Evaluation of measurement uncertainty of rotor flowmeter indication error

1 Overview

(1) Measurement basis: jjg257-1994 "Rotameter Calibration Procedure"

(2) Measurement environment conditions: ambient temperature (20±2)℃, relative humidity (30～90)%

(3) Measurement standard: Bell jar gas flow standard device

(4) Measurement process: Connect the rotor flowmeter to the pipeline of the bell-shaped gas flow standard device, adjust the flow calibration point according to the regulations, record the volume flow rate discharged from the bell-shaped gas flow standard device within a certain period of time, record the gauge pressure in front of the standard device and the flowmeter, and make corrections for temperature. Compare the scale flow rate of the rotor flowmeter with the standard flow rate obtained by the volume-time method of the bell-shaped gas flow standard device to obtain the indication error of the rotor flowmeter.

2 Mathematical model

Bell jar volume flow calculation formula

Where: v is the volume discharged (flowed) from the gas standard device, m3;

t —— discharge (inflow) time, s;

qv——The actual flow rate of the flowmeter in the calibration state; the relationship between the calibration state and the calibration state is as follows:

Where: tn, ts, tm - the thermodynamic temperature of the gas in the standard device and before the flow meter under standard conditions, tn = 293.15K

pn, ps, pm - the absolute pressure of gas in the standard device and before the flow meter under standard conditions, pn = 101325 pa

Where: pa - local atmospheric pressure, pa;

pes——standard internal pressure, pa

pem - flow meter front gauge pressure, pa

Flow meter basic error expression:

3 Variance and coefficients

The mathematical model is integrated, and the relationship between each influencing quantity is listed. Under laboratory environmental conditions, the calibration volume is 500L bell jar, the pressure inside the bell jar is 1000Pa, the average atmospheric pressure in Nanning is 99.8kPa, and the laboratory temperature is controlled at (20±2)℃. The formula can be simplified, and the influence of temperature and pressure can be ignored.

4 Evaluation of standard uncertainty of input quantity

4.1 Relative standard uncertainty of flow meter indication u(qm)

The rotor flowmeter needs to be calibrated at 5 flow points, and each flow is tested 4 times. The standard deviation of the average value of each flow point is calculated, and the largest value is selected as the repeatability. Select a maximum flow range of 30m3/h, repeat the measurement 10 times, and the test volume is 500l. The test time is as follows

During the calibration, the average of two measurements is taken as the measurement result of the scale point. Therefore, the standard uncertainty of the flow meter measurement repeatability is

4.2 Relative standard uncertainty of bell jar indication u(v)

The accuracy of our bell-shaped gas flow standard device is 0.5%, normal distribution, k=3, then the standard uncertainty is:

4.3 Relative standard uncertainty of the timer u(t)

The time t when the device discharges (inflows) gas, taking into account the time start-stop effect and control capability, the time error is ±0.1s, and is considered according to the rectangular distribution. In the experiment, t=60.91s, so the standard uncertainty introduced by time is

5 Evaluation of combined uncertainty

5.2 Calculation of combined standard uncertainty

6 Evaluation of expanded uncertainty

Take k=2 and we get

7 Reporting and Expression of Measurement Uncertainty

The uncertainty of the rotor flowmeter test results using the bell-shaped gas flow standard device is expressed as follows:

8 Calibration measurement capability

8.1 Optimal measurement capability of rotor flowmeter indication

A rotor flowmeter (level 1) with good measurement repeatability is selected for continuous measurement. In actual measurement, the average value of two measurements is taken as the measurement result. The measurement repeatability uncertainty component of the gas meter is

8.2 Summary of standard uncertainties

8.3 Calculation of combined standard uncertainty

8.4 Calibration measurement capability

ur can be expressed with an expanded uncertainty of k=2

【references】

[1] jjf1059-1999, Evaluation and expression of measurement uncertainty[s].

[2] jjf1001-1998, General metrological terms and definitions[s].