Ifm Mass Flow Meter: Calibrating Thermal Gas Insertion Probes for Direct Mass Flow Calculations

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Calibrating Thermal Gas Insertion Probes for Direct Mass Flow Calculations

Quick Answer: A thermal gas insertion probe gives direct mass flow only when the calibration constants match your actual gas mix, pipe size, temperature range, and pressure range. If you use an Ifm mass flow meter or a Silver Instruments thermal gas probe with default air calibration on biogas or mixed fuel gas, the reading can drift by more than 5 percent. Send us your gas composition, pressure, temperature, and pipe size to get a calibration curve matched to your process.

How a Thermal Gas Insertion Probe Calculates Mass Flow

A thermal gas insertion probe uses two PT100 sensors. One sensor is heated. The other sensor measures the gas temperature. Gas flows past the heated sensor and removes heat. Higher mass flow removes more heat. The transmitter converts the heat loss into a flow signal. This signal becomes velocity in Nm3/h or mass flow in kg/h after the pipe area and gas properties are applied.

Direct mass flow calculation is simple in principle. The probe measures heat transfer. The transmitter multiplies the signal by the pipe cross section and a gas conversion factor. That factor depends on thermal conductivity, density, and viscosity. For dry air or nitrogen, factory calibration is straightforward. For other gases, the factor changes.

Gas Composition Changes the Calibration Curve

This is the part most engineers skip. A thermal probe calibrated on air reads high or low when you put it in a different gas. For example, carbon dioxide has different thermal conductivity than air. Digester gas from a wastewater plant in Thailand often contains 55 to 65 percent methane and 35 to 45 percent carbon dioxide. If you use air calibration on that gas, the mass flow error can reach 8 to 12 percent. We have seen this on customer sites many times.

Ifm mass flow meter models and Silver Instruments insertion probes share the same requirement. The calibration constants must match the gas species. You cannot simply change the gas type in the transmitter menu and expect accurate mass flow. The conversion factor must be derived from the gas composition at operating conditions.

Parameters You Need for Accurate Calibration

We ask for five values on every thermal gas insertion probe order. These are gas composition in volume percent, moisture content, operating pressure in bar, operating temperature in °C, and pipe inner diameter in DN. The flow range in kg/h or Nm3/h also matters. A probe calibrated for 0 to 500 Nm3/h of air will not give good resolution on a 0 to 50 Nm3/h biogas line.

Do not forget the pipe inner diameter. A DN80 schedule 40 pipe has an inner diameter of 77.9 mm. A DN80 schedule 80 pipe has an inner diameter of 73.7 mm. That difference changes the flow area by about 12 percent. If the transmitter uses the wrong inner diameter, the mass flow output is wrong from the start. This is not a sensor problem. It is a configuration problem.

Factory Calibration vs Field Adjustment

Most thermal gas insertion probes are calibrated in a flow rig or wind tunnel with the actual gas or a gas with similar thermal properties. Silver Instruments calibrates its TMF-100 insertion probe with a reference flow standard. The test points cover the full flow range from 0.5 to 60 Nm/s. A calibration certificate comes with the probe.

Field adjustment is possible but only with a reliable reference meter. A clamp on ultrasonic flow meter or a calibrated inline meter can work. In practice, most field adjustments fail because the reference meter is not accurate or the flow profile is not stable. A biogas plant in Vietnam tried to adjust a probe against a handheld pitot tube. The result was about 15 percent error at low flow. After factory calibration, the error dropped below 2 percent.

Installation Depth Affects Direct Mass Flow Output

Insertion depth is not a minor detail. The probe tip must sit in the center of the pipe or at the point where velocity equals the average flow. For a DN100 pipe, the tip should be about 50 mm from the inner wall. For larger pipes, the probe must be long enough to reach the center. If the probe is too short, it reads the boundary layer. If it is too long, it can create vortex effects.

Straight run matt

Recommended Configuration for Direct Mass Flow

For clean and dry gas lines, we recommend the Silver Instruments TMF-100 thermal gas insertion probe with actual gas calibration. It covers pipe sizes DN40 to DN300, flow range 0.5 to 60 Nm/s, process temperature from -20 °C to 200 °C, and pressure up to 16 bar. Output is 4-20 mA HART and RS485 Modbus. The sensor is PT100. An ATEX Zone 1 option is available for biogas, gas processing, and chemical plants.

For a 2 inch biogas line at 0.3 bar and 40 °C, the same probe can output kg/h directly. For a DN150 compressed air line at 7 bar and 25 °C, you get Nm3/h without a separate pressure transmitter. The output matches your DCS or PLC input. You do not need extra flow computers for most dry gas applications.

When You Need Pressure and Temperature Compensation

A thermal probe measures mass flow directly. It does not need pressure and temperature compensation for stable dry gas composition. But saturated gas, wet gas, or gas with changing moisture content can cause errors. In those cases, add a pressure transmitter and a temperature sensor. The transmitter can use them to correct the mass flow output for gas density changes.

We do not recommend thermal insertion probes for steam. A vortex flow meter or differential pressure meter is better for saturated steam. For wet methane or landfill gas with high moisture, use a thermal probe with an integrated moisture correction curve.

Cost and Lead Time

Calibration adds cost but it is lower than most engineers expect. A standard thermal gas insertion probe with air calibration costs less than one with actual gas calibration. The difference is typically 150 to 400 USD depending on gas type and test points. Lead time for factory calibration is usually 7 to 12 working days. If you need an expedited calibration, ask our team. We have shipped calibrated probes to Colombia, Indonesia, and Saudi Arabia within 5 working days.

Get a Quote for Your Gas Line

Send us your gas composition, pressure in bar, temperature in °C, pipe size in DN, and flow range. We will propose a calibrated thermal gas insertion probe with the correct output for your system. You can reach Silver Instruments at Tel: +86-135-1213-1526, Whatsapp: 86-135-12131526, or WeChat: +86 13512131526. We respond to most technical quotes within 24 hours.

FAQ

Can an Ifm mass flow meter probe be recalibrated for a different gas?

Yes, many thermal insertion probes can be recalibrated if the transmitter allows new calibration factors. You can send the probe to a calibration lab or replace it with a probe that was calibrated from the start for your gas. Recalibration works best when you provide the full gas composition and operating conditions.

What parameters affect direct mass flow calculation?

Gas composition, moisture content, pipe inner diameter, operating pressure, operating temperature, and insertion depth all affect the calculation. Pipe inner diameter errors are the most common issue we see on customer sites.

Do I need pressure and temperature compensation for a thermal gas probe?

For dry and clean gas lines with stable composition, a properly calibrated thermal probe does not need separate pressure and temperature compensation. For wet gas or gas with changing density, add a pressure transmitter and a temperature sensor.

What is the typical accuracy after actual gas calibration?

For a thermal gas insertion probe with actual gas calibration, accuracy is around plus or minus 1.5 percent of reading plus 0.5 percent of full scale. Air calibration used on other gases can produce errors from 5 percent to 15 percent or worse.

What pipe sizes work with insertion probes?

DN40 to DN300 is the common range for insertion thermal gas probes. Larger ducts up to DN600 or rectangular stacks are possible with longer probes and a flow profile study. For smaller lines, an inline thermal mass flow meter is a better choice.

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