Surge Warning System

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CALLAB TECHNICAL NOTE TN209

ENGINE MEASUREMENT SERIES

Design &Development of LowDifferential PressureCalibration Standard


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GAS TURBINE RESE ARCH ESTABLISHMENT

Design & Development of Low

Differential Pressure Calibration

Standard

Gas Turbine Research EstablishmentPost bag 9302

C.V. Raman NagarBANGALORE 5600923

Phone: 91-080-25040501 Fax: 91-080-25241507


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Guidelines for Calibration Of Pressure Transducers

List of Contents

INTRODUCTION :............................................................................................................................ 4

1 SYSTEM REQUIREMENT.............................................................................................................4

1.1 GENERALREQUIREMENT:.......................................................................................................................4

1.2 PERFORMANCE:....................................................................................................................................4

1.3 ELECTRICAL: ......................................................................................................................................5

1.4 MECHANICAL: ...................................................................................................................................5

1.5 ENVIRONMENTAL: ...............................................................................................................................5

1.6 FUNCTIONS: ........................................................................................................................................5

1.7 FEATURES: ..........................................................................................................................................6

2 PRINCIPLE OF OPERATION.......................................................................................................7

2.1 QUARTZ RESONANT PRESSURE TRANSDUCERS: .......................................................................................7

2.1.1 INTRODUCTION: .................................................................................................................................7

2.1.2 CONSTRUCTIONAND OPERATION:........................................................................................................7

2.1.3 PRECISION PRESSURE VOLUME ADJUSTER: .........................................................................................9

2.1.4 PRINCIPLEOFPRESSUREGENERATION ................................................................................................10

3 SYSTEM DESIGN..........................................................................................................................11

3.1 OVERALL SYSTEMDESCRIPTION .......................................................................................................11

3.2 PNEUMATIC SUBSYSTEMDESIGN ...........................................................................................................12

3.3 MEASUREMENTHARDWARE: ................................................................................................................12

3.4 SYSTEMSOFTWARE..............................................................................................................................13

3.5 MEASUREMENT...................................................................................................................................14

4 SYSTEM INTEGRATION............................................................................................................16

4.1 SYSTEM CAPABILITY .........................................................................................................................16

4.1.1 FUNCTIONS:.....................................................................................................................................16

4.1.2 PRESSUREGENERATION: ...................................................................................................................16

4.1.3 PRESSURE MEASUREMENT: ..............................................................................................................16

4.1.4 DISPLAYCAPABILITY: ......................................................................................................................16

4.1.5 FEATURES........................................................................................................................................17

4.2 INTERFACINGA DUT..........................................................................................................................17

5 CONCLUSION...............................................................................................................................18

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List of figures

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INTRODUCTION :

To establish the surge margin of developmental aircraft gas turbine engines using

experimental method requires the engine to be operated to the threshold of aerodynamic

instability while avoiding subjecting the engine to deep surge, which may be seriouslydetrimental to engine health. A surge warning system is developed to monitor aerodynamic

disturbances in the compressor, detect the onset of surge and issue a warning signal that can

be used for shutting down the engine. The system uses a bi-directional total pressure probe tosimultaneously sense the total pressures in the direction of flow and opposite to it. The differential

pressure resulting from total pressures in forward and reverse direction is measured and monitored.

The measurement hardware is dynamic signal acquisition and analysis system that acquires

the signal and the onset of surge is detected by the software using pre defined thresholds. The

systems detects the surge condition and issues a solid state relay potential free contact within

30 ms to enable the control system to cut off the fuel flow to the engine to prevent a sustained

surge and thereby safeguarding the engine.

1 SYSTEM REQUIREMENT

Differential pressure transducers used for the purpose of measuring engine intake

pressure are Piezoresistive pressure transducers. A typical such transducer has a range of up

to 15 kPa differential and operates at a static pressure level of the local atmospheric

pressure which is around 91 kPa. These transducers have a static error band of 0.1 % of FS

and a thermal error band of 1.5 % of FS over a 100 C band, typically from 20 C to 80 C.

Sometimes capacitance gauge transducers are used for measurement of intake pressure at low

rated tests where pressure range is limited to a range of 3.5 kPa or 7 kPa differential. These

transducers typically have an accuracy of 0.25 % of the FS.

Since this measurement is of paramount importance to the engine performanceevaluation, it is necessary that these transducers are calibrated periodically to ensure the

accuracy of measured data. At the time when the need for precise calibration of these sensors

arose, no commercially available system was suitable for this purpose. Therefore it was

decided to indigenously develop a low differential pressure calibration system.

1.1 General requirement:

It was necessary for the system to be able to calibrate various transducers used for air

intake pressure measurement at different ratings, i.e. the system shall be able to calibrate

differential pressure sensors in the range of 3.5 kPa differential full scale to 15 kPa

differential. The system was expected to perform necessary operations involved in a lowdifferential pressure calibration.

1.2 Performance:

The system performance is expressed in terms of quality of measurement, i.e. range,

accuracy, resolution, long-term stability and temperature effect.

Range (differential pressure): 15 kPa bi-directional

Range (static pressure or common mode pressure): 80 kPa to 110 kPa (commonly used

atmospheric pressure range)

Accuracy: The required accuracy was 0.01 % of FS keeping in view that measurement

accuracies for intake pressure are absolutely critical and the same reference can be used for

transducers ranging from 3.5 kPa to 15 kPa full scale.

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Resolution: The system needed to have a resolution of 1 ppm (1 part in 10 6) to be enable to

observe the effect of pressure variation, especially while operating at the low end of the scale

Long term stability: Long-term stability of 0.01 % of FS per year was needed to ensure

quality of measurement between annual calibrations

Temperature effects: It was necessary to eliminate the temperature effect by suitable

compensation since that is a major source of error.

1.3 Electrical:

The system needed to use electrical transducers for the purpose of processing,

displaying and storage of calibration information. The electrical subsystem needed to include

all the necessary means of powering the transducers, measurement of transducer output and

any other electrical signal required apart from generating any control signal required for

implementation of any desired features (described later).

1.4 Mechanical:

The entire pneumatic subsystem assembly needed to be packed in a compact tabletop

instrument enclosure (19 inch rack mount with 4 U height). The pneumatic subsystem

assembly was inclusive of reference transducer(s), any utility sensor used, pressure generator,

switching valves, interconnecting tubing, mechanical and electrical connectors. All electrical

connections between the pneumatic subsystem and the measurement hardware needed to be

carried out using circular connectors. The pressure connections provided to connect a device

under test need to be of Swagelok type.

1.5 Environmental:

The target system was intended to be used in a calibration lab with environmental

control. The system was expected to correct effects of temperature by suitable compensation.

Since the lab environment was free from extreme humidity condition, static electricity,

EMI/RFI, vibration and acoustic noise the system was free from these design constraints.

However, it was necessary to ensure that the system does not generate EMI/RFI, acoustic

noise or vibration levels that would affect other measurements in the lab.

1.6 Functions:

The system under discussion needed to provide the following operational functions to

the operator.(i) Display of barometric pressure, differential pressure, their minimum and

maximum values, over pressure and over temperature warnings, power supply

limit checks, DUT output etc.

(ii) Control of equalizing and vent valves through software front panel

(iii) Manual control of equalizing and vent valves on instrument front panel

(iv) Viewing and editing of calibration coefficients and instrument configuration

(v) Viewing on line plot of barometric pressure since the equipment was switched on

(vi) Viewing of unscaled (raw) outputs of the sensors

(vii) Manual adjustment of static and differential pressure from front panel

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2 PRINCIPLE OF OPERATION

The principle of operation of such a device is based on generation and precise

measurement of reference pressure and comparing it with the response of the device under

calibration.The principle of reference pressure measurement is based on measurement of low

differential pressure using a quartz resonant pressure transducer and carrying out the

necessary corrections. The corrections include corrections for temperature effects and

common mode pressure. Temperature is measured using a temperature sensor built into the

pressure transducer and the compensation is carried out by the software using a polynomial

expression. Common mode pressure measured using a quartz resonant barometric pressure

transducer and the compensation is carried out using an equation.

Pressure generation technique is based on a manually operated precision pressure

volume adjuster to set the common mode as well as differential pressure in conjunction with

equalizing and vent valves.

2.1 Quartz Resonant Pressure Transducers:

2.1.1 Introduction:

Accuracy, stability, and reliable performance under difficult environmental conditions

are key performance requirements for transducers used in calibration systems. Transducers

employing quartz crystal resonator technology meet these requirements. The design and

performance requirements include

(1) Inherently digital outputs,(2) Accuracy comparable to the primary standards,

(3) Highly reliable and simple design,

(4) Minimum size, weight and power consumption,

(5) Insensitivity to environmental factors, and

(6) Long-term stability.

2.1.2 Construction and Operation:

The resonant quartz crystal transducers are designed to have resolution better than

0.0001 % and a precision of better than 0.01% of reading maintained even under difficult

environmental conditions. The remarkable performance is achieved through the use of aprecision quartz crystal resonator whose frequency of oscillation varies with pressure induced

stress. Quartz crystals are chosen for the sensing elements because of their remarkable

repeatability, low hysteresis, and excellent stability. The resonant frequency outputs are

maintained and detected with oscillator electronics similar to those used in precision clocks

and counters. Several flexurally-vibrating, single or dual beam, load-sensitive resonators are

used for different designs. The Double-Ended Tuning Fork consists of two identical beams

driven piezoelectrically in 180o phase opposition such that very little energy is transmitted to

the mounting pads. The high Q resonant frequency, like that of a violin string, is a function of

the applied load increasing with tension and decreasing with compressive forces.

The digital temperature sensor consists of piezoelectrically driven, torsionally

oscillating tines whose resonant frequency is a function of temperature. Its output is used tothermally compensate the calculated pressure and achieve high accuracy over a wide range of

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temperatures.The barometer mechanisms employ bellows as the pressure-to-load generators.Pressure acts on the effective area of the bellows to generate a force and torque about the

pivot and compressively stress the resonator. The change in frequency of the quartz crystal

oscillator is a measure of the applied pressure. Temperature sensitive crystals are used for

thermal compensation.

The mechanisms are acceleration compensated with balance weights to reduce theeffects of shock and vibration. The transducers are hermetically sealed and evacuated to

eliminate air damping and maximize the Q of the resonators. The internal vacuum also serves

as an excellent reference for the absolute pressure transducer configurations. Since any

changes in the reference vacuum directly affect the barometric output, great care is taken to

ensure that there are no leaks and minimal outgassing in the evacuated housing. Because the

quartz crystal constrains total mechanism movement to several microns full scale,

reproducibility is excellent.

Figure 1 Load and Temperature resonators

Figure 2 Barometer mechanism

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Figure 3 Paroscientific quartz transducer

2.1.3 Precision Pressure Volume Adjuster:

Precision-pressure volume controllers provide a quick-and-easy method for precisely

setting a pressure in a closed pneumatic system. Heise HVC 1000 is used for pressure

adjustment. Once the HVC unit is connected to a pneumatic system, the volume of the

chamber becomes part of the volume of the system. The pressure-adjust knob at the front of

the unit repositions the piston within the chamber through interaction with a precision-

machined lead screw. Piston movement within the chamber increases or decreases the

volume of the system, depending on the direction of movement. In a closed system where gas

cannot leak out upon compression or be drawn in upon expansion, this volume change results

in a change in the internal pressure. Increasing the volume by moving the piston toward thefront of the HVC unit will decrease the pressure. Conversely, decreasing the volume by

moving the piston toward the rear of the unit will increase the pressure. The pressure change

generated by a given amount of piston travel is proportional to the change in volume as

compared to the total system volume.

An integral balance valve provides a means for equalizing pressure on both sides of

the piston prior to making the final adjustments when setting the pressure. This minimizes the

resistance encountered when repositioning the piston and assures ease of pressure setting.

The balance valve also serves as a pressure- relief valve, assuring that the differential

pressure across the piston does not reach unsafe levels.

HVC units can also be used without a compressed air source for the generation of

moderate levels of positive pressure and vacuum. The high resolution of the HVC, combinedwith the ability to generate pressure and vacuum, make it an ideal tool for low pressure

calibration and test as well as higher pressure calibration and test activities.

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Figure 4Pressure volume adjuster

2.1.4 Principle of pressure generation

The pressure generation of pressure can be divided in two steps. First the static

pressure is set in the system and then the differential pressure is generated using Precision

Pressure Adjuster. At first the whole unit is vented to atmosphere. The equalizing valve

between positive and negative pressure port of differential pressure sensor is kept open so

that the static pressure level in the system can be set. The static pressure level is adjusted now

using precision pressure adjuster keeping in view of the static pressure limits, however over

pressure or under pressure protection is taken care by software through software controlled

solenoid valve.

The second step is to set the differential pressure. To set the differential

pressure first the equalizing valve is closed to isolate the positive and negative pressure port.

The precision pressure volume adjuster is adjusted now to generate the desired differentialpressure.

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3 SYSTEM DESIGN

3.1 Overall System descriptionThe low differential pressure standard uses three major subsystems, the pneumatic subsystem including thesensor assembly, measurement hardware and the system software. Block diagram of the full system is

shown below:

Figure 5 System Block Diagram

Figure Captions:

1) Optional insulated storage tank for positive reference pressure stability

2) Variable bi-directional volume control for +/- 15 kPa (2 psig) pressure generation

3) Shunt valve (open to set static pressure, closed to set differential pressure)

4) Static (absolute) pressure monitor (a Paroscientific Model 216B)

5) Dynamic overpressure shunt valve (low cost pressure monitor that opens shunt valve

outside differential pressure range)

6) High-precision low-pressure transfer standard 202BG

7) Optional shutoff valve

8) Positive reference pressure outlet

9) Vent or static pressure inlet (80 to 110 kPa)

10) Inlet shutoff valve

11) Optional static pressure storage tank for prolonged stability

12) Optional shutoff valve

13) Static pressure outlet

14) Barometric Overpressure sensor and shutoff valve (low cost pressure monitor that opensshunt valve outside barometric pressure range)

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15) Data Logger

16) Buffer-tubes for shock isolation of sensitive sensors

17) Computer

18) Counter

3.2 Pneumatic Subsystem design

As shown in figure 6 both the low and high-pressure lines are connected to two

thermally insulated reservoirs to maintain thermal stability of pressure. A precision pressure

volume adjuster is used to generate the pressure differential between high and low lines. The

pressure applied to the low line before setting the differential pressure is the common mode

pressure. The common mode pressure is measured using a Paroscientific 216B quartz

resonant transducer. To monitor the over pressure condition one utility sensor, a Keller 33X

transducer is used. Differential pressure is measured using a Paroscientific 202BG quartz

resonant transducer. Over pressure condition is monitored using a Druck PMP 4060, which isa utility sensor. An ON/OFF valve is used as a manually operated equalizing valve and a

normal closed solenoid valve is used as an automatic equalizing valve, primarily used for

safety purposes while setting the differential pressure. Second ON/OFF valve is used as a

manually operated vent valve and another normally closed solenoid valve is used as

automatic vent, primarily for safety purposes. The picture of the pneumatic subsystem is

shown below

Figure 6 Picture of Pneumatic System

3.3 Measurement hardware:

The measurement hardware is used for measuring the output signals of the standard

utility sensors, the sensors under calibration and the power supply voltage and currents. An

Agilent 34970A data logger is used for multiplexing the frequency signals corresponding to

the pressure and temperature outputs of the quartz sensors and the multiplexed output is

measured by an Agilent 53132A, 12 digit frequency counter with ultra high stability OCXO

time base. The counter is integrated to the system using a GPIB to Ethernet interface

converter. The voltage and current outputs for the utility sensors and the sensors under test is

multiplexed and measured in the data logger using its internal 6 digit DMM. The power

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supply voltage and currents are measured directly from the power supply via the serial

Ethernet converter. The picture of the measurement hardware is shown below

Figure 7 Picture of Electronic System

3.4 System software

The system software runs on a host PC that communicates with the instruments and hosts the web server.

The counter is connected to Ethernet via a GPIB Ethernet connecter. The power supplies and the data

logger are connected to Ethernet via a 4 port serial to Ethernet converter. Any PC connected to the networkcan be either the host PC or the remote PC. The system software displays the various measured values and

alarm conditions as well as it takes necessary user inputs. A separate configuration screen allows user to

modify the settings of the instrument and calibration coefficients of the transducers. A baro graph displaysthe barometric pressure graph form the time the instrument is switched on till the current time. Thesoftware also generates the report in MS excel format using the set/measured values and necessary user

inputs.

Figure 8 Software front Panel

As shown in the picture the reference baro pressure, reference differential pressure and their minimum and

maximum values, utility baro pressure, utility differential pressure, power supply voltage and currentstatus, over pressure status, and test sensor output are available on the screen. Buttons available on thefront panel allow user to go to individual screens like raw signal values, configuration screen or Baro

graph screen. The raw signal shows the un-scaled voltage or current values of sensors, primarily used for

debugging purpose in case of any problem. The configuration displays the various instrument setting and

calibration co-efficient of sensors and allows user to modify it if required. The following picture shows theconfiguration screen

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Figure 9configuration screen

Baro graph displays the barometric pressure variation over time, from the time the instrument is switchedon till current time. The following picture shows the barograph

Figure 10 Baro graph screen

3.5 Measurement

The quartz measurement sensors provide two frequency outputs, one for pressure and one for temperature.For barometric pressure the frequency to pressure converter is carried out using the following equation:

Temperature Co-efficient:

X = temperature period (sec)

U = X-U0

Temperature: (deg C)

Temp = Y1U+Y2U2+Y3U3

Pressure co-efficient:T = pressure period (sec)

C = C1+C2U+C3U2

D = D1+ D2UT0 = T1+T2U+T3U2+T4U3+T5U4

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Pressure: (psia)

=

2

2

0

2

2

0111

T

TD

T

TCP

Where, U0, Y1, Y2, Y3, C1, C2, C3, D1, D2, T1, T2, T3, T4 & T5 are coefficients obtained from calibration.

For accurate measurement of low differential pressure the knowledge of common mode pressure is

required. Computation of differential pressure requires the frequency output conversion to pressure and

temperature and the value of common mode pressure and uses the following equation.

Temperature Co-Efficient:

X = temperature period (sec)U = X-U0

Temperature: (deg C)

Temp = Y1U+Y2U2+Y3U3

Pressure co-efficient:

T = pressure period (sec)C = C1+C2U+C3U2

D = D1+ D2U

T0 = T1+T2U+T3U2+T4U3+T5U4

Pressure: (psia)

=

2

2

0

2

2

0111

T

TD

T

TCP

Pcorr=P+a1(Ps-Po)+a2(Ps-Po)2+b1(Ps-Po)P

Where, U0, Y1, Y2, Y3, C1, C2, C3, D1, D2, T1, T2, T3, T4 & T5 are coefficients obtained from calibration.

a1, a2, b1 & Po are common mode coefficients.Pcorr= Sensor pressure using common mode correction

P = Indicated pressure using standard CD equation

Ps = Static pressurePo = Reference static pressure

a1, a2 = Zero common-mode correction terms

b1 = Span common-mode correction term

The outputs of the piezoresistive utility sensors are converted to pressure values a polynomial of second

degree as shown below

P=Ax2 + Bx + CWhere, P= pressure being measured

x = sensor output in voltage/currentA, B & C are calibration co-efficient

The sensor under calibration is characterized by generating a second degree polynomial from the generated

reference pressure and from the measured sensor output. The software has a provision for calibration of a

barometric test sensor adjusting the barometric reference pressure.

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4 SYSTEM INTEGRATION

The low differential pressure calibration standard system can be divided into

pneumatic sub system and electronic sub system like power supply, counter and Ethernet

servers. The pneumatic subsystem is integrated in a 19 inch portable 3 U rack. The portabledesktop rack houses the reference sensors, utility sensors, precision pressure volume adjuster,

thermal stabilization chambers, manual and solenoid valves for vent and equalizing

interconnected using various types of tubes and tube fittings. The electrical interface is

provided in the form of circular connecter at the back end of the rack. This connector is

connected to power supply, counter unit and data logger for measurements externally. The

electronic unit i.e. power supply, counter and data logger is connected to Ethernet using

GPIB ENET Ethernet server and four port serial server. The counter is connected to Ethernet

using GPIB ENET server and other electronic devices are connected using four port serial

server.

The Pneumatic system is pressures with the valves closed and leak check is done. The

pneumatic system is placed inside the rack with pressure port and vent port provided outsidefor user accessibility. The full system is integrated with a personal computer using

indigenously developed software. The full control and status monitoring of the system can be

done from the front panel except pressure generation.

4.1 System Capability

4.1.1 Functions:

This system uses an innovation whereby it doubles up as a low differential pressure

calibration standard as well as a calibration standard for barometric pressure. The hardwareand software capability built in is designed in such a way that the system can be operated in

any one of the modes described above. In the following section the ability of the device to

serve as a differential pressure and barometric pressure standard will be described together.

4.1.2 Pressure generation:

The system is capable of manual generation of differential as well as static pressure. Static

pressure can be generated in the range of 80 kPa to 110 kPa and bidirectional differential

pressure can be generated in the range of 7 X 10 -3 Pa and 15 kPa. Pressure generation has a

resolution of 7 X 10 -3 Pa.

4.1.3 Pressure Measurement:

The system carries out measurement of differential as well as common mode pressure and

displays them on the screen. Static pressure can be measured in the range of 80 kPa to 110

kPa with a resolution of 0.005 kPa and bidirectional differential pressure can be generated in

the range of 7 X 10 -3 Pa and 15 kPa with a resolution of 7 X 10 -3 Pa.

4.1.4 Display capability:

The system displays the instantaneous values of barometric and differential pressure

measured by reference sensors, minimum and maximum values of these since instrument

switch on, barometric and differential pressure measured by utility sensors, over range

indications for the measured parameter for system health monitoring and DUT output. It also

allows the operator to view the calibration coefficients of sensors, unprocessed outputs of

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reference and utility sensors and a graph of barometric pressure since the time instrument was

switched on.

4.1.5 Features

Automatic overpressure protection: This system has built in automatic overpressure

protection. The overpressure capability of the quartz sensors are only 1.2 times the full scalepressure. Therefore they need to be protected from accidental overpressure. Two utility

sensors, one in the barometric range and the other in the differential are used for monitoring

the pressure levels. In the event of overpressure the equalizing solenoid valve and the vent

solenoid valve are opened by the software. This helps in equalizing and venting the system,

relieving the transducers of any possible overpressure.

Thermal Stabilization chambers: Both sides of the differential pressure arrangement are

connected to independent thermal stabilization chamber. These chambers serve the purpose

of stabilizing the pressure fluctuations or variations that could be caused by change in

ambient temperature or operation of the equipment. In low pressure measurement

stabilization is extremely important without which high resolution can not be obtained.

Web enabled: The measurement hardware used for measurement of outputs of sensors arecommercially available off the shelf products. These are controlled by the system software

running on a PC. These instruments are fitted with serial (RS 232) or GPIB interfaces.

However, unlike traditional instrument control approach, here connectivity between the PC

and the devices is not using dedicated bus, and is via Ethernet. The instruments use interface

converters that enable them to get connected to the existing LAN. Any PC running the

software over LAN will then be able to communicate and control these instruments. Except

pressure generation all other features are automated and can be controlled with appropriate

authorization over LAN/internet.

4.2 Interfacing a DUT

This system is capable of automatic calibration by integrating the DUT into the

calibration process i.e. the system can measure the DUT output with reference to the applied

pressure and can generate a calibration report. The hardware is capable of accepting all

available type of DUTs and provides user selectable and configurable measurement of DUT

voltage, current or frequency. Operator can select the option of calibrating a DUT for low

differential or barometric range. The software can generate a report based on the calibration

performed.

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5 CONCLUSION

This calibration standard is used as a working standard and an uncertainty of 100 ppm

is achieved which is the best available from electronic transducer. This performance is

comparable to laboratory grade differential dead weight tester with the added advantage of

ease of an electronic instrument and automated calibration. Remote control and monitoringvia web enables effective supervision of the calibration process. The modular design

approach enables the system to be built at the fraction of the cost of such commercial systems

and provide enormous flexibility by adding the software features that cannot be implemented

by standalone instruments. This revolutionary approach to develop web enabled virtual

instrument based calibration setups is likely to hold the key to the future of calibration

systems.

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Surge Warning System