Calibration Procedure
Current meter and other devices calibration and their operation are carried out in strict accordance with ISO 3455 Hydrometry – Calibration of current-meters in straight open tanks and ISO 2537 Hydrometry – Rotating-element current-meters.
Standard ISO 3455 specifies the procedure of calibration of current meters of rotating-element type as well as stationary-sensor type (electromagnetic type) in straight open tanks. It also specifies the types of tank, rating carriage and equipment to be used and the method of presenting the results.
Standard ISO 2537 specifies the operational requirements, construction, calibration, and maintenance of rotating-element devices for the measurement of flow velocities in open channels. Czech version of ISO 748 gives information on the use of these devices.
Object of the calibration procedure is to determine a relationship between liquid velocity v and either the rate of revolution of the rotating element (propeller) or the liquid velocity v directly indicated by the current meter. For this purpose, the current meter is mounted on a rating carriage and drawn through still water contained in a straight tank of a uniform cross section at a number of steady speeds of the rating carriage. Simultaneous measurements of the speed of the rating carriage and the rate of revolution of the rotating element or the velocity indicated by the current meter are made. In the case of rotating-element current meters, the two parameters are related by one or more equations, the limits of validity of which are stated. In the case of stationary-sensor type current meters, the velocity indicated by its display unit is compared with the corresponding carriage speed to know the error in measurement.
The current meter has to be attached to the towing carriage in the same way as it will be used for the measurement. During every calibration run, a carriage speed v [m/s], and – in the case of rotating-element current meters – a rotational frequency n [1/s] of the propeller are determined from measured distance l [m] covered by the carriage, number of revolution of the propeller N [-] and a relevant time interval t [s]. The relationship between the carriage speed and the rotational frequency of the propeller is then expressed by the calibration equation.
Rotating-element Current Meters
The procedure starts by setting a demanded number of propeller revolutions and carriage speeds through user interface of the control computer. Then the carriage starts moving, and after the demanded speed is reached and stable, the control computer starts a data acquisition. The first impulse from the current meter switches the measuring of the covered distance and the time on. Simultaneously, the impulses from the meter are counted. When their predetermined number is reached, the measuring of the distance and time is terminated and the data are transferred to the central computer in the control room through Wi-Fi LAN.
Measuring of the covered distance is based on recording the number of impulses from a distance gauge. The gauge is made as a metal strip with slits of spacing of 0.1 m, scanning is provided as an optical one. This arrangement enables to reach an uncertainty and resolution of 0.1 % for typical distance of 50 m covered by the carriage. The total distance covered by the rating carriage is the product of the amount of track impulses and unit distance. Time measurement is performed by a software frequency counter; the frequency is derived from a crystal generator of the computer. Measuring instruments used at calibration in CCSCM are metrologically traced to the main standards in the accredited calibration laboratories.
Recommended number of calibration points (number of runs) is 8 to 12 for calibration up to 2.5 m/s, 12 to 18 for calibration up to 5 m/s and 18 to 24 for calibration up to 8 m/s, while the distance between the points somewhat increases with increasing speed. Because during every run of the carriage the water in the calibration tank gets turbulent (more for higher velocities, it also depends on the current meter suspension – a cable-suspended current meter with a sinker weight causes more turbulence than a rod-suspended current meter), there are stilling times between single runs set down for specific types of current meters and their attachments. During these waiting times the water in the tank should regain its stillness.
After all runs are carried out and the demanded number of calibration points is reached, data are processed by the computer and the calibration equation is determined. The equation has a shape of several linear equations prescribed by ISO 3455 [1]
where v [m/s] is the towing speed of the current meter computed from the covered distance and the time interval, n [1/s] is rotational frequency of the meter’s propeller computed from the predetermined number of propeller revolutions and the time interval, and αi and βi are empirical calibration constants valid in specified range of rotational frequencies n and i is number of the linear parts of the equation, whilst i ≤ 3. The constant αi roughly corresponds to a water velocity, when propeller begins to rotate, and the constant βi is close to the propeller pitch.
The calibration equation is determined by the least square method. The software assigns also position of potential break points, but the operator carrying out the evaluation can optimize their position in case of need. For this purpose, an 
auxiliary chart (0,64 MB, Adobe Acrobat document) is used, into which the differences
are plotted, where k [m] is the geometrical pitch of the propeller. The break points can be determined from this graph much more easily than from the standard calibration chart v = f(n). Potential outlaying points can also be easily determined from this chart. This chart is recommended by the Standard for this reason. This auxiliary chart has no other meaning.
It is possible to discard outlaying points during the calibration data processing (and during calibration as well) and repeat the calibration run at a given speed.
An electronic calibration record contains, apart from the calibration data, also the other data used for automatic generation of Calibration Certificates that customers are provided with.
All these data are put into the central computer in the control room before the calibration procedure starts during electronic calibration record establishing.
Current Meters with Direct Reading
Calibration of electromagnetic or ultrasonic devices is also performed in accordance with ISO 3455.
[1] This shape of the calibration equation is based on an analysis by L. Ott (Ott, L.: Teorie und Konstantenbestimmung des hydrometrischen Flügels. Berlin 1925).