Flow meter
WATER FLOW CALIBRATION LABORATORY
Introduction
Water flow calibration laboratory of SMG is the laboratory in Thailand, which has been accredited according to ISO/IEC 17025:2517. The flow calibration facility uses the primary method of measuring the water flow by gravimetric principle. Flow calibrations are arranged by collecting a measured mass of the water flowing through the meter being calibrated over a measured time interval under steady condition (i.e., flow rate, temperature and density). Constant flow of system is reached with constant head tank assembly. The flow rate measured by the static gravimetric method is computed along with the average of the flow rate output signal of unit under calibration (UUC) during the collection interval. Calibrations consist of three sets of 10 measurements determined at essentially the same set point.
Four types of flow calibration in SMG:
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Volume flow rate:
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Mass flow rate:
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Totalized volume:
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Totalized mass:
(1)
(2)
(3)
(4)
Where is the collected mass, T (s) is themeasured time interval, ) is the water density.
Schematic Layout
T1 Water Reservoir
T2 Overhead Constant Tank
T3,T4,T5 Surge Suppressor Vessel
L1 Pump Line
L2 Overflow Return Line
L3,L4 Measuring Section Line
L5 Circulation Line
D1,D2,D3 Diverter System
W1,W2,W3 Weight Tank
P Discharge Pump
UUC Unit Under Calibration
Figure 1. Layout of the water flow calibration laboratory.
Figure 2. Overview of the water flow calibration laboratory at SMG.
Operation
Water in the reservoir is drawn by four pumps and supplies it to the elevated tank that has an overflow structure to eliminate variations in flow rate caused by changes in pump speed. When the constant tank overfills, the water is then divided into two separate parts by the weir constructed. The excess water which flows via the weir constructed is returned back to the reservoir without passing through the meter calibration section. The water in the constant tank falls freely under the action of gravity and then the water passes the meter calibration section including a surge tank, flow meter under calibration, control valve, nozzle and diverter into the reservoir. The system is operated until steady state is reached. During this time the water is returned backed to the reservoir. The measurement process is started by diverting the flow from the bypass circulation toward the weigh tank and starting the recording of the output signal of the Unit under calibration (flow meter) and synchronously the readings of the time measurement. After the weighing tank is filled for the predetermined period of time, the diverter is started again to redirect the flow to the bypass circulation position, and stops synchronously the readings of the time measurement. The mass of water the in the weighing tank is measured. When the standard flow rate value calculation, the buoyancy correction must be taken into account.
PC base Data Acquisitions and Control System
During the measurement process, both of Unit under calibration and instruments of water flow calibration facility have been automated in the Flow Lab SMG software (the base of software is visual basic 6) to controls the system and to put data into the Excel spreadsheet template and it also generates the report using a PC associated hardware. The data acquisition and control system consists of a NI GPIB-USB-HS interface adapter IEEE-488 and a RS-232 serial to USB.
The Data acquisition diagram is shown below (Figure 3)
Figure 3. Data Acquisition Chart of the Water Flow Calibration at SMG.
