NBSIR 74-562

Space Shuttle Pogo Pressure

Measuring System

A Progress Report

John S. Hilten and Paul S. Lederer

Electronic Technology Division

Institute for Applied Technology

National Bureau of Standards

Washington, D. C. 20234

July 15, 1974

Progress Report Covering Period

January 15, 1973 to September 15, 1973

Prepared for

NASA George C. Marshall Space Flic^t Center

Huntsviile, Alabama 35812

NBSIR 74-562

SPACE SHUTTLE POGO PRESSURE

MEASURING SYSTEM

A PROGRESS REPORT

John S. Hilten and Paul S. Lederer

Electronic Technology Division

Institute for Applied Technology

National Bureau of Standards

Washington, D. C. 20234

July 15, 1974

Progress Report Covering Period

January 15, 1973 to September 15, 1973

This is a progress report. The work is incomplete and is continuing.

Results and conclusions are not necessarily those that will be included

in the final report. Performance Test Data were obtained from one or two samples of several transducer types, and do not necessarily re- present the characteristics of all transducers of a given type.

Prepared for NASA George C. Marshall Space Flight Center

Huntsville, Alabama 35812

U. S. DEPARTMENT OF COMMERCE, Frederick B. Dent. Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts. Director

ABSTRACT

This progress report describes a variety of experimental approaches

to the dynamic calibration of pogo pressure transducers for the measurement

of oscillatory pressures generated in the propulsion system of the space

shuttle. The requirements are for the generation of a known (5^ or better)

sinusoidal pressure perturbation of l40 kPa (approx. 20 psi) peak-topeak at bias pressures up to 55 MPa (8000 psi) over a frequency range

from 1 Hz to 100 Hz. Vibrating a liquid column in a vertical plane

is one technique developed that achieves 53 kPa (7-7 psi) peak-topeak from 30 Hz to 150 Hz at all bias pressures, with smaller amplitudes

at lower frequencies, reaching about 6 kPa (0.9 psi) peak-to-peak at

10 Hz. Another technique, rotating a liquid column in a vertical plane,

shows promise at frequencies from 10 Hz down.

KEY WORDS

Calibration; differential pressure; dynamic; high pressure; orbiter

vehicle; pogo; pressure; sinusoidal; space shuttle; transducer.

Space Shuttle Pogo Pressure Measuring System

Progress Report for the Period

from January 15, 1973 to September 15, 1973

to the

George C. Marshall Space Flight Center

NASA Order #H-92100A

NBS Project 4253^^8

Prepared by

John S. Hilten

and

Paul S. Lederer

1. Introduction

The objectives of this project include the development of a dynamic pressure calibration technique for pogo pressure transducers,

for the measurement of oscillatory pressures generated in the propulsion system of the space shuttle and the evaluation of several com- mercial transducers for suitability for the measurement of these

pressures. The requirements for this measurement specify a full-scale

range of l40 kPa (approx. 20 psi) [1] peak-to-peak at bias pressures

up to 55 MPa (approx. 8000 psi) over a frequency range from 1 Hz to

100 Hz. Propulsion system tenperatures in the space shuttle are expected to range from about -253°C (-425°F) to about 482°C (900°F);

the transducers used for the pogo measurements will be required to

operate at some tenperature vd.thin that range, depending on the final

choice of their location within the propulsion system of the space

shuttle orbiter vehicle. The definition of objectives will be suitably modified at a later date to establish the actual operating con- ditions for these transducers

The current project efforts are directed toward the development

of a dynamic pressure calibration technique suitable for use at ambient

laboratory tenperature, the procurement of promising commercial offthe-shelf (or suitably modified) transducers, and the evaluation of

such transducers. A dynamic pressure corrparator is under development

by the Cryogenic Metrology Section of IJBS Boulder to make it possible

to compare the response of a pogo pressure transducer at cryogenic

terrperatures with the response of a similar transducer at laboratory

ambient conditions.

2. Development of Dynamic Calibration Techniques

[1] 1 kPa = 0.145 psi, 1 MPa = l45 psi

[2] See Reference 1
As indicated in the previous progress report [1], feasibility

has been demonstrated of a calibration procedure which subjects

pressure transducers to sinusoidal pressure variations at amplitudes

ranging up to 53 kPa (7-7 psi) peak-to-peak, and at frequencies from

10 Hz to 150 Hz in the presence '^f bias pressures from 0 to 55 MPa

(0 to approx. 8000 psi). The procedure, which involves a liquid-filled

tube mounted on an electrodynamlc vibration exciter (see Figure 1),

does not extend to the desired low-frequency limit of 1 Hz. Subseauent

efforts described in this report have largely been directed towards ex- tending calibration capabilities to lower frequencies. In addition,

certain problems encountered with the above technique were investigated,

and further work was done on a technique involving a double-ended

piston-cylinder driven by a vibration exciter.

There were three problems with the liquid filled tube calibration

system: (1) The halving of the generated pressure airplitude when the

tube is capped to superimpose a bias pressure with the consequence

that either the tube height, the liquid density, or the applied acceleration must be increased to obtain the same pressure, (2) observed

inconsistencies in generated pressure anplitudes with variations in

bias pressure up to l4 MPa (approx. 2000 psi), and (3) the inability

to cover the lower frequencies (1 - 10 Hz) because of displacement

limitations of available vibration exciters: for Instance, to achieve

even 20^ of the specified l40 kPa (approx. 20 psi) peak-to-peak pressure at 1 Hz would require 5-3 m of displacement using a 32-cm column

of oil, and 0.3 m of displacement with a mercury-filled tube.

In the case of the double-ended piston-cylinder calibrator,

frequency response capability, bias pressure capability, and modulation

pressure considerations looked promising in preliminary experimentation,

but the apparent sensitivity of the transducer calibrated with it, as determined from calculations using the test data, was only about ho%

of the predicted value in a number of tests.

In this report work done along these two calibration approaches

and four additional calibration approaches will be described.

2.1 Liquid Filled Vertical Tube Calibrator

2.1.1 Pressure Halving In Closed Tubes

The halving of the pressure amplitude generated with the closed

tube relative to the open tube is explained by the following considerations: In the open tube, the upper surface of the liquid is unconstrained and hence is the locus of a pressure node . At the lower

end of the tube the velocity is constrained to be zero (in the acceleration frame of reference) and acceleration (and hence pressure) is a maximum. Pressure is proportional, therefore, with distance from

the node (open end). When the tube is capped, the upper surface of

the liquid is, like the lower surface, constrained to zero velocity

2
(again in the acceleration frame of reference) and equal pressure •• maxima occur at the ends. From symmetry considerations, it can be seen that the pressure node must be located halfway between the

ends so that the pressures in the liquid column will range from zero

at the center to equal maxima, determined by the acceleration amplitude,

at the ends. These maximia, however, created by the liquid "Head"

over one-half of the tube length, are then only one-half of the value

for the open tube. The same considerations lead to the doubling of

resonance frequency of the closed tube system relative to the open

tube

To confirm our understanding of the halving of the generated pressure anplitude in a capped or closed system, a new tube was fabricated

in which the transducer was mounted 12 cm from the bottom of a 31-cm

tube (about hO% up from the bottom of the tube). Calibrations were

performed with the tube capped and with the tube open. The test was repeated with the transducer 6.1 cm from the bottom of the tube (about

20 ^ up from the bottom of the tube). The experimental data indicate,

as expected, that when the tube is open the dynamic pressure level is

determined by the height of the liquid column above the center of the

transducer multiplied by the density of the liquid and by the shaker

acceleration level. When the tube is capped, however, the dynamic

pressure level must be calculated by measuring height from the mid

point of the entire liquid column to the center of the pressure transducer. The transducer mounted at a point ^0% from the bottom of the

tube sees six times the pressure in the open tube that it sees in the

' closed tube. Figure 2 shows the data in graphical form for Transducer

A-3. The expected doubling of natural frequency for the capped tube

system was also observed.

2.1.2 Data Variation Due to Voids in Liquid-Filled Tubes

In many previous tests performed at bias pressure levels up to

l4 MPa there was a considerable scatter in data. It is now clear

that considerable care must be taken to see that the capped tube does

not contain a void. If this condition exists, the generated pressure

. at the bottom of the tube can vary anywhere from that in an open tube

to the half pressure (see previous discussion) generated in a corrpletely

filled tube. Even when the tube is properly filled, care must be taken

at low bias pressures. It can be seen, for instance, that with zero bias pressure in a capped tube which is properly filled, a slight

change in tenperature will produce either a positive bias pressure or

a cavity due to unequal expansion or contraction of liquid and tube.

The warmth generated by holding the tube in one's hand for a twominute period was sufficient to drive the bias pressure up approximately 3^0 kPa. As will be pointed out in a later section, the

liquid-filled system resonance frequency has been observed to be a smooth function of the pressure in the tube; this variation in resonance can be used to monitor bias pressure readily to within an estimated +170 kPa.

3
In an experiment where 6.89 MPa bias pressure was applied to a properly filled and capped tube and then reduced in successive steps

to 6.21, 5.52, 4.83, 4.14, 3.45, 2.76, 2.07, 1.38, 9.69, and 0.21 MPa

the total transducer sensitivity vaz^iation at 50 Hz was within 1.5%,

well within the estimated limit of error for the calibration. Table I

shows the values of sensitivity and resonance frequency as a function

of bias pressure. The transducer tested v/as Transducer A-3.

2.1.3 Transducer Response as a Function of Bias Pressure 15-150 Hz

A new calibrator tube was fabricated to permit the simultaneous

calibration of two pressure transducers. It is shown in Figure 1. In

this calibrator each transducer under test is mounted flush in each

of two flat steel plates, 4.1 cm square and 0.95 cm thick. These

plates in turn, are fastened with four bolts to machined flats on opposite sides of the block holding the vertical liquid-filled tube.

The seals between plates and flats are achieved with metal seal rings.

Transducers A-1 and A-2 were calibrated simultaneously in three

consecutive calibrations at bias pressures ranging from 6.9 to 55 MPa

over a frequency range fram 15 to I50 Hz and at pressure levels up

to 55 kPa peak-to-peak. These calibrations were performed at laboratory ambient conditions. No sealing tape was used for the transducers.

Representative values of sensitivity for the transducers at 6.9

MPa bias pressure, 50 Hz, are 3.98 mV/kPa for transducer A-1, and

3.71 mV/kPa for transducer A-2 (27.1 mV/psi and 25.7 mV/psi respectively). These measured sensitivities over three calibrations show

a spread of 0.37% for A-1 and 0.68% for A-2. Sensitivity values at

pressures from 6.9 MPa to 55.2 MPa show a spread of 3-9% and 1.8%

respectively. Our calibrations have an estimated total error of

+3.80%, consisting of an estimated total systematic rms error of +1.85%

plus three times an estimated total random rms error of +0.65%. Table

2 shows a tabulation of the data while figures 3 and 4 show the data

graphically. Table 3 is a compilation of the sources of error of

these calibrations, performed in the NBS transducer laboratory.

When the tube, liquid height and terrperature are unchanged, the

resonance frequency of this system is observed to be a smooth function

of the bias pressure, as shown in Figure 5. The value of resonance

frequency is thus a sensitive indicator of changes in the value of

bias pressure resulting from leakage or temperature change. It is

estimated that a change of +170 kPa can be detected. This change in

resonance frequency with bias pressure can not be accounted for wholly

by the change in liquid density (and thus the speed of sound in a

liquid); it is possibly a function of tube geometry.

It is planned to use this particular calibrator for the dynamic

calibration of all test transducers in the range from 10 Hz to 150 Hz.

To cover the low-frequency region between 10 Hz and 1 Hz, we are ih4
vestigating several schemes, of which the following one appears most

promising.

2.2 Low Frequency Calibration Techniques

2.2.1 The "Windmill" Calibrator

A brass tube, 1.6 cm inside diameter, and 2.2 cm outside diameter,

and 38 cm long, plugged at both ends, was attached to a similar tube

by means of a stud to make a 76-cm long structure. Transducer A-3

was then mounted in the end of one of the tubes after it was filled

with 35.6 cm of oil; this left an air gap of about 2.5 cm. The other

end of the structure was provided with an adjustable screw that was used for balancing purposes. The 76-cm structure was then mounted at

the end of a shaft that lies in a horizontal plane so that as the

shaft rotates, the 76 cm long structure rotates in a vertical plane.

See Figure 6. This shaft, supported on air bearings, equipped with

slip rings, and driven by a low-speed motor, is part of the prototype

Earth's Field I>ynamic Calibrator for Accelerometers [3] developed by

this laboratory.

During rotation the pressure developed at the diaphragm of the

pressure transducer at any constant rotational speed (frequency) is

conposed of both a static and a gravity conponent. The peak-to-peak

arrplitude of the gravity conponent is conputed from the height and

density of the liquid column multiplied by 2; the static, or steady

state, conponent is determined by the centripetal force as measured

at the center of the oil column and density of the oil. A correction

must be made for a transducer that responds to static pressure, but

for a piezoelectric transducer no correction is necessary.

Using this apparatus, transducer A-3 was calibrated from about

2 Hz to 9-5 Hz, and then the transducer tube was removed and remounted on the shaker for a liquid-filled-tube calibration from

10 Hz to 150 Hz. The two calibration methods agreed to within 1% at

the junction of the two sets of data. The dynamic pressure level was

5.9 kPa peak-to-peak; no bias pressure was used in this test. The

3-dB down point was extrapolated to be at 1.42 Hz. Figure 7 shows

the data graphically with the sensitivity at 25 Hz assigned a response

ratio of 1.0.

In a similar experiment, but using a shorter tube, the oil was replaced with mercury to generate a nearly sixteen-fold increase in

pressure range. While this was a preliminary investigation covering

only a frequency range from 2 Hz to 6 Hz, the results look promising.

The sensitivities determined using the mercury column agree with those

determined using the oil column to within 2%. A typical waveform is

shown in Figure 8.

It is planned to apply the full 55 MPa bias pressure to the oil

[3] See Reference 2

5
column in the next series of tests. The use of mercury will follow

the successful conclusion of this experiment.

To use this modified version of the Earth's Field Dynamic Calibrator below 2 Hz (rps) will require a slower, geared down, motor to provide smooth operation; to operate above 8 or 10 Hz (rps) will require

a better balancing system than the adjustable screw used in the end

of the nonactive tube.

This appears to be the most promdsing technique for generating

low frequency sinusoidal pressures and will be pursued further.

2.2.2 Extra Long Liquid-Filled Vertical Tube Calibrator

The maximum amplitude of the sinusoidal variations which can be

generated by vertical tube technique depends on three parameters:

liquid height, liquid density, and the acceleration amplitude to which

the column can be subjected. The maximum acceleration anplitude is

limited by the displacement capability of the electrodynamic vibration

exciter used. Since acceleration anplitude is proportional to the

product of the displacement and the square of the frequency, acceleration levels drop off rapidly with decreasing frequencies at constant

displacement. The vibrator available for this task is limited to a maximum displacement of 1.27 cm peak-to-peak; this assumes that the

weight of the calibrator can be conpensated by a spring so that the table

position of the vibrator can be centered within the mechanical limits

of table displacement. For a 30.5-cm column of oil of specific

gravity 0.86 the maximum peak-to-peak pressure variations which can

be generated with this shaker range from about 73 kPa at 20 Hz to

4.6 kPa at 5 Hz.

Corresponding to the three constraints on generated pressure am- plitude cited above, there are three ways of increasing this anplitude: (1) increasing displacement, (2) use of a liquid of greater

density, and (3) use of a longer tube. We have used the last in a series of tests in which column length was increased by a factor of

4.5 to 137 cm. In this test water was used primarily as a convenience

rather than for its slightly higher specific density, and the tube was

not capped. The tube was vibrated on the elctrodynamic vibrator at

frequencies ranging from 5 Hz to 20 Hz. A piezoresistive pressure

transducer B-1 was used at the bottom of the tube and a, static calibra,-

tion was performed with the transducer in place by superimposing a pneumatic bias pressure of first 6.9 kPa and then-6.9 kPa. The

average dynamic calibration value agreed with the static calibration

value to within Vfo and with the exception of one point the scatter of

the sixteen data points was within 0.5%. The data are shown in the

Table 4. It should be noted that at 5 Hz the generated pressure was

12.8 kPa peak-to-peak, about 2.8 times the pressure generated with the

short tube. More careful centering of the vibration shake table allowing full use of the double displacement would Increase the generated

6
pressure toward the theorectical 4.5 ratio in anplitude.

Calibrations were not performed below 5 Hz because of vibrator

waveshape limitations; calibrations were not performed above 20 Hz

because of the low, theoretical, 243 Hz column resonance and its

effect on the anplitude frequency response.

2.2.3 Liquid Filled Tube with Generator of Large Sinusoidal Displacement

In Section 2:2.2 it was noted that in a liquid tube system, the

range of generated pressure could be increased by larger shaker displacements. While there are commercially available shakers with

displacements up to 30 cm, they are expensive, have a limited frequency

range, and have long delivery dates. We decided to build a mechanical

device that would iirpart a large, rectilinear, sinusoidal displacement

to a liquid-filled tube. A 137-cm tube was constructed and mounted

vertically using two air bearings (the tube weight was counterbalanced

by an elastic cord) . A motor with an eccentric was then used to drive

the tube up and down by means of a connecting rod. Adjustment of the

connecting rod on the eccentric wheel provided peak-to-peak displacements ranging from 5-1 cm to 11.4 cm in 1.27-cm increments. The upper

bearing on the connecting rod was an air bearing; previous experience

in developing the Earth's Field Dynamic Calibrator for Accelerometers

[4] strongly suggested the use of air bearings to isolate the tube from

motor-, gear- and bearing-vibration; mechanical vibrations transmitted

to the liquid-filled tube create spurious pressures which are superiirposed on the desired sinusoidal pressure.

At this stage of development the pressure signal generated Is too

noisy to be of use; when the motor was disconnected and the tube displaced by hand a clean signal was generated, verifying that the drive

mechanism is creating the problem. Figure -14 shows a picture of the

motorized calibrator; no data are shown because of the poor waveform

obtained. Some efforts may be directed toward inproving generated

wave shapes, if, contrary to e^q^ectations , all the other approaches

should prove to be unsuccessful.

2.2.4 Liquid Filled Tube on the Dual Centrifuge

The dual centrifu^G- developed for the calibration of accelerometers [5] w'as raodll'led in an experimental set-up to calibrate pressure transducers. The centrifuge consists of a 6l-cm diameter main

turntable mounted in a horizontal plane and a second smaller turntable, 12.7 cm in diameter, mounted eccentrically on the larger

turntable. By means of gears and belts, both turntables are constrained

[4] see reference 2

[5] see reference 3

7
to rotate at the same speed. The speed of rotation can be adjusted

from 0.5 rps to 25 rps. The distance between the paralled shafts

of the two turntables can be set to any value between 0 and 30.5 cm. The dual centrifuge was modified by attaching an oil-filled tube to

the smaller turntable in a horizontal plane; this oil-filled tube,

pressurized to 6,9 MPa, had a pressure transducer in one end and was capped at the other end. With the centrifuge in motion at constant speed the acceleration at the center of mass of the tube can

be calculated from the square of the rotational speed and the distance

of the center of mass from the center of rotation of the main turntable. Prom this the pressure seen by the transducer is calculate

as with the vertical tube. The generated pressure is a majcimum with

the tube in a radial position with respect to the main turntable and

the pressure transducer at the far end of the tube; l80 degrees later,

with the tube again in a radial position but with the pressure transducer close to the center of rotation of the main turntable, the

generated pressure is a minimum. Transducer C-1 was thus calibrated

at TOtational speeds from 2 rps to 6.5 rps (2 to 6.5 Hz). Figure

9 shows the wavefom at 3 rps and 6 rps. Since the generated pressure is four times as great at 6 rps as it is at 3 rps, the signal

to noise ratio is much more favorable at 6 rps; the noise is electrical in nature rather than vibrational. T^e calculated values of

peak-to-peak pressure range from abour 9.0 kPa at 2 rps to 92.4 kPa

at 6.5 rps.

The experimental data are given in Table 5. As in all other

tests, the transducer sensitivity is corrputed by dividing the measured transducer output by the value of pressure calculated from

acceleration, tube length and liquid density. Since the transducer

response for this type is described by the manufacturer as being "down

3 dB at 1 Hz", the data were corrected fro this. If one assumes a

siirple RC roll-off, a response which is 3 dB (30^) down at 1 Hz will

be down at 6 Hz. The corrected experimental values were referred

to the value measured at 6 Hz and the resulting curve plotted in

Figure 10. Also shown is a theoretical curve for a system "down

3-dB at 1 Hz". Extrapolation of the cui^es suggests that the actual

3-dB frequency for this transducer system is close to lv2 Hz.

It should be noted that experimental values of sensitivity obtained by this method were about 8^ lower than those reported by the

manufacturer. This discrepancy is still under investigation. Because of this and the more conplicated test equipment, this technique

is not likely to be as satisfactory as the use of the sinple, vertical tube or the "windmill" technique.

2.3 Modified Double-Ended Piston-Cylinder Calibrator

i Attempts to use a double-ended piston-cylinder calibrator were

[6] See Reference 1

8
described in the last report [6]. The concept appeared promising

initially due to its ability to use the vibration exciter's blockedmotion power output at very low frequencies. Experimental data showed

large discrepancies, which were thought due to remediable design flaws.

Accordingly a calibrator was designed and fabricated with four

major modifications from the previous model:

(1) the clearance between the piston and cylinder was reduced

from 2.5^ x 10^ cm to 2.5^ x 10^ cm;

(2) the piston and cylinder were made of steel, hardedned, and

ground;

(3) the piston diameter was reduced from 3.8l cm to 1.27 cm;

(4) the "0" rings on the piston were elljrn.nated.

A sketch of the modified device is shown in Figure 11 and a picture is

shown in Figure 12.

The force exerted by the shaker on the yoke was measured by a load cell sandwiched between the yoke and the shaker head. The generated pressure was corrputed from the known diameter of the piston.

Transducer A-3 was then mounted on the calibrator and a calibration

attenpted. The results were disappointing.

The waveform at all frequencies was poor; rotating the piston

back and forth by hand radically improved the waveform suggesting

that friction is still a problem. There were changes from 1% to 235? in transducer output, with load cell output and bias pressure held

constant, at frequencies from 10" to 50 Hz. There were 171^ to 46^

changes in pressure transducer output v;hen the load cell output and

frequency were held constant, and the bias pressure was changed from

0.69 MPa to 6.9 MPa. There were ^1% to 104^ changes in pressure

transducer output when the bias pressure and frequency were held

constant and the load cell output was increased by 40^. The graph

in Figure 13 shows the data. Even when frequency, bias pressure, and

load cell output were held constant, the output of the pressure transducer measured at one minute intervals over a period of 15 minutes

continuously increased. We have decided to abandon this approach to

the dynamic calibration of pressure transducers since we have found

more promising techniques (see 2.1.3 and 2.2.1).

3. Transducer Procurement

In the previous Progress Report we noted that a prototype pressure-sensing system using a semiconductor strain gage pressure transducer had been orxiered in February 1973 with delivery expected in early

August 1973. Delays in the receipt of materials and conponents by

the manufacturer, have delayed delivery.

In May 1973, a contract was let for a prototype pressure sensing

system using a variable-impedance transducer, and suitable for use

9
at high tenperatures . This system was received in early September

1973. A cursory dynamic calibration showed the expected low output

of about 1 mV for a O.69 MPa peak-to-peak pressure fluctuation.

Further evaluation was deferred, pending a static calibration of the

system over the full range of bias pressures.

A semiconductor strain-gage pressure transducer was orxiered in

March 1973 with delivery promised for Aiogust 1973. The delivery date

currently promised is for late November 1973.

Finally, in view of the satisfactory performance exhibited by

the piezo-electric pressure transducers previously obtained, two

additional ones were ordered. They were delivered early in September.

They have not yet been tested.

4. Future Plans

We plan to calibrate all transducers by means of the liquid-filled

vertical calibrator (see 2.1.3) in its latest version which permits

simultaneous calibration of two transducers. We hope to use the "windmill" calibrator (see 2.2.1) for extending the low-frequency calibration capability, thus covering the entire frequency range of interest.

When this has been accoirplished at pressure amplitudes below the desired levels, we plan to use mercury or other high density fluids to

extend the amplitude capability of the calibration technique.

Upon the successful demonstration of the capability of the "windmill" generator at high bias pressures, we expect to redesign and

rebuild this device to meet the calibration objectives of this task.

A mathematical treatment of the capped tube arrplitude-halving and

frequency-doubling phenomena is being developed.

As soon as the dynamic pressure comparator in the NBS Cryogenic

Metrology Division at Boulder is completed, we will send the previously

evaluated transducers to that laboratory for tests at cryogenic tenperatures.

5. References

1. Space Shuttle Pogo Pressure Measuring System. Prop-ress RePO-»^t ^^or

the period endir^ Januarv 15 > 1973, Report 135, Instrumentation

Applications Section, NBS, February 15, 1973

2. Accelerometer Calibrations with the Earth's Field Dynamic Calibrator,

NBS Technical Note 517, March 1970

3. A Dual Centrifuge for Generating Low Rrequencv Sinusoidal Accelerations, NBS Report #5730, 195o

10
TABLE I

CALIBRATION OF TRANSDUCER A-3

Sensitivity at 50 Hz and Resonant Frequency Variation

Prom 6.89 MPa to 0.21 T^Pa (1000 psi to 30 psi)

Resonant Frequency

Hz

Bias Pressure

MPa psi

Transducer Output

at +10 g, TiiV rms

2227 6.89 1000 32.12

2224 6.21 900 32.28

2222 5.52 800 32.38

2217 4.83 700 32.46

2212 4.14 600 32.56

2208 3.45 500 32.60

1 2206 2.76 400 32.58

2200 2.07 300 32.52

2196 1.38 200 32.50

2193 0.69 100 32.42

2188 0.21 30 32.36

2233 6.89 1000 32.32

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TABLE 3

HYDRAULIC SINUSOIDAL CALIBRATION

ESTIMATED CALIBRATION UNCERTAINTIES

PCB TRANSDUCERS AT + 20 g, 50 Hz

Source of Error Systematic

Percentage

of reading

Random

Et'T'dt' Percentage

of reading

Static Pressure

Estimated error, height of liquid

+0.08 cm, 30.5 cm Estimated error in liquid density due to

+2°C tenperature variation,

"(petroleum oil, specific gravity O.858)

+0.26

+0.15

Acceleration

Accelerometer sensitivity: 7.23 mV/g

Charge anplifier gain value

Noise:

Output voltage: 2000 mV (manuf. value)

Accuracy of calibration: +(0.05% reading

+ 0.05% range)

Repeatability + resolution,

estimated +5mV

+1.00

+1.50

+0.30

+0.50

J-O OCT +0 . 25

Transducer output

Output voltage: 70.00 mV

Accuracy of calibration: +(0.05% reading + 0.05% range) manuf.

Repeatability + resolution, estimated +0.2 mV

+0.12

+0.29

Estimated total systematic error rms= Estimated total r-andon error, rms=

Estjjnated total error (S.E. + 3 R.E.)=+3.80%

+1.85%

+0.65%

13
TABLE 4

DYNAMIC CALIBRATION OF PIEZORESISTIVE PRESSURE TRANSDUCER

B-1

USING A 137-cm WAIER-FILLED TUBE

Frequency

Hz

Generated

kPa

Pressure

psi

Transducer Sensitivity

mV/kPa mV/psl

5 12.8 1.86 1.19 8.21

6 15.6 2.26 1.18 8.11

7 18.3 2.66 1.17 8.07

8 21.2 3.07 1.17 8.05

9 23.9 3.47 1.17 8.04

10 26.8 3.88 1.17 8,05

11 29.5 4.28 1.17 8.06

12. 32.4 4.70 1.17 8.04

13 35.2 5.11 1.17 8.04

14 38.1 5.52 1.17 8.06

15 40.7 5.91 1.17 8.09

15 43.6 6.32 1.17 8.07

17 46.3 6.71 1.17 8.09

18 48.9 7.09 1.18 8.12

19 51.5 7.47 1.18 8.12

20 53.9 7.82 1.18 8.11

1.17 8.08

Average Average

Static calibration 1.16 mV/kPa 8.0 mV/psi

System theoretical resonance frequency, 243 Hz

14
TABLE 5

CALIBRATION OF PIEZOELECTRIC

PRESSURE TRANSDUCER C-I ON THE DUAL CENTRIFUGE

rps

(Hz)

Transducer

Output

mV rms

Conputed Pressure

kPa psi

peak-to-peak

Sensitivity

mV/kPa mv/psi

Frequency

Response

%

2 16.6 0.88 1.28 532 36.7 86.9

2.5 27.2 1.37 1.99 561 38.7 91.6

3 40.07 1.98 2.87 573 39.5 93.6

3.5 55.00 2.69 3.90 578 39.9 94.5

72.60 3.52 5.10 584 40.3 95.4

4.5 93.20 4.45 6.45 593 40.9 96.9

5 115.5 5.49 7.96 595 41.0 97.3

5.5 140.7 6.64 9.63 599 41.3 98.0

' 6 168.5 7.90 11.46 603 41.6 98.6*

6.5 198.2 9.27 13.45 604 41.7 98.8

Note: This test was performed at a bias pressure of 6.9 MPa

*The value for 6 Hz is based on the manufacturer's data for frequency

response

Values at the other frequencies are relative to the 98.6% response at

6 Hz.

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6 Hz (6 rps)

WAVE SHAPE OF PRESSURE GENERATED IN OIL FILLED TUBE

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FIGURE 12: MODIFIED DOUBLE ENDED PISTON-CYLINDER DYNAFIIC

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GENERATOR

DRIVEN DISPLACEMENT
NBS-1 14A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. puhi.ic:ation or ri:por r no. NBSIR 74-562

2. Gov't Accession

No.

3. Recipient's Accession No.

4. Trri.i-: and SLiin'iTi.i:

SPACE SHUTHZ POGO PRESSURE MEASURING SYSTEM

A PROGRESS REPORT

5. Publication Date

// K^.J/ 1 1 —

6. Performing Organization ( ode

7. AU rHOR(S)

John S. Hilten and Paul S. Lederer

8. Performing Organ. Report No. NBSIR 74-S6?

9. PICRFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 4253448

11. C^ontract/Grant No.

NASA ORDER #H92100A

12. Sponsoring Organization Name and ("omplete Address (Street, City, State, ZIP)

NASA G. C. Marshall Space Flight Center

Huntsville, Alabama 35812

1 o. 1 ype or Keport & Keriou Covered interliTi

1/15/73 to 9/15/73

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

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

bibliography or literature survey, mention it here.)

This progress report describes a variety of experimental approaches

to the dynamic calibration of pogo pressure transducers for the

measurement of oscillatory pressures generated in the propulsion

system of the space shuttle. The requirements are for the generation of a known (5% or better) sinusoidal pressure perturbation of

l40 kPa (approx. 20 psi) peak-to-peak at bias pressures up to 55 MPa

(approx. 8000 psi), over a frequency range from 1 Hz to 100 Hz,

Vibrating a liquid column in a vertical plane is one technique

developed that achieves 53 kPa (7-7 psi) peak-to-peak from 30 Hz to

150 Hz at all bias pressures, with smaller amplitudes at lower

frequencies, reaching about 6 kPa (0.9 psi) peak-to-peak at 10 Hz.

Another technique, rotating a liquid column in a vertical plane,

shows promise at frequencies from 10 Hz down.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons

)

Calibration; differential pressure; dynamic; high pressure; orblter vehicle;

pogo; pressure; sinusoidal; space shuttle; transducer.

18. AVAILABILITY r_ Unlimited 19. SECURITY CLASS

(THIS REPORT)

21. NO. OF PAGES

For Official Distribution. Do Not Release to NTIS

UNCL ASSIFIED

31

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Washington, D.C. 20402. SD Cat. No. CH

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