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Gas Flow Meter Totalizer Programming: Converting Continuous Mass Frequencies into Accumulated Standard Volumes
Quick Answer: A gas flow totalizer takes a pulse or frequency signal from the flow meter and turns it into accumulated standard volume. For a Coriolis mass flow meter, first convert the pulse total to mass in kg. Then divide the mass total by the gas standard density in kg per Sm3 or kg per Nm3. Set the same base conditions in the flow meter, PLC, flow computer, and SCADA.
Gas buyers and plant engineers care about standard volume because gas density changes with pressure and temperature. A cubic metre at 7 bar is not the same as a cubic metre at 0.3 bar. Standard volume removes that variable. It makes compressor runtime, fuel gas billing, and emission reporting repeatable.
In practice, we see two base conditions on customer sites. Normal cubic metre is usually 0 °C and 101.325 kPa abs. Standard cubic metre is usually 15 °C and 101.325 kPa abs. Some contracts in Latin America use 20 °C. Check the contract before you program the totalizer. A 5 °C difference changes accumulated volume by about 5.5 percent. That is enough to upset a gas buyer in Australia or a landfill operator in Vietnam.
What the Totalizer Actually Counts
A totalizer does not measure flow. It counts pulses and multiplies the count by a scaling factor. If the flow meter sends 1000 pulses per kg, then 5000 pulses equals 5 kg. If the PLC or flow computer counts those pulses for one minute, the average mass flow is 5 kg per minute. The totalizer keeps adding those mass units. That accumulated mass becomes volume after density correction.
Input Signal Types from the Gas Flow Meter
Most gas mass flow meters from Silver Instruments offer a pulse output or a frequency output. A Coriolis meter may send 0 to 10 kHz proportional to mass flow. A thermal mass flow meter may send 4-20 mA HART plus pulse. Some vortex meters send pulses proportional to actual volume. Those need separate pressure and temperature transmitters for standard volume.
Set the totalizer input for frequency first, not analogue. Pulse signals avoid zero drift. At low flow, a 4-20 mA signal at 4.1 mA may still read a small flow, but PLC analogue filtering can hide it. Frequency counting is cleaner for batching and fiscal records.
Converting Mass Pulse Frequency into Mass Flow
Read the meter datasheet first. A typical Silver Instruments Coriolis gas meter with DN15 line size may output 1000 pulses per kg. If the PLC receives 5000 pulses in 60 seconds, mass flow is 5 kg per minute or 300 kg/h. Scale the pulse frequency in the PLC using a simple linear equation. Mass flow equals frequency divided by the K-factor.
Here is the thing. Many engineers stop at mass flow. Mass flow is fine for control loops. But the totalizer needs a different unit because gas contracts and invoices are often written in Nm3 or Sm3. So the next step is density correction.
Converting Mass Flow to Accumulated Standard Volume
Take the mass flow total in kg and divide it by the standard density of the gas. Standard density is gas-specific. For dry natural gas, many sites use 0.82 kg per Sm3 at 15 °C and 101.325 kPa abs. For biogas with 60 percent methane, the value may be 1.0 to 1.1 kg per Sm3 depending on CO2 content. You need a gas composition reading or a fixed reference density from the operator.
The formula is simple. Standard volume equals mass total divided by standard density. If a biogas plant in Selangor accumulates 820 kg of methane in one hour, that equals about 1000 Sm3 at a density of 0.82 kg per Sm3. If the siloxane filter inlet pressure is 0.4 bar, the actual cubic metre per hour is higher. Do not use actual volume for tariff meters unless both parties agree in writing.
Temperature and Pressure Compensation in the Totalizer
For a Coriolis mass flow meter, pressure and temperature compensation is not needed for mass. Mass
For a vortex flow meter or a differential pressure meter, the pulse output is actual volume. You need live pressure and temperature inputs. Use a pressure transmitter with 4-20 mA HART and a PT100 RTD. Program the flow computer as V_standard equals V_actual times P_abs divided by P_base times T_base divided by T_abs. Use absolute units, not gauge. A plant in Saudi Arabia made this mistake once. They used gauge pressure at 0.8 bar and the totalizer underscaled by 20 percent.
PLC Totalizer Example
A customer in Indonesia asked us last year to configure a PLC for a thermal mass flow meter on a natural gas line. The meter sent 0 to 200 Hz for 0 to 400 kg/h. The PLC input was set to high-speed counter mode. The totalizer logic divided the pulse count by 0.5 pulses per kg. Then it divided the mass total by 0.82 kg per Sm3. The result was accumulated Sm3. The plant SCADA showed the same value as the Silver Instruments display.
Most engineers skip this part. They wire the pulse output and expect the totalizer to read Nm3 automatically. It does not. The meter may show mass or actual volume. The totalizer must be told which base condition and which gas density to use.
Recommended Silver Instruments Models for Gas Totalizer Jobs
For line sizes from DN15 to DN50, the Silver Instruments CMF series Coriolis mass flow meter works well for natural gas, LPG, and biogas. It offers 0 to 10 kHz pulse output, 4-20 mA HART, and a local totalizer. For DN80 to DN200, the VF series vortex flow meter with inline temperature and pressure sensors gives actual volume plus standard volume if the flow computer is configured correctly. For low pressure air or flue gas, a thermal mass flow meter is often the lower cost option.
Match the output type to the control system. If the PLC is a Siemens S7-1200 or Allen-Bradley CompactLogix, use the high-speed counter input and a shielded twisted pair cable. Do not route pulse wiring in the same tray as VFD motor cables. We have seen customer sites lose pulse counts from VFD noise many times.
Frequently Asked Questions
What is the difference between Nm3 and Sm3? Nm3 usually means normal cubic metre at 0 °C and 101.325 kPa abs. Sm3 usually means standard cubic metre at 15 °C and 101.325 kPa abs. Some contracts in Latin America use 20 °C. Always check the gas sales agreement.
Can I use a 4-20 mA signal instead of a pulse for totalizing? You can, but accuracy at low flow suffers. Analogue inputs have filter time and offset. A pulse or frequency input gives better low-flow resolution and no zero drift.
How do I handle changing gas composition? Use a gas chromatograph or a Coriolis meter with density output. If the gas composition changes daily, program a live density or standard density input into the flow computer. Fixed density works only for stable gas from a single source.
What K-factor should I use? The K-factor depends on the meter model and size. A DN15 Silver Instruments Coriolis meter may output 1000 pulses per kg. A DN25 unit may output 500 pulses per kg. Always read the calibration sheet, not only the brochure.
Do I need an explosion proof rating for gas flow totalizer installations? Yes, if the area is classified. For biogas, natural gas, and LPG, specify ATEX Zone 1 or Zone 2 depending on the area classification. The flow meter and the wiring must match the local electrical code.
Need help with a gas flow totalizer program? Send us your gas type, pressure in bar, temperature in °C, pipe size in DN, flow range, and the expected standard condition. Our engineers will recommend a Silver Instruments flow meter with pulse output and the correct totalizer settings. Tel: +86-25-68650347, WhatsApp: +86-25-52155837, WeChat: +86 15365082610.
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