🟦 Part 1. Introduction and Purpose of the Secondary Instrumentation Cabinet

🔹 1.1. Why the Secondary Instrumentation Cabinet is Needed

In Custody Transfer systems, the secondary instrumentation cabinet is the central element that connects measuring sensors and primary devices (flowmeters, temperature transducers, densitometers, level gauges) with computing devices (flow computers, correction modules, and archivers).

Its main functions are:

• ⚙️ to accumulate all incoming signals;
• 🔄 to convert them into formats convenient for processing;
• 🛡️ to ensure reliability, safety, and legal protection of accounting;
• 🛠️ to create a convenient platform for operation and maintenance.

In other words, it is in the cabinet that all channels are “brought together” into a single accounting system. The accuracy of accounting, the legal validity of custody transfer acts, and the convenience of commissioning engineers in daily work all depend on how competently it is designed.

🔹 1.2. The Role of “Olimps” in Design

The experience of SIA Olimps has shown: a proper cabinet is not only electronics and cables, but also a philosophy of maintenance.
Over the years, the company’s engineers have developed a number of practical solutions that ensure:

• ⚡ Ease of commissioning: breakable terminal blocks arranged in logical sequence, access to each signal point possible without disconnecting wiring;
• 🛡️ Legal protection (Legal Shield): all critical circuits are sealed, archival data is stored in compliance with API, OIML, and national standards;
• 🔋 Durability and reliability: reserve in power supply, DIN rails, and ventilation allows cabinets to operate for decades without redesign;
• 🙌 Acknowledgment from commissioning engineers: minimization of manual operations during calibrations and service.

🔹 1.3. Why This is Critical

The design of the secondary instrumentation cabinet directly affects:

1. 🎯 Measurement accuracy – minimization of losses and signal distortions, protection from interference.
2. 📑 Legal validity of data – presence of archives, sealing, independence of channels.
3. 🛡️ Safety – application of intrinsically safe barriers, separation of Ex circuits.
4. 🛠️ Operation – ease of calibration, absence of cable chaos, minimal time for diagnostics.
5. 💰 Cost of ownership – the fewer errors and redesigns, the cheaper the maintenance.

🟦 Part 2. Main Tasks and Architecture of the Secondary Instrumentation Cabinet

🔹 2.1. Main Tasks of the Cabinet

The secondary instrumentation cabinet is not just a “box with wires.”
Its purpose is much broader and includes:

1. 📡 Signal collection
   • reception of analog (4–20 mA, RTD, thermocouples), discrete, pulse, and frequency signals;
   • integration of digital protocols (Modbus, HART, RS-485/422, Ethernet).
2. 🛡️ Conversion and protection
   • galvanic isolation, intrinsically safe barriers (Ex i);
   • protection against overvoltage, impulse noise, and lightning surges.
3. ⚖️ Ensuring legal protection (Legal Shield)
   • sealing of terminals, archives, and settings;
   • audit trail, events, intervention logs;
   • ensuring metrological traceability.
4. 🔧 Service convenience
   • breakable terminal blocks for easy verification and calibration;
   • modular structure allowing new channels to be added without redesign;
   • minimal cable lengths for connecting reference instruments (≤1.5 m), ensuring reliable protection against interference.
5. 🔁 Redundancy and reliability
   • reserve in power supply, DIN rails, and modules;
   • duplication of flow computers (N+1 scheme);
   • presence of service ports and convenient access to each block.

🔹 2.2. Cabinet Architecture by Sections

The cabinet is divided into logical sections. Such layout has proven its efficiency in Olimps projects:

1. ⚡ Power Section
   • Input 230 VAC / 24 VDC;
   • power supplies, UPS/DC-UPS;
   • surge and lightning protection;
   • circuit breakers.
2. 🟦 Intrinsically Safe Section (Ex i)
   • barriers and galvanic isolators;
   • separate blue-colored DIN rails;
   • physical separation from “ordinary” circuits.
3. 📑 Signal Section (Marshalling)
   • AI, AO, DI, DO terminal blocks;
   • separate groups for pulse/frequency inputs;
   • RTD/TC;
   • digital interfaces (RS-485, Modbus, HART).
4. 💻 Computing and Communication Section
   • flow computers, correctors;
   • Ethernet switches, RS-485/422 converters;
   • time synchronization (GPS or optical link);
   •  
5. 🖥️ Operator / Service Area
   • HMI panel or service ports;
   • USB/Ethernet connectors for maintenance;
   • sealable doors and locks.

🟦 Part 3. Practical Design Features (Olimps’ Experience)

🔹 3.1. Olimps’ Approach to Secondary Instrumentation Cabinets

Practice has shown: it is the design details that determine the ease of operation and trust in the system.
Therefore, cabinets designed by Olimps engineers are valued by commissioning and operating personnel. These cabinets not only comply with standards but also solve real tasks on site — from quick connection of instruments to protection against unauthorized interventions.

🔹 3.2. Breakable Terminal Blocks

• 🔌 Breakable terminal blocks are used instead of regular ones.
• During calibration or verification, the engineer can simply open the terminal without disconnecting the wire and without disturbing the installation.
• This significantly reduces service time and minimizes the risk of cable damage.

Advantage: commissioning engineers have noted for years that this saves hours of work and reduces the number of errors when connecting reference instruments.

🔹 3.3. Minimum Cable Lengths Inside the Cabinet

• 📏 Cables inside the cabinet should not exceed ~1.5 m.
• This approach eliminates the need for “extensions,” reduces interference and losses.
• Everything is arranged compactly, within easy reach.

Result: fewer failure points, simpler signal tracing, higher measurement reliability.

🔹 3.4. Intrinsically Safe Barriers

• 🔷 All circuits from hazardous areas are connected via Ex i barriers.
• They are mounted on blue DIN rails for visual identification.
• Barriers are physically separated from other terminals and modules, simplifying checks and eliminating wiring errors.

🔹 3.5. Sealing

• 🔒 Breakable terminals and critical channels are sealed after calibration.
• This prevents unauthorized interventions and guarantees the legal validity of the data.
• Seals are recorded in protocols, and any violation is immediately visible.

Legal Shield: sealing is a key element of legal protection.

🔹 3.6. Marking

Proper marking is one of the key factors determining the ease of operating a secondary instrumentation cabinet.

Marking tasks:

• 📖 Provide quick orientation for a commissioning engineer in the cabinet without the need to leaf through multi-volume project documentation.
• 🔄 Standardize the designation system so that work can be carried out even by an engineer from another organization without special training for the specific site.
• 🚫 Prevent errors during calibration, verification, and repair.
• 🔧 Simplify the introduction of new channels and system modernization.

Olimps practice:

• All terminals, cables, and modules have clear and readable marking plates.
• Standard industrial marking systems are used (e.g., DIN EN 81346, IEC 60617 for structural designations, ISO 14617 for graphic symbols).
• Color codes are applied:
  • 🟦 blue for intrinsically safe circuits (Ex i);
  • 🔴 red for emergency signals;
  • 🟨 yellow for discrete inputs/outputs;
  • 🟩 green for grounding and protective circuits.
• Markings are resistant to external conditions: thermal transfer inscriptions resistant to UV and moisture.

Result:

• Any engineer quickly finds the required signal.
• Errors when connecting reference instruments are eliminated.
• Diagnostic and maintenance time is reduced.
• Legal protection (Legal Shield) is enhanced, as the risk of “human factor” due to misidentification of circuits is eliminated.

📌 Operating experience confirms: proper marking means time savings and increased reliability for decades.

🔹 3.7. Redundancy and Reserve

• 🔄 In design, Olimps always leaves a reserve:
  • free DIN rails;
  • spare power sources;
  • additional terminals.
• This allows new instruments or channels to be added without redesign.

Advantage: the cabinet operates for decades without requiring complete replacement during modernization.

🔹 3.8. Service Convenience

• 🏷️ All terminals and modules are signed with readable markings.
• Access to each circuit is free, without “forests” of wires.
• Service connectors and HMI are placed in a separate area so that the engineer can connect without opening the entire cabinet.

📌 Summary of Part 3:
Olimps’ practical design approach ensures ease of calibration, minimizes risks of errors, simplifies service, and provides legal protection — all contributing to the long-term reliability of secondary instrumentation cabinets.

🟦 Part 4. Signal Validity Algorithms

🔹 4.1. Why This is Important

In Custody Transfer systems, one cannot rely on “raw” data — each channel must be verified for validity.
If distortion or loss of a signal is allowed, custody transfer acts lose their legal force, and the parties may suffer serious losses.
Therefore, in the secondary instrumentation cabinet and in flow computers, channel validity algorithms are implemented.

🔹 4.2. Basic Validity Conditions

For a channel to be considered valid, two conditions must be met simultaneously:

1. ▶️ Line in operation
   • product is actually moving through the pipeline;
   • the technological line is open, the flowmeter is engaged;
   • confirmation is received from the system (e.g., “line in operation” signal from SCADA/PLC).
2. 🔗 Channel is not broken
   • the signal is physically present at the terminals;
   • cable is intact, terminals are closed, no break or loss of frequency/pulses;
   • check via signal level monitoring (e.g., presence of pulses above threshold, current within 4–20 mA range).

🔹 4.3. Processing Rule

• ✅ If both conditions are met → data is considered valid, archived, and included in calculations.
• ❌ If at least one condition is not met → the channel is blocked as invalid, data is excluded from calculations.

🔹 4.4. Practical Implementation (Olimps’ Experience)

• Impulse channels are routed through special modules with frequency control.
• Analog signals pass through modules with diagnostics for open/short circuits.
• Discrete circuits have test terminals and breaks for verification.
• Flow computers receive not only the signal, but also a validity flag.
• Archives record all events:
  • “channel invalid”,
  • “line not in operation”,
  • “signal break”.

📌 Legal Shield: this guarantees that in the future it can be proven which specific data was used in calculations, and why.

🔹 4.5. Examples

• Case 1: line open, product flowing, but cable breaks.
  • Result: channel is blocked, archive records “signal break”.
• Case 2: cable intact, signal present, but the line is technologically stopped.
  • Result: channel is blocked, archive records “line not in operation”.
• Case 3: both conditions are met.
  • Result: data is considered valid and included in Custody Transfer.

🔹 4.6. Recommendations on Dual Channel (A / B) and Signal Validity Protection

To improve reliability and legal protection of measurements in Custody Transfer practice, duplication of impulse signals is applied.
From one flowmeter (turbine, mass, etc.), two independent channels (A and B) can be output.

Essence of the method:

• Channels may be phase-shifted (e.g., by 90 or 180 degrees) relative to each other.
• The system compares them, detecting discrepancies, pulse dropouts, failures, or interference.
• If one channel fails, data is blocked or the system switches to the backup channel.

Advantages:

• 📈 Higher validity of accounting.
• ⚖️ Reduced risk of errors and disputes.
• 🛡️ Strengthened legal protection of measurements.

Regulatory Basis:

• ISO 6551:1982 — Petroleum liquids and gases — Fidelity and security of dynamic measurement — Cabled transmission of electric and/or electronic pulsed data.
  • Defines requirements for reliable transmission of pulse signals, including redundancy and channel protection.
• ISO 17089-1:2010 / 2019 — Measurement of fluid flow in closed conduits — Ultrasonic meters for gas — Part 1: Meters for custody transfer and allocation measurement.
  • Establishes requirements for output signal validity, redundancy, and self-diagnostic methods in ultrasonic gas meters for Custody Transfer.

📌 Thus, the use of dual channel (A and B) with phase shift or independent pulse generation is a best practice, consistent with ISO 6551 and ISO 17089-1, significantly increasing the reliability and legal force of custody transfer acts.

🟦 Part 5. Legal Component (Legal Shield)

🔹 5.1. Why “Legal Shield” is Critical

In Custody Transfer, measurements are not just technical — they are the legal basis of the transaction.
Any error, intervention, or data loss leads to conflicts between the parties: seller, buyer, operator.
Therefore, the secondary instrumentation cabinet must be designed to:

• 🚫 exclude the possibility of hidden intervention;
• 🛡️ ensure legal evidence of results;
• 📏 comply with the requirements of API, OIML, ISO, GOST, and national rules.

🔹 5.2. Sealing and Protection Against Interventions

• 🔒 All terminal blocks through which custody transfer channels pass must have sealable covers.
• Breakable terminals after calibration are sealed.
• Access to flow computer settings is restricted, key parameters are protected by passwords and seals.
• Any seal violation is immediately recorded in protocols and constitutes a legal fact.

🔹 5.3. Archives and Audit Trail

• Flow computers and the cabinet as a whole must provide non-volatile data storage.
• The archive includes:
  • 📊 measured values (flow, volume, mass, temperature, pressure, density);
  • ⚠️ events (“channel invalid”, “signal break”, “power loss”);
  • 👨‍💻 operator actions (parameter changes, interventions).
• Audit trail records: who, when, and what was changed.
• The archive is protected from modification or deletion.

🔹 5.4. Independence of Channels

• Each channel must be independent so that the failure of one does not affect the validity of others.
• If necessary, duplicate computers (N+1) are used.
• This eliminates situations where a single error leads to the complete loss of legal force of the protocol.

🔹 5.5. Traceability and Compliance with Standards

• All cabinet elements must have metrological traceability: certified instruments, calibration, entries in passports.
• The system must comply with standards:
  • 📏 OIML R117 (liquid measurement);
  • 📏 API MPMS (measurement methods);
  • 📏 ISO/IEC 17025 (laboratories and calibration);
  • 📏 national standards (e.g., GOST 8.586).

Legal Shield: such compliance guarantees that the data can be used in court or arbitration without additional expertise.

🔹 5.6. Practical Result

Olimps’ experience has shown:

• ✅ Customers appreciate when all channels are sealed.
• ✅ Lawyers feel confident, as the protocols have legal validity.
• ✅ Commissioning engineers are grateful because the system is simple and transparent.

The cabinet is designed not only as an engineering device, but also as a shield of legal protection for all parties to the transaction.

🟦 Part 6. Operating Experience and Acknowledgment from Commissioning Engineers

🔹 6.1. Practical Value for Commissioning Engineers

Cabinets built according to the Olimps approach have been appreciated by operating personnel for years. Commissioning engineers note:

• ⏱️ Time savings: calibration and verification require minimal effort thanks to breakable terminals and convenient access to each signal.
• 🔍 Ease of diagnostics: each channel is logically labeled and physically accessible, reducing the likelihood of errors during checks.
• 😌 Reduced stress: less “manual work” with wires, lower risk of accidental breaks or short circuits.

🔹 6.2. Reliability in Operation

• 🕑 Cabinets operate for decades without the need for complete modernization.
• 🔄 Reserve in power supply and free DIN rails allow painless system expansion.
• 🛡️ Intrinsically safe barriers and circuit separation minimize accident risk.
• 💾 Archives and audit trail ensure data protection in any power failure.

🔹 6.3. Legal Confidence for the Customer

• 📑 Data from the cabinet and flow computers has legal validity and is protected from interventions.
• ⚖️ In dispute situations, archives and audit trail can be presented, recording all events and changes.
• 🤝 Such an approach reduces the risks of conflicts between seller and buyer.

🔹 6.4. Gratitude from All Parties

• 👷 Commissioning engineers: value convenience and predictability.
• 📏 Metrology engineers: receive valid data and transparent protocols.
• ⚖️ Lawyers and contract departments: are confident that custody transfer acts are legally protected.
• 🏢 Customers: understand that the equipment will last long and will not require frequent redesign.

🔹 6.5. Conclusion

The secondary instrumentation cabinet is not just an electrical panel.
It is the heart of the custody transfer unit, where the interests of engineers, lawyers, and operating staff meet.

Olimps’ experience has shown: if the cabinet is designed properly, it provides:

• 🎯 measurement accuracy,
• 🛡️ legal protection of data,
• 🛠️ ease of maintenance,
• ⏳ reliability for decades.

Therefore, proper cabinet design is an investment in trust between the parties of the transaction, in the peace of mind of the personnel, and in the sustainability of the entire Custody Transfer system.

🟦 Part 7. Examples of Cabinet Implementations

In this book we present only an overview of possible solutions for designing secondary instrumentation cabinets.

Olimps and Pro meter cabinets are manufactured in various configurations, depending on the application environment and customer requirements. In practice, these may be:

• 🛢️ Cabinets for oil and petroleum products — with enhanced archiving systems, API standards support, and N+1 redundancy of flow computers.
• 💨 Cabinets for gas — with high-frequency pulse inputs, digital interfaces for chromatographs, and extended signal diagnostics functions.
• ⚗️ Cabinets for chemical industries — with special attention to intrinsic safety, additional Ex i barriers, and zoning separation of circuits.
• 🏭 Cabinets for tank farms — with integration of level gauges, temperature strings, density systems, and specialized modules for loading/unloading accounting.

Each project is carried out individually, taking into account:

• the application environment,
• regulatory requirements,
• customer’s tasks,
• requirements for Legal Shield (legal protection).

📌 Detailed layout diagrams, drawing examples, and real cabinet photos are provided upon request.

🟦 Final Conclusion

The secondary instrumentation cabinet is the central element of any custody transfer unit.
Its design determines not only the accuracy of measurements, but also:

• 🛡️ the legal protection of accounting,
• 🔧 convenience of maintenance,
• ⏳ long-term reliability of the entire system.

Experience of Olimps and Pro meter has shown:

• proper design guarantees trust between parties,
• ensures calm work for engineers and operators,
• and provides lawyers and customers with full confidence in the legal force of custody transfer protocols.

🌐 For full, detailed information on cabinet design and custody transfer unit development, please visit:
👉 www.prometer.pro