ABB 268D/V User manual

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Operating instruction
IM/268D_1 2600T Series Pressure Transmitters
Models 268D/V/P

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Health and Safety
To ensure that our products are safe and without risk to health, the following points must be noted:
1. The relevant sections of these instructions must be read carefully before proceeding.
2. Warning labels on containers and packages must be observed.
3. Installation, operation, maintenance and servicing must only be carried out by suitably trained personnel and in accordance with the
information given. Any deviation from these instructions, will transfer the complete liability to the user.
4. Normal safety precautions must be taken to avoid the possibility of an accident occurring when operating in conditions of high
pressure and/or temperature.
5. Chemicals must be stored away from heat, protected from temperature extremes and powders kept dry. Normal safe handling
procedures must be used.
6. When disposing of chemicals ensure that no two chemicals are mixed.
Safety advice concerning the use of the equipment described in this manual or any relevant hazard data sheets (where applicable) may
be obtained from the Company address on the back cover, together with servicing and spares information.
The Company
ABB is an established world force in the design and manufacture of
instrumentation for industrial process control, flow measurement, gas and
liquid analysis and environmental applications.
As a part of ABB, a world leader in process automation technology, we offer
customers application expertise, service and support worldwide.
We are committed to teamwork, high quality manufacturing, advanced
technology and unrivalled service and support.
The quality, accuracy and performance of the Company’s products result from
over 100 years experience, combined with a continuous program of innovative
design and development to incorporate the latest technology.
The NAMAS Calibration Laboratory No. 0255(B) is just one of the ten flow
calibration plants operated by the Company, and is indicative of ABB
dedication to quality and accuracy.
Use of Instructions
Warning.
An instruction that draws attention to the risk of injury or
death.
Caution.
An instruction that draws attention to the risk of damage
to the product, process or surroundings.
Although Warning hazards are related to personal injury, and Caution hazards are associated with equipment or property
damage, it must be understood that operation of damaged equipment could, under certain operational conditions, result in
degraded process system performance leading to personal injury or death. Therefore, comply fully with all Warning and
Caution notices.
Information in this manual is intended only to assist our customers in the efficient operation of our equipment. Use of this manual
for any other purpose is specifically prohibited and its contents are not to be reproduced in full or part without prior approval
of Technical Communications Department, ABB.
✶
Note.
Clarification of an instruction or additional information.
Information.
Further reference for more detailed information or
technical details.
ABB
EN ISO 9001: 1994
Cert. No. Q5907
ISO 9001: 2000
Cert. No. 9/90A
Cert. No. 0255
0255

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Section Page
INTRODUCTION.............................................................. 3
TRANSPORT, STORAGE, HANDLING AND
PRODUCT IDENTIFICATION .......................................... 4
SAFETY PHILOSOPHY, MANAGEMENT OF
FUNCTIONAL SAFETY AND INFORMATION
REQUIREMENTS ............................................................ 5
LIFE-CYCLE ACTIVITIES ................................................ 6
FAULTS OUTSIDE THE FUNCTIONAL SAFETY ........... 8
PRINCIPLE OF OPERATION .......................................... 9
INSTALLATION.............................................................. 12
ELECTRICAL CONNECTIONS ..................................... 13
ELECTRICAL REQUIREMENTS .................................. 15
COMMISSIONING AND CONFIGURATION
ISSUES .......................................................................... 16
CALIBRATION ............................................................... 17
PRE-STARTUP ACCEPTANCE TEST,
PROOF TESTS AND
SAFETY RELATED PARAMETERS .............................. 19
DISMANTLING AND REASSEMBLY ............................ 20
SIMPLE FAULT FINDING .............................................. 22
RETURNING FORM ...................................................... 23
ADDENDUM FOR "METERS" OPTION OF
THE TRANSMITTERS ................................................... 24
ADDENDUM FOR COMETER OR INDICATOR
WITH HART PROGRAMMING CAPABILITY AND
PROMETER - PROGRAMMABLE INDICATOR ............ 25
ADDENDUM FOR PV-SCALING OPERATION ............. 30
ADDENDUM FOR "SURGE PROTECTOR" OPTION
OF THE TRANSMITTERS ............................................. 31
ADDENDUM USE OF HARDWARE LINKS
ON THE SECONDARY ELECTRONIC.......................... 34
ADDENDUM FOR DIFFERENTIAL PRESSURE
TRANSMITTERS : SELECTABLE OUTPUT
FUNCTIONS .................................................................. 35
ADDENDUM FOR FLANGE-MOUNTED
TRANSMITTERS ........................................................... 41
ADDENDUM FOR "EX SAFETY" ASPECTS
AND "IP" PROTECTION (EUROPE) ............................. 47
CONTENTS INTRODUCTION
The 2600T series is a modular range of field mounted,
microprocessor based electronic transmitters, using a unique
inductive sensing element. Accurate and reliable
measurement of differential pressure, gauge and absolute
pressure, flow and liquid level is provided, in the even most
difficult and hazardous industrial environments.
Now a Safety pressure Transmitter is included in the 2600T
Series, with its analog output signal plus HART digital
communication.
The HART digital protocol allows remote re-ranging,
calibration and diagnostics, without any interference with the
standard 4-20 mA analog output signal.
This operating instructions manual describes the Safety
version of the 2600T Series transmitters and specify all
information necessary to safely connect the Safety 2600T
pressure transmitter in a Safety Instrumented System.
It details also how the signals from the input field device should
be interpreted.
Refer to the shortened contents of this manual, here in this
pagefor addressing thesection ofyour interest,and alsoto the
supplementary documentation for additional remarks.
Reference information on remote seals and configuration of
the transmitter can be found in the following documents:
SS/S26 Remote Seal Specification
SS/268xx Data Sheets
IM / 691HT Hand-Held Communicator
Online HELP SMART VISION Configuration Program
IEC 61508 Functional Safety of e/e/pe Safety-related systems
ISA S84.01 Application of Safety Instrumented Systems for
the Process Industries
NE43 Standardization of the signal level for the breakdown
information of digital transmitters
Other help ful or general information can be found in the ABB
web site, at www.abb.com
SUPPLEMENTARY DOCUMENTATION

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Important - The instrument serial number must always be quoted when making enquiries.
PRODUCT IDENTIFICATION
The instrument is identified by the data plates shown in
Figure 1.
The Nameplate (ref.A) provides information concerning the
code number, maximum process working pressure, range and
span limits, power supply and output signal. See code/
specification sheet for detailed information. This plate also
shows the transmitter serial number.
Please refer to this number when making enquiries.
A dedicated label (ref. B) is welded as standard to the primary
unit, carrying specific details of the transducer (diaphragms
material, fill fluid, range limit and identification number).
A Safety Marking plate ( ref. C) is fitted when the transmitter
is required to comply with hazardous area regulations, e.g.
flameproof or intrinsic safety protection. Additionally Tag plate
(ref. D) provides the customer tag number and calibrated
range, maximum process working pressure (PS) and tempera-
ture (TS).
The instrument may be used as a safety accessory (category
IV) as defined by the Pressure Equipment Directive 97/23/EC.
In this case, near the CE mark, there is the number of the
notified body (0474) that verified the compliance.
Fig. 1 - Product identification
Ref. B
Primary Unit
TRANSPORT
After final calibration, the instrument is packed in a carton
(Type 2 to ANSI/ASME N45.2.2-1978), intended to provide
protection from physical damage.
STORAGE
The instrument does not require any special treatment if
stored as despatched and within the specified ambient
conditions level (Type 2 to ANSI/ASME N45.2.2-1978).
There is no limit to the storage period, although the terms of
guarantee remain as agreed with the Company and as given
in the order acknowledgement.
HANDLING
The instrument does not require any special precautions
during handling although normal good practice should be
observed.
Ref. C
Ref. D
Ref. A
DIN TYPE
HOUSING
BARREL TYPE
HOUSING
SERIAL
NUMBER
URL
DIAPHRAGM
MATERIAL
FILL
FLUID
3rd and 4th numerals show
the year of construction
Ref. D
(Ex) - - X X - - - -
Nameplate
Ref. A

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SAFETY PHILOSOPHY
The Safety 2600T Pressure Transmitters are field devices
designed according to the requirements of the standard
IEC61508 for the Safety Related Systems. Standard currently
used focus on individual parts of all the safe instrumentation
used to implement a safety function. The IEC61508 defines
requirements related to all the system that normally comprises
initiating devices, logic solver and final elements. It also
introducesthe conceptof Safetylifecycledefining thesequence
of activities involved in the implementation of the safety
instrumented system from conception through
decommissioning. For a single component it is not correct to
define a SIL level. The term SIL (Safety Integrity Level) refers
to the complete safety loop therefore the single device shall be
designed in order to be suitable to achieve the desired SIL level
in the entire Safety Loop.
Application
The Safety 2600T Pressure Transmitters are intended to be
applied for safety relevant application in the process industry.
They are suitable to be used in SIL2 applications. Special
attention has to be given to the separation of safety and non-
safety relevant use.
Physical Environment
The transmitter is designed for use in industrial field
environments and must be operated within the specified
environmental limits as indicated in the Transmitter Data
Sheet.
Role an Responsibilities
All the people, departments and organisations involved in the
life-cycle phases which are responsible for carrying out and
reviewingthe applicableoverall,E/E/PES (Electrical/Electronic/
Programmable Electronic System) or software safety lifecycle
phases of a Safety Instrumented System shall be identified. All
those specified as responsible for management of functional
safetyactivitiesshallbeinformedof theresponsibilitiesassigned
to them. All persons involved in any overall, E/E/PES or
software safety lifecycle activity, including management
activities, should have the appropriate training, technical
knowledge,experienceandqualificationsrelevantto thespecific
duties they have to perform.
MANAGEMENT OF FUNCTIONAL
SAFETY
For each application the installer of the owner of a safety
systemmust preparea SafetyPlanning whichmust beupdated
throughout the Safety Life-cycle of the Safety Instrumented
System. The requirements for the management of functional
safety shall run in parallel with the overall safety lifecycle
phases.
Safety Planning
The Safety Planning shall consider:
• policies and strategies for achieving safety;
• safety life-cycle activities to be applied, including names of
responsible persons and departments;
• procedures relevant to the various life-cycle phases;
• audits and procedures for follow up.
INFORMATION REQUIREMENTS
The information shall comprehensively describe the system
installation and its use in order that all phases of the overall
safety lifecycles, the management of functional safety,
verification and the functional safety assessment can be
effectively performed.
Overall Safety Life-cycle Information
The overall safety lifecycle shall be used as the basis for
claiming conformance to the standard IEC61508. The lifecycle
phases consider all the activities related to the Safety
Instrumented System (SIS) from the initial concept through
design, implementation, operation and maintenance to
decommissioning.

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LIFE-CYCLE ACTIVITIES
Application Scope
Definition of the Application Target
The process equipment shall be described in order to define
clearly the application target with its hazard potential.
Applicable LAWS and Standards
All applicable general Laws and Standards related to the
allowed operations of the equipment, as EU-Directives shall
be collected. The plant owner shall produce a Regulatory
Requirements List document.
Definition of the Application Scope
The scope for the safety-related application shall be fully
described in order to produce the following documentation:
- Safety Integrity Level classification;
- Functional safety requirements of the equipment under
control
Necessarysteps forthedefinition oftheabove listeddocuments
are:
-Detailed investigation about which potential hazards of the
process equipment have been reduced by design or an
independent layer of protection.
-Checking of the necessary functional requirements required
by the applicable laws and Standards.
-Determination of the Safety Integrity Level with a specific risk
reduction method.
-Specification of each functional risk reduction by its physical
risk,properties tobemeasured, itssafeactiontobe performed
Functional Safety Requirements of the Target
Equipment
Safety Functions
The documents:
-Safety Requirement Specification;
-Piping and Instrument Diagram;
Shall be produced in order to fully define the safety functions
of the Safety Instrumented System. Necessary steps for the
definition of the above listed documents are:
-Definition of the required Safety Functions.
-List of all the process conditions under which the safe action
is required.
-Investigation of the effect of common cause failures.
-Specification of the actions required for the process
measurement failures which are not covered by the Safety
Functions.
-Identification if the required safe actions are dependent on
operating states or are effective under all operating states.
-Transformation of the verbal functional requirements into a
graphical form.
Process Interface
The documents:
- Functional Requirement Specification;
- Piping and Instrument Diagram;
- Functional Diagram
Shall be produced in order to fully describe the process
interface and connections. Necessary steps for the definition
of the above listed documents are:
-Definition of the Process Interface requirements.
-Identification of the instrumentation for every physical risk
property (input) and define their fail safe signal.
-Definition of the required amount of instruments and
certifications according the SIL requirements
-Identification of the type of actuator and definition of their fail
safe position for the required safe action
-Definition of the required redundancy and certification
-Completion of the functional diagram with instrumentation
details
-Definition of the necessity of a regulatory body approval;
System Safety Requirement Assignment
I/O System Response Time
The total system response time is determined by the following
elements:
- Sensor detection time,
- Logic solver time;
- Actuator response time;
The total system response time must be less than the process
safety time. To ensure a safe operation of the system, the scan
rate of each section of the logic solver multiplied by the number
of channels must be less than the safety time less actuator and
sensor response time.
I/O System Selection
The I/O system selection is mainly dictated by the required
logicsolvertime.Appropriateselectionprocedures andanalysis
shall be used.
System Structure
System configuration drawings shall be available to describe
the equipment and interfaces required for a complete
operational system. The system must be fully operational
before start-up.
Safety Requirement Allocation
Each safety function, with its associated safety integrity
requirement, shall be allocated to the designated safety-
relatedsystems takingintoaccount theriskreductionsachieved
by the other technology safety-related systems and external
risk reduction facilities, so the necessary risk reduction for that
safety function is achieved. The allocation indicated shall be
done in such a way that all safety functions are allocated and
the safety integrity requirements are met for each safety
function.
Programming Environment
Computer system which provides the necessary software to
program, compile, and load an application shall be separated.
Safety Routines
Safety additional requirements may be defined in order to
ensure the correct functionality of sequences in the Safety
Instrumented System.
Safety Templates
Safety Templates must be followed for particular applications.
(e.g. SIL 2 and burner management applications have certified
"Templates" that adhere to all the rules spelled out by the
applicable regulations).
Separation of Safety Functions
Each safety function shall be separated in a different
programming section.

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Application Software Development
Programming Environment
The application software of the Safety 2600T has been
developed in ANSI C language. Emulation and system testing
have been performed with the support of Mitsubishi ICE
development system.
Program Structure for Safety Applications
Thecomplete softwarehas beenseparated ina safetyrelevant
and a non-safety relevant sections. The safety relevant area is
constituted by a set of modules and functions which are
rigorously separated and checked in their correct execution.
Safety Logic Programming
A specific document has been developed to define the basic
rules for C-Programming in safety related system applications
in compliance with what defined by the IEC 61508-3. The
software development of the Safety 2600T has been carried
out following the restrictions and recommendation contained
in the above mentioned documents.
Program Compilation
Specialaccuracyhave beenused inthe softwaredevelopment
in order to avoid any error and warnings.
Application Software Testing
A Safety 2600T transmitter functional test report document
has been issued after the operational and the safety related
program have gone through their initial check. It verifies that
the program will perform as desired and specified.
Application Software Safety Validation
The Safety 2600T Application Software testing has been
carried out and audited by TUV. A Test Report document
approved by TUV states that the system reacted in each test
as expected and that the safety related program fulfil the
Safety Requirement Specification
Installation
Environmental Requirements
The Safety 2600T pressure transmitter has been designed to
operate in a wide range of environmental conditions typical of
industrial field and in hazardous environments. The
environmentalconditionsunder whichthemeasuringequipment
is designed to operate within its specified accuracy limits and
withoutimpairment ofits operatingcharacteristicsarespecified
in the "Specification Sheet" document.
Mechanical installation and System completion
All the necessary operations to correctly installing the device
in order to assure operator and plant safety are described in
the section "installation" of the present manual.
System Wiring
The procedures to safely make the electrical connections of
the device are described in the section "electrical connections"
of the present manual. For installation in hazardous areas,
compliance with safety information on the safety marking plate
shall be ensured.
Commissioning
Field Instrument Functionality
All the necessary activities to assure that the process sensor
orfinal elementare operatingtogetherandperformthe required
function are described in the "Electrical connections" and
"Calibration" sections of the present document.
Overall System Functionality
The activities to validate the required safety functionality of the
system together with the target equipment according to the
Safety Requirement Specification are Pre-Startup Acceptance
test section of the present document.
Operation
System Operating Discipline
A Plant policy guideline document containing the specific plant
policy guideline for the daily safe operation has to be produced
and periodically reviewed by representatives of the Process
Control Service.
Maintenance
Maintenance is defined as the routine activities which are
carried out to detect unrevealed faults.
Preventive and Routine Maintenance
Preventive and routine maintenance activities are defined in
the maintenance section of the present manual.
Function-unit Replacement
In case of hardware failure corrective actions may be carried
out. In case of transmitter replacement all the operations
described in "Electrical Connection", "Calibration" and "Pre-
Startup Acceptance tests" shall be conducted.
All maintenance activities shall be documented in the system
documentation.Possible safetycriticalfailures shallbereported
using the Incident Report process.
Function-unit Repair
The transmitter is constituted of two main units (transducer and
electronics). It can be repaired following the information
contained in the Dismantling and Reassembly section of the
present manual.
Central repair shall maintain a record of detected failures,
calculate actual failure rates and compare with the expected
failure rate. Extensive failure rates shall be communicated to
the supplier.
Modification Request
Request of modification due to possible safety critical failures
and performance deviations shall be reported to the factory.
Modificationsshall followthecompanymodificationprocedures.
Management of Change
All process changes or SIL category change shall follow the
procedures defined in the safety life-cycle of the system and
shall be reviewed and validated by the external competent
body for a new functional safety assessment.
Management of change Process Components and Roles
Each process component needs to be defined in details
accordingto therequirements andthe relevantdocumentation.
Each process component change shall follow the activities
defined in the overall safety life cycle.
Management of change Documentation and Training
Requirements
TheManagementof Changeprocessshallfollow documentation
andtrainingrequirementsdefinedinthe systemimplementation.
. . . . LIFE-CYCLE ACTIVITIES

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FAULTS OUTSIDE THE FUNCTIONAL SAFETY
The redundant algorithms and the electronics are designed to detect all the internal hardware faults therefore the transmitter
diagnostic is not able to detect faults related to the process and to the installation configuration. In the following table the known
weaknesses resulting from the transducer FMEA (Failure Mode and Effect Analysis) are listed.
Assembledmaterialat thepipesofthe transmitter,
blockage of pipe.
Application outside specified temperature range.
Excess of temperature
Assembledgasatthetransmitter,if thetransmitter
is mounted above the process line
Overload pressure, high peak pressure pulses in
process lines
Penetration of hydrogen, diaphragm crack in
applications with hydrogen process medium.
Thin walled diaphragm, leaky diaphragm in
applications with abrasive medium.
Thin walled diaphragm, leaky diaphragm in
applications with corrosive medium.
Higher diaphragm stiffness, crack in application
with contamination of metal ions
Mechanical damage through cleaning, damage
of the coating, corrosion.
Other considerations
The alarm levels of the transmitter (down-scale or up-scale)
can be selected by the user. For some faults (e.g. crystal
breakdown), the output will latch at 22 mA even if the down
scale alarm level is selected.
failure failure effect Comments
∆p-level
measurement is wrong
wrong measurement
insensitive, wrong
measurement
wrong measurement after
compression stress
insensitive measurement,
breakdown
wrong measurement,
breakdown
wrong measurement,
breakdown
insensitive
measurement
faulty or insensitive
measurement,breakdown
Piping should be periodically
inspected and cleaned.
The transmitter should operate
inside the specified temperature
ranges.
Transmitter should be installed
properly as specified in this
manual.
The transmitter should operate
inside the specified temperature
ranges.
Hydrogenserviceallowedwiththe
application of a special grace on
diaphragms or by using gold
plated diaphragms.
Transmitter manual specifies the
preventiveperiodicmaintenance.
Appropriate materials should be
selected for corrosive
applications.
Appropriate materials should be
selected for particular
applications.
Transmitter manual specifies
correct maintenance
procedures.

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The instrument consists of two functional units:
- Primary Unit
- Secondary Unit
The Primary Unit includes the process interface, the sensor
and the Primary electronics; the Secondary Unit includes the
electronics, the terminal block and the housing. The two units
are mechanically coupled by a threaded joint. Electronics are
based on custom integrated components (Application Specific
Integrated Circuit - ASIC).
PRINCIPLE OF OPERATION
The principle of operation of the Primary Unit is as follows. The
process fluid ( liquid, gas or vapour ) exerts pressure on to the
sensor diaphragm via flexible, corrosion-resistant isolating
diaphragms and capillary tubing containing the fill fluid (see
Fig. 2a).
As the sensor diaphragm deflects in response to differential
pressure changes, it simultaneously produces variations in the
gap between two fixed magnetic circuits (comprising coil and
ferrite core) positioned on both sides of the measuring
diaphragm. As a result, the inductance of each coil changes.
The two inductance values L1 and L2, and the sensor
temperature ST are combined in the primary electronics to
provide a proprietary standardized signal.
In the manufacturing process the sensor output characteristics
are compared with reference pressures and temperatures: the
"mapped"parameters arethen storedin thememory ofPrimary
electronics.
Themeasuredvalues andthesensorparameters aretransferred
to the Secondary Unit, where a microprocessor computes
precise primary output linearisation, compensating for the
combined effects of sensor non linearity, of static pressure and
temperature changes. In the permanent memory of the
secondary electronics are stored the transmitter specific
information:
-non modifiable data such as the serial number, the UID
(UniqueIdentifier),the manufacturer'sname anddevice type,
the hardware and software version of the electronics.
-the modifiable data such as the final trimming and calibration,
in other words, all data that can be changed by the user
through the configuration devices.
External Zero/Span
adjustments
Fig. 2b - Secondary Unit
Output meter
(option)
Surge protector
(option)
Terminal
block
Housing Electronics
RFI filter
Resin potting
Primary Electronics
Printed Circuit
Sensor Diaphragm
with Ferrite Disks
Process
chamber
Capillary tubing
Inductance Coils
& Magnetic
Cores
Isolating
diaphragm
Fig. 2a - Primary Unit
Capillary tubing
Process
chamber
Isolating
diaphragm

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. . . PRINCIPLE OF OPERATION
2600T Safety Transmitter takes advantage of the intrinsic
redundancy of the 2600T series differential inductive sensor.
The two inductive signals are separately detected in the
primary unit by two independent ASICs and separately
elaborated internally to the electronics. Calculations follow
independentflows andthey arecomparedin themicrocontroller
in order to validate the output pressure signal. If a difference
between the two measurements is detected the analog output
is driven to a safety condition. Internal diagnostics algorithms
are implemented to check correctness and validity of all
processing variables and the correct working of memories.
A supplementary shut down circuitry provides a safe shut down
when a fault occurs in the analog section of the electronics. The
output stage is also checked by reading back the analog output
signal. The feedback loop is obtained by an additional A/D
converter put at the end of the output stage, which translates
the 4-20 mA signal into a digital form suitable to be compared
by the microcontroller.
HARDWARE DESCRIPTION
General hardware description
The electronic hardware structure is described in the following
figure.
Pressure Sensor
The pressure sensor gives the primary input signal to the
electronics. The input pressure is converted in a (micro)
displacement"d"of ametal diaphragm(measuring diaphragm)
whose stiffness determines the URL of the sensor. The
diaphragm displacement changes the gap of a magnetic
circuit,generating thevariation ofthe inductivepick-up detector
constituted of two inductances called L1 and L2. One of the
inductance values increases the other decreases. The
inductance value is measured by forming an oscillator with an
extra capacitor (C1,C2). The oscillation is excited by a pulse
and simultaneously measured by two ASIC5 (see the picture
above). The fundamental frequency of oscillation relates to the
inductance values with the following law: (T=2PI*SQR(LC)).
Temperature Sensor
The temperature sensor measures the temperature of the
pressure sensor. The resulting value is used by the µP for
temperature compensation purposes.
SAFETY 2600T Pressure Transmitters
block diagram
ASIC A
Primary
Electronics
EEPROM
EEPROM
A/D
converter
ASIC7
µP
Secondary
Electronics "µP"
ASIC B
T5 Generator
1.watchdog
2.watchdog
&
L1
L2
P1
P1
Pressure
Sensor
Temperature
Sensor
PWM
Filter
TX2
TX1
Calibration
Parameter
Sensor
Parameter
Display
Reset
Reset
HART
modem
Tx
4...20 mA
reading
HART
modem
Rx
Power
supply
protection
GND
const.
current
generator
4÷20 mA
second
shut
down
>20 mA
4...20 mA
10.5...42 V
Key
Span
Key
Zero
Secondary
Electronics "PS"
VCC
D0...7 A0...9
Pressure sensor
Ferrite
L2
L1
P2
P1
Measuring
diaphragm
d1 (gap)
d2 (gap)