C.4.4 Current Meter Counters and Timing Devices
This section describes common methods of obtaining the revolution count from the Price 622AA fitted with a standard penta chamber.
In situations where the rotor is visible and the rate of rotation is not more than two revolutions per second, a visual count may be timed with a stopwatch. Paint half of one cup with a bright colour. Start and end count as the open face of the cup passes through the yoke.
Suppliers of current meters usually offer battery powered headset kits as an option. When a low-voltage battery is connected to the meter, an electrical pulse generated by either the single or the penta eccentrics making contact with their respective sensor wires may be used to provide an audio signal or a digital counter input. At low-to-medium velocities, the sensor wires dragging over the eccentrics will usually produce multiple signals during the course of one contact resulting in gross errors in the digital displays of most counter units. A counter for use with a penta head Price meter requires a square wavelength input signal the length of which can be varied to suit the rotor speed.
The direct connection to the Price 622AA or Pygmy meter is accomplished by attaching one spade connector (or bare wire) to the meter/rod mounting screw (or the hanger mounting screw) while the other connector is attached to either terminal on the penta chamber. Connecting to the top terminal should generate 1 click/rev, connecting to the lower terminal produces one click per five revolutions. The Pygmy has one single count terminal.
Note: An audio counter may be easily assembled using the following off-the-shelf-components: mini headphones with mini phone jack, phone jack socket, plastic pocket flashlight, length of lamp cable, and two or more open-ended #10 spade connectors.
Assembly: Remove flashlight bulb socket and replace with mini phone jack socket. Disable flashlight on/off switch. Split one end of the lamp cable and connect it to either pole of the on /off switch. The other end of the cable can be fitted with the spade connectors or any other type of connector to match the meter suspension to be used. Solder all modified connections.
Testing: Plug the headphones into the socket and touch the outer ends of the cable together several times. Matching clicks should be heard in the headphones as the circuit opens and closes.
The two following methods of modification result in providing clean signals to digital counters.
The so-called Low Velocity Price Meter is a standard 622AA with the penta chamber sensor replaced by a revolving magnet that activates a reed switch. This alternative sensor unit can be purchased as retrofit or supplied complete with the current meter, i.e. current meter "AA" Magnetic Head Model 1215. This type of current meter may be connected to a variety of revolution counting devices such as powered headphones, digital counters (including the impeller types described below), velocity display calculating units, and field computer interfaces.
Note: All revolution indicators passing current through magnetic reed switches should have a spark suppression diode included with the unit or wired into the positive pole of the connector.
Note: Velocity display units must employ time averaging over at least 40 seconds and should be capable of applying changeable current meter calibration equations.
The operational capabilities of the Photo-Fibre-Optic Swoffer 2200 quartz-timed counter/retrofit kit have already been described. This control unit, powered by a standard 9-V transistor battery, provides for input from both the retrofitted Price 622AA and Pygmy meters.
There are several types of counters available that are primarily designed to display the faster rate of pulse transmission generated by impeller type meters. The models are described below in ascending order of capability (and price).
Note: The inexpensive and simple counter and stopwatch option is an awkward combination for streamflow measurement because the technician must simultaneously operate the start and stop buttons on both the stopwatch and counter with one hand while holding the rod or handline steadily in position with the other.
Every pulse generated by the current meter is counted. The instrument counts a maximum of 10 pulses/s and is thus suited for operation in conjunction with meters generating a signal at every revolution of the propeller. A button is provided for resetting to zero.
The OTT F4 is powered by two 1.5-V flashlight batteries, and the OTT Z21 is powered by four 1.5-V cells. Both instruments are used in conjunction with a stop watch.
The CMC 20 and CMC 200 counters (Figure C-26 and Table C-8) are self-contained instruments featuring solid state circuitry, a quartz timebase, and LCD. Both counters are powered by six AA zinc carbon or alkaline cells.
An internal buzzer sounds at each contact closure during the timing period to indicate correct operation. The displays indicate time and count with a resolution of 0.1 seconds and one count respectively, to a maximum of 1000 seconds and 10 000 pulses. See Table C-9 for troubleshooting.
Note: A small variable error is introduced when a counter is operated in the pre-set time mode. While the time count starts when the first pulse from the meter is received, the count stops at the end of the selected time period. If the counter "times out" between pulses, some portion of the pulse cycle will be ignored. This error will not occur in the CMC 200 or OTT Z210 when the pre-set pulse mode is selected.
The simpler of the counters, the CMC 20, displays the accumulated pulse count for a range of pre-set time periods. In addition an "integrate" range is included for use when integrating depth measurements are made with a ground-feeler weight. In the "integrate" mode the buzzer sounds continuously while the contacts are closed and counting continues until interrupted manually by the STOP switch. The use of a supplementary timer is necessary in this mode. The OTT Z210 has similar capabilities.
The CMC 200 counter includes the features of the CMC-20 combined with the choice of counter operation in fixed count mode. Dual displays indicate both time and count. The OTT Z200 has similar capabilities.
The Braystoke (BFN current meters) counter has capabilities similar to the CMC 200, although it is somewhat heavier.
Table C-8. Features of the CMC-20 and CMC-200 counters.
Action |
CMC-20 |
CMC-200 |
Pulses per second |
N/A |
Counter allows choice of fixed time or fixed count. |
Start |
Resets display and commences pre-set time interval. |
When PULSES/SECOND switch is set to SECONDS, resets display and commences pre-set time interval.
|
Stop |
Stops further counting and allows the counter to be resent and restarted by the START switch. |
In SECONDS mode, operation of the STOP switch disables the internal counter on the next whole second after STOP switch operation.
|
Rotary switch |
Selects a range of re-set time periods and the integrate mode for use with ground-feeler weights. |
Selects a range of pre-set time periods for pre-set counts, depending on the position of the PULSES/SECONDS switch. |
Display |
Displays pulses on one display. |
Displays pulses and time in 1/10 seconds on two displays. |
Annual servicing |
Replace batteries. | |
Table C-9. Troubleshooting faults: CMC-20 and CMC-200 counters.
Symptom |
Possible Cause |
Action |
No count or audible signal while meter propeller turning. |
Faulty leads or connections at one terminal. |
Check loads for continuity. |
All decimal points showing on display. |
Low battery voltage. |
Check voltage. Low level is 5.5 V. |
Figure C-26. Panel layout for the CMC-200 (top) and CMC-20 (bottom) counters.
