How the SDAC1200 Bomb Calorimeter Supports ISO 1928:2025 for Gross Calorific Value Testing

Understanding the Requirements of ISO 1928

The gross calorific value of coal and coke is a fundamental parameter in fuel trading, boiler efficiency evaluation, energy accounting and combustion control. To ensure that results obtained by different laboratories are comparable, ISO 1928 establishes a standardized bomb calorimetric method for determining gross calorific value.

The current edition, ISO 1928:2025, Coal and coke — Determination of gross calorific value, specifies that the analysis sample is combusted at constant volume in a combustion vessel calorimeter. The calorimeter is calibrated through the combustion of certified benzoic acid, and the result is referenced to 25°C. The standard also describes calibration, fuel testing, corrected temperature-rise calculation, precision evaluation, reporting and the calculation of net calorific value.

The Sundy SDAC1200 is an automatic isoperibol oxygen bomb calorimeter designed around this measurement principle. Its combustion system, temperature-control architecture, heat-capacity management and automated operating functions provide the technical foundation required for carrying out ISO 1928 testing.

1. Constant-Volume Combustion in an Oxygen Bomb

ISO 1928 determines gross calorific value by burning the sample inside a sealed combustion vessel under constant-volume conditions. The heat released by combustion is transferred to the calorimetric water, and the resulting temperature rise is used to calculate the sample’s energy content.

The SDAC1200 incorporates a 250 mL stainless-steel oxygen bomb designed for constant-volume combustion. The bomb is rated for a maximum pressure of 20 MPa and is used with oxygen of at least 99.5% purity. This arrangement provides the oxygen-rich, enclosed combustion environment required for the complete combustion of coal and coke samples.

Because the volume of the combustion vessel remains essentially constant during the experiment, the directly measured result corresponds to the gross calorific value at constant volume. This is the primary calorific-value result defined by ISO 1928.

2. Isoperibol Calorimeter Design

ISO 1928 includes procedures applicable to different calorimeter configurations, including adiabatic, isoperibol, static-jacket and automated calorimeters. The standard contains specific informative annexes for isoperibol and automated systems.

The SDAC1200 is an automatic isoperibol calorimeter. In an isoperibol system, the temperature of the calorimeter jacket is maintained under controlled conditions while the temperature change of the inner bucket is continuously measured. Any heat exchange between the bucket and its surroundings must then be minimized, controlled or mathematically accounted for.

To create a stable thermal environment, the SDAC1200 uses a multi-point isothermal jacket design incorporating a helical circulation tube. According to the manufacturer’s specifications, the temperature difference within the jacket and jacket lid is controlled to within 0.05°C. This reduces the influence of laboratory temperature fluctuations and air movement on the calorimetric bucket.

This controlled thermal boundary is important because ISO 1928 calculations depend not simply on the maximum temperature reached, but on an accurately determined and corrected temperature rise.

3. Calibration with Certified Benzoic Acid

One of the central requirements of ISO 1928 is that the calorimeter must be calibrated by combusting certified benzoic acid. Since the certified heat of combustion of the reference material is known, the effective heat capacity of the complete calorimetric system can be calculated.

The effective heat capacity includes the combined thermal response of components such as:

  • Calorimetric water
  • Inner bucket
  • Oxygen bomb
  • Thermometer or temperature sensor
  • Stirrer and immersed components
  • Other parts participating in heat absorption

The SDAC1200 is designed for calibration using approximately 1 g of benzoic acid. Its published specifications state a relative standard deviation of no more than 0.05% based on the analysis of 1 g benzoic acid, together with heat-capacity stability of no more than 0.2% over 12 months.

These specifications indicate that the instrument provides the repeatability and thermal stability needed for establishing and maintaining the effective heat-capacity value required by ISO 1928.

However, instrument performance alone does not replace the calibration procedure. The laboratory must still use certified benzoic acid, follow the prescribed calibration conditions and periodically verify or redetermine the effective heat capacity.

4. Precise Measurement of Temperature Rise

The temperature rise caused by combustion is usually only a few degrees. A very small error in temperature measurement can therefore produce a measurable error in the reported calorific value.

The SDAC1200 uses a PT1000 temperature sensor with a stated temperature resolution of 0.0001 K. Its average temperature rise during a normal test is approximately 2°C.

High temperature resolution allows the system to characterize the complete temperature-time curve, including:

  • The initial temperature period
  • The rapid temperature rise after ignition
  • The final temperature period
  • Heat exchange before and after combustion
  • The corrected calorimetric temperature rise

ISO 1928 specifically addresses observed temperature rise, corrections for isoperibol calorimeters, thermometer corrections and conversion to the 25°C reference temperature. The high-resolution temperature-measurement system of the SDAC1200 provides the raw measurement capability required for these calculations.

5. Stable and Reproducible Water Quantity

The effective heat capacity of a calorimeter depends strongly on the quantity of water in the inner bucket. If the water quantity changes between calibration and sample testing, the thermal response of the system also changes.

The SDAC1200 uses an automatic bucket-filling and water-volume determination system. Its inner-bucket circulation arrangement is designed to maintain a consistent calorimetric water quantity for each test. The bucket-water system and jacket-water system are independent, preventing the filling and draining of the inner bucket from disturbing the jacket temperature.

After a test, the inner-bucket water returns to the dedicated water tank rather than mixing directly with the jacket circuit. This separation helps maintain:

  • Stable jacket temperature
  • Consistent bucket water quantity
  • Reproducible effective heat capacity
  • Reliable performance during consecutive tests

This design is particularly relevant to the ISO 1928 procedures based on a constant mass of calorimeter water or a constant total calorimeter mass.

6. Uniform Heat Transfer Through Efficient Stirring

After ignition, heat must be transferred rapidly and uniformly from the oxygen bomb to the calorimetric water. Insufficient or inconsistent stirring can produce temperature gradients and delay thermal equilibrium.

The SDAC1200 uses propeller stirring to improve water circulation around the oxygen bomb. Efficient circulation accelerates heat transfer and helps the temperature sensor measure a representative bucket-water temperature rather than a localized temperature.

Combined with the metal bucket and controlled water circulation, the stirring system contributes to a more clearly defined temperature-rise curve and improves repeatability between calibration and fuel tests.

7. Controlled Oxygen Filling and Combustion Reliability

Complete combustion is essential for an accurate gross-calorific-value result. Incomplete combustion causes part of the sample energy to remain unreleased, producing a result that is lower than the true value.

The SDAC1200 automatically performs oxygen filling and releasing. It also monitors oxygen-filling pressure and checks the oxygen bomb for leakage before testing. These functions reduce variations that can arise from manual filling and help identify conditions that could compromise combustion.

The instrument supports ignition by either nickel ignition wire or cotton thread. This is also relevant to the 2025 revision of ISO 1928, which expanded its treatment of alternative ignition methods.

The laboratory must nevertheless confirm complete combustion after each test. Any unburned sample, soot formation or abnormal combustion condition should be investigated in accordance with the laboratory procedure.

8. Corrections and Calorific-Value Calculation

The temperature rise measured by a bomb calorimeter is not generated exclusively by combustion of the sample. Other energy contributions may arise from the ignition material, combustion aid and formation of acidic combustion products.

ISO 1928 therefore includes procedures for determining the corrected temperature rise, accounting for ancillary energy contributions and calculating gross calorific value. The standard also provides formulas for converting results to other reporting bases and for calculating net calorific value.

The SDAC1200 software supports advanced calculation, analysis reports, statistical reports and system monitoring. These functions allow test data, calibration data and calculated results to be managed within the instrument system.

For full method compliance, the laboratory must ensure that the software settings and entered correction values correspond to the applicable ISO 1928 procedure. The instrument cannot independently determine every external value required for net-calorific-value calculation. Parameters such as sample moisture, hydrogen and other relevant analytical data may need to be obtained using separate standardized methods.

9. Precision and Long-Term Stability

ISO 1928 addresses both repeatability and reproducibility. Good repeatability depends on the combined control of calibration, sample preparation, combustion conditions, water quantity, temperature measurement and data calculation.

The SDAC1200 has a stated test-result precision of RSD ≤0.08%, with benzoic-acid precision of RSD ≤0.05% and annual heat-capacity variation of no more than 0.2%.

These specifications support the precision requirements of routine calorific-value analysis. Nevertheless, an individual laboratory must establish its own method performance through measures such as:

  • Replicate analysis
  • Certified reference materials
  • Control charts
  • Periodic benzoic-acid verification
  • Interlaboratory comparison or proficiency testing
  • Calibration and maintenance records

A product specification demonstrates instrument capability; actual ISO 1928 conformity must be verified under the laboratory’s own operating conditions.

10. Automation Without Changing the Standard Principle

The SDAC1200 automatically performs several operations that are traditionally carried out manually, including:

  • Raising and lowering the oxygen bomb
  • Filling and releasing oxygen
  • Checking oxygen-filling pressure
  • Checking bomb air-tightness
  • Filling the calorimetric bucket
  • Adjusting bucket-water temperature
  • Flushing pipelines
  • Recording temperature data
  • Calculating and reporting results

The complete test time is less than 10 minutes in fast mode, less than 12 minutes in standard mode and less than 14 minutes in the default precision mode.

This automation does not alter the fundamental ISO 1928 measurement principle. Instead, it standardizes repetitive operations that can otherwise introduce operator-dependent variation. The benefit is not simply faster testing, but improved consistency between successive measurements.

11. Combustion-Vessel Safety

ISO 1928:2025 places explicit emphasis on combustion-vessel safety and includes guidance covering safe use, maintenance and testing of calorimeter combustion vessels. The standard warns that improper operation or a vessel in poor mechanical condition can create a risk of explosive rupture or blow-out.

The SDAC1200 contributes to safer operation through automatic pressure monitoring, air-tightness detection, automatic oxygen release, automatic bomb lifting and system self-diagnostics. These features reduce direct operator involvement during pressurization and combustion.

They do not, however, eliminate the need for scheduled bomb inspection, seal replacement, pressure testing, cleaning and maintenance. Laboratories should establish a documented combustion-vessel safety program based on the manufacturer’s instructions and ISO 1928 guidance.

What “Conforms to ISO 1928” Actually Means

The statement that the SDAC1200 conforms to ISO 1928 means that its measurement principle, calorimeter configuration and major operating functions are suitable for implementing the standard method.

It does not mean that every result produced by the instrument is automatically ISO-compliant. A compliant test also depends on:

  • Representative sampling
  • Correct sample preparation
  • Moisture determination
  • Certified calibration materials
  • Proper sample mass
  • Complete combustion
  • Correct correction values
  • Appropriate calculation settings
  • Quality-control procedures
  • Maintenance and operator competence

In other words, the SDAC1200 provides the measurement platform, while the laboratory remains responsible for implementing the complete ISO 1928 method.

Conclusion

The SDAC1200 supports ISO 1928:2025 through the combination of a constant-volume oxygen bomb, isoperibol calorimeter design, certified-benzoic-acid calibration capability, high-resolution temperature measurement, stable bucket-water control and automated oxygen handling.

Its independent bucket and jacket water circuits, controlled thermal environment, automatic pressure and leakage checks, efficient stirring and heat-capacity stability address the principal factors that determine the accuracy and repeatability of bomb calorimetric measurements.

When operated with appropriate sampling, calibration, correction and quality-control procedures, the SDAC1200 provides laboratories with an efficient and reliable platform for determining the gross calorific value of coal and coke in accordance with ISO 1928:2025.