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In order to quantitatively describe the effect of changes in these parameters on the calibration of ultrasonic flowmeters, a series of carefully controlled calibration experiments were performed. The first step of the experiment involved calibrating a 200 mm (8 inch) and a 300 mm (12 inch) ultrasonic flowmeter using a 2.8 MPa (400 psi) natural gas medium in the Southwest Research Institute (SwRI) high pressure circuit. As an additional reference, 200mm and 300mm turbine meters are also used in the circuit. The fluid medium was then changed to nitrogen and the sound speed was changed by 16%, which is numerically equivalent to the natural gas pressure of 4.6 MPa (677 psi).
To further test the effect of pressure on the calibration of the ultrasonic flowmeter, a series of statistical sound velocity measurements were performed on a 300 mm diameter flowmeter using nitrogen gas at pressures ranging from 1.4 MPa (200 psi) to 7 MPa (1000 psi). The measured values ​​indicate that the change in sound speed within this pressure range is within 0.03% of the calculated value.
In addition, further experiments were conducted on changes in the speed of sound caused by changes in temperature and fluid media. Calibration experiments with natural gas at 21°C (70°F) and 10°C (50°F) were performed with nitrogen at 21°C (70°F) and 32°C (90°F). For each series of calibrations, compare the average calibration curve and derive the effect of the change on the calibration. Under conditions that satisfy the desired device and flowmeter reproducibility, calibration of the ultrasonic flowmeter does not respond to changes in speed of sound, changes in temperature and pressure. When the fluid medium used for calibration changes from natural gas to nitrogen, the small changes observed are due to different equations of state for the two gases.
These test results prove that if the ultrasonic flowmeter is feasible under a set of conditions, it can be used under other conditions, including using different gaseous media.
2. Introduction The principle of the ultrasonic flow meter for natural gas metering used in related transactions is to measure the ultrasonic propagation time of gas. When the ultrasonic wave coincides with the flow direction of the fluid, the propagation time is smaller than that when the ultrasonic wave is reversed. The difference in propagation time between the two states is used to calculate the average rate of gas flow. The actual volumetric flow can be expressed by the following formula: 
Here K = meter factor of the flow meter, ΔT = difference in propagation time, T1 = propagation time in the forward flow, T2 = propagation time in reverse flow Since this flow equation only includes the physical structure size and propagation time of the flow meter, It is independent of the speed of sound (SOS) in the flowing gas. Therefore, it can be assumed that the measurement of the gas flow rate is independent of factors that affect the speed of sound in the gas, such as temperature, pressure, and gas composition. If this assumption is incorrect, the effectiveness of the ultrasonic flowmeter in calibration other than in field operating conditions is worth considering.
First, the measurement of the gas velocity of the ultrasonic flowmeter is independent of the speed of sound, but there may be some minor effects due to the following reasons.
The acoustic resistance changes the coupling of the signal to the gas;
Reynolds number changes. Reynolds number is proportional to the ratio of specific gravity (SG) to viscosity;
The wavelength (WL) of the signal changes as the gas composition changes.
It is of great interest to pay attention to these parameters in different media. The following table is for standard conditions. Note that the Reynolds number is almost constant for a given pipe size and flow rate.
Table 1 Gas properties 
When changing from natural gas to air or nitrogen, many gas properties change, such as sound speed, specific gravity, and viscosity. However, because they also vary with temperature and pressure, ultrasonic flow meters have imaginable overlap in gas properties over the range of operating conditions, indicating that deviations from standard conditions are less important.
3. Research Objectives The objective of the research set forth in this paper is to determine the effects of changes in temperature, pressure, and fluid media on gas ultrasonic flowmeters. In addition to testing ultrasonic flow meter technology, the suite also supports the use of nitrogen gas or air calibration for these natural gas flow meters.
Currently, only two sets of equipment in North America can be used to calibrate ultrasonic flowmeters larger than 200 mm in the out-of-flow range. The installation of these flow meters is growing at a rate of more than 10% per year. In the future, calibration equipment will become very limited. These calibration devices may need to be recalibrated in less than a few years.
If the calibration of the ultrasonic flowmeter is limited to natural gas installations, the possibility of constructing a new installation may be limited due to location and cost. However, if it can be demonstrated that calibration with other media is equivalent, the possibility of building a new device is greatly increased. Calibrating natural gas flow meters with air is not just about ultrasonic flow meters. Almost all turbine meters used in natural gas metering and residential gas meters can be calibrated with air.
4. Flow Loop Calibration To achieve the goal of this procedure, experiments were performed under zero flow and other flow conditions. Zero flow is used to determine the possible effect of pressure on the calibration.
4.1 Flow circuit experiments Since there is now no pressurized flow calibration device that can be calibrated with both natural gas and air in the same test loop, the best option is to use inert gas nitrogen, which has properties similar to air (air contains 78% % of nitrogen). Southwest Research Institute has a circulatory system (not part of the gas pipeline), which is the only device in North America that can use the same set of instruments in the same loop for both nitrogen and natural gas testing. Although the sonic nozzle device is a calibration device that uses a weighing system to determine the mass flow rate, the sonic nozzle flow rate calculation requires a gas composition and a state equation to determine the gas properties. It was also decided to use reference turbine flowmeters in the loop to compare two volumetric flowmeters (such as turbines and ultrasonic flowmeters). This reference table method compares the flow meter with the sonic nozzle compared to the gas composition has been small.
5. Concluding remarks A series of standard calibrations were performed using nitrogen and natural gas as test fluids in the high pressure circuit of the Southwest Research Institute. In addition, changing the temperature changes the speed of sound accordingly. In these calibration experiments, the test equipment including instrumentation, pipeline layout, and data acquisition system did not change.
1. Summary Nowadays, the calibration of an ultrasonic flow meter for natural gas metering is performed on a flow calibration device as much as possible. Since almost all of these devices use natural gas flowing through the pipeline, it is not usually possible to change parameters that affect sound velocity, such as temperature, pressure, and gas composition. When the ultrasonic flow meter is used, can these parameters work if they are different from the values ​​in the calibration case?