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Technical Note (National Research Council of Canada. Division of Building Research), 1963-10-01
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A Remote Reading Anemometer Counter
McGuire, J. H.
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DIVISION OF BUILDING RESEARCH
NATIONAL RESEARCH COUNCIL OF CANADA
·-'f
E
C
1HIN
J[
CAlL
NOTlE
No.
403
PREPARED BY J.H. McGuir e CHECKED BY GWS APPROVED BY NBH
PREPARED FOR record purposes
SUBJECT A REMmE READING ANEMOMETER COUNTER
To carry out certain studies in the Fire Section of the Division of Building Research a remote reading anemometer operating in the range
1 to 20 ft/sec was required. A hot wire type was not acceptable due to
inaccuracies associated with variation of the gas temperature. It was
decided that a simple vane anemometer would prove satisfactory provided it was capable of being read remotely.
At about this time a small. inexpensive. transistorized capacitance transducer came on the market and it became apparent that an anemometer utilizing the transducer as a tachometer would prove less expensive than
one offering remote reading by the use of a built-in dynamo. It would also
have less drag•
. This Note describes the circuit associated with the capacitance
transducer to conve:J;"t it into a tachometer.
CAPACITANCE TRANSDUCER
The transducer (model CTI0l) is manufactured by G. I. D. Ltd••
14 Oriental Street, London, E.14, England. The product information
leaflet states: "The Transducer. which is encapsulated into a block
slightly more than 1 in. cube. consists of a transistor oscillator feeding a sensing circuit. this circuit providing a D. C. output level of an amplitude determined by the capacity between a probe terminal and earth. "
The specification states that there is a normal d-c output level of -28 v and that this will vary by about 700 mv per picofarad variation of probe capacity (over the first 15 picofarads).
2
-The transducer is bolted to the anemometer and an appropriate probe was found to be a strip of 22-gauge copper which, in the immediate
vicinity of the anemometer vanes. is
*
in. wide and 1 in. long. Figure 1illustrates the arrangement, which could be improved by securing the free end of the probe to the framework with a material such as plexiglass, to give a more robust assembly.
CIRCUIT ARRANGEMENT
A schematic ｾｲ｡ｷｩｮｧ of the circuit is shown in Figure 2. The
waveform of the transducer output is something between a sawtooth and a sine wave and the object of the first four stages is to generate a positive
going pulse train from this waveform. The first stage is a simple
amplifier and the second and third are squaring stages. Differentiation of
the output of the third stage already provides a pulse train. The fourth
stage is a pulse amplifier. which serves to give an adequate pulse amplitude and to clip the noise.
The object of the succeeding stages is merely to facilitate counting
the pulses appearing at the output of the fourth stage. Each binary stage
divides by tVio and the final output is applied to a counter drive circuit which merely lengthens the pulse to meet the specification for the input to the electro-mechanical counter.
The counter drive input may be taken from either the fourth
(divide by 16) or the sixth binary stages (divide by 64) and as the anemometer has eight vanes a count corresponds to either two or eight revolutions.
POWER UNIT
The arrangements illustrated for deriving the d-c supply voltages
are not ideal. It was originally intended to provide only one from the 7-0-7 v
winding via a full wave rectifier. It was found. however, that the supply to
the transducer had to be exceptionally stable to avoid excessive 60 cps in the
output. To achieve substantial smoothing by means of resistance capacitance
networks a higher potential was therefore derived by the use of bridge
rectification. It was then found that the current consumed by the relay
operating the counter introduced a ripple in the d-c line and this circuit was therefore powered from a separate rectifier bridge.
In fact. of cour se, the two bridges are not completely divorced, for
the lower halves of each are in parallel. The circuit performs satisfactorily.
principally because of the extravagant use of high capacity electrolytic condenser s.
A much more aesthetically satisfactory circuit would be given by using a transformer with a higher voltage secondary, say 12 -0 -12 v, and
3
-would probably be adequate, one for the final relay stage and one for all
the remaining stages. The only circuits that would then require two stage
smoothing would be the transducer and the fir st stage.
PERFORMANCE
With the selector switch set to "divide by 64, " the counter responds satisfactorily when the anemometer is s\).bjected to the highest
wind velocity that it will accept. Below 1. Z ft/sec the pulse output from
the squaring amplifier. is insufficient to operate the first binary stage and
the circuit fails, completely. This is of little importance however, as the
anemometer itself is not very reliable at this low wind speed. The
anemometer registers approximately 1.71 ft per revolution and with a
registered velocity of 1.Z ft/sec the pulse repetition frequency is about
5.6 per second.
The temperature dependence of the circuit has not been analyzed and it is possible that the very first stage might fail at high temperatures. Should this be the case the problem can readily be eliminated by the use of a more refined base input circuit which might, in turn, involve the inclusion of one mor e stage of gain.
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