 0.11
2 0.2 to 1.6 2.5 to 25 1500 7:1 .17
3 0.02 to 0.20 0.25 to 2.5 1300 2:1 .13
Low range 4 0.05 to 0.6 0.75 to 9 1600 16:1 0.11
5 0.005 to 0.05 0.075 to 0.75 1500 1.2:1 .16
6 0.0004 to 0.005 0.007 to 0.075 1260 2:1 .39
aThe ratio of full scale frequency (F fs ) to the frequency at the point where K
deviates from Kfs by ±0.5 percent.
bThe average deviation of data points from the mean curve drawn through three separate calibrations.
4
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V	 ,
F -
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READOUT AND
FLOW VALVE
CONTROLLER
FLOW
I	 ^
STRAIGHTENERS -^ \I
FE	
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i II	 ^	 I
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/--- FEi	 ^— J
I	 ^^	 I
—^ MASTER	 I
~	 FLOWMETERS
FLOW	 FE ^	 \ IELECTRICALLY CON-
TROLLED AIR -OPERATED\VI
BALL VALVES
Figure 1. - Schematic of one of the two manifold systems.
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rh1FTER 3
h1FTFR 6- \
	^
METER 2,
METER 5—^
METER 1 
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METER d
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LARGE
MANIFOLD=
BALL VAIVI Q of 41J
Figure 2. - Complete manifold system.
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PRIMARY STANDARD
-----------1
BACK /|	 /-
r^E|GH	
~ PR[SBUR[	 HOT
TANK	 ALVE	 COLD FTEST
—4 v^
	I METER
HEAT	
SENSOR
EXCHANGER
.	 .
'	 |	 '
|
|
|	 "^~ ' FE
VA	
'
^	
~^
^
v'RESERVOIR. 	
^F^|	 .' /	 |L ^^'_~-__---
L	 / ^-- — — — — — -^/--
 SECONDARY
	 STANDARD
FLOW
CONTROL
VALVES -j
Figure 3. - Schematic of one rz:,ge of complete flow loop.
LOO
^ .95
---- LARG[5TMET[R.
2U LITERS/SEC FULL SCALE
------ SMALLEST METER,
K005L|TER/5[C FULL SCALE
500	 1000	 180
FREQUENCY, Hz
Figum4.
	 Linearity of largest and smallest size meters.
v^
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TEST
FLOWMETER
I
COUNTER
i
FLOW CON-
TROLLER AND
COUNTER
MASTER
FLOWMETER
REMOTE TIME
SHARING SYSTEM
COMPUTER TERMINAL
VOLUME FLOW CALCU-
LATED FROM MASTER,
AND K FACTOR CAL-
CULATED FOR TEST
METER
r	 '^
s
F
A
Y
TABLE OF
K, & F
GENERATEDTABLE AND
GRAPH TO
USER
PLOT OF
K VS. F
Figure 5.
	
- Block diagram of data acquisition system.
DATE 01/15/76 MFG.	 %YZ	 CO. S/N	 1234	 S17F	 3/4"
MODEL	 76-001-3/4	 J.O.	 Y0nGF,06	 FOIL	 SCAIF	 20	 GPM	 1500	 HFPTZ
FREQ.(HERTZ) K	 PUISFS/(.AL, r)	 GPM
1500.0 4367,6 20, F,QfS
1400.0 4371.3 19,21:3
1300.0 4370,6 11, A1017
1200.0 43(,9,7 1e.4770
1100.0 4374.7 15.08x,9
1000,0 4314.6 13.715'
g 00.0 4315.2 12.3424
800.0 4379.6 10,959
700.0 4375.6 9.5'l'r
600.0 4367.1 F.^423
700.0 435A.5 i., eA31
400.0 4331.4 5.541'1
300.0 4319.5 4.1x:71
200.0 4226.5
120.0 42(^(	 ,3 I.f,A7L
Figure 6.	 - A typical table produced by computer.
I
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NASA-L-
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An automated secondary standard for calibrating liquid flowmeters

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