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GP-HR Counter, Flow and Frequency

Application Notes
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The Fast Pulse Digital input (Channel 4) of the GP-HR Logger can be configured as a Counter, Flow or Frequency input. Incoming pulses can be from switch closures or electronic circuits. Please see Notes on Signal Conditioning for Fast Pulse Digital Inputs to determine what conditioning will be required for your application. A Breakout Probeset B-PS could be used to connect the logger to pulse sources. 

Note 1: Some Flow sensors have an analog output. These can be used on channel 1,2 and 3 of the  GP-HR Logger 
Note 2: The Fast Pulse Digital input is used to count pulses. If you need to log the state of a switch (On or Off), the switch should be read by one of the analog channels (Channel 1,2 or 3). See GP-HR Switch Application Notes 

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Counter

In Counter mode, the logger counts the number of incoming pulses per logging interval. This is useful when the exact total number of incoming pulses is required. For example, we might be monitoring the amount of milk being pumped from a vat into a tanker. Our sensor gives us two pulses per litre. We are not primarily interested in the flow rate (how many litres per minute) but in the total amount of milk pumped (how many litres). We run the Fast Pulse Digital input (Channel 4) of the GP-HR Logger in counter mode. We can see the volume of milk pumped in every logging period and total the exact amount over the whole operation in the Statistics and Period Reports. Other applications of Counter mode include:

As the counting of Tips from a tipping bucket rain gauge is a very common application for this logger, we have defined a special Rainfall mode for the Fast Pulse Digital input (Channel 4) of the GP-HR Logger. This operates as a standard counter but the channel  can easily be setup to display rain fall in  mm or inches.     

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Flow

In Flow mode, the logger counts the number of incoming pulses per second and averages these for the logging period. So flow mode is actually measuring the RATE of the incoming pulses, not totaling the number of incoming pulses. Flow readings are in units per second. A Scale Gain factor can be used to scale these readings to units per minute and units per hour. For example, we might be monitoring the amount of water being pumped from a stream into an irrigation canal. We must pump at least 30 litres per minute but must not exceed 200 litres per minute. Our sensor gives us five pulses a litre. We are not primarily interested in the total amount of water pumped (how many litres) but in the flow rate (how many litres per minute). We run the Fast Pulse Digital input (Channel 4) of the GP-HR Logger in Flow mode. The incoming pulses are counted every second. Each second in the logging period is averaged and this average is recorded by the logger. The average is in pulses per second. The Omnilog software then converts this to Litres per minute so the flow can be monitored in the Realtime, Values, Graph and Statistics screens. Because we know the average flow rate for each logging period and the number of logging periods, we can calculate the number of litres that have been pumped. The Statistics and Period Reports show a total amount but it must be realised that this is based on the average flow rate and the number of logging periods so there may be averaging and rounding inaccuracies in the total amount calculated.   

As the measuring of windspeed from an anemometer is a very common application for this logger, we have defined a special Windspeed mode for the Fast Pulse Digital input (Channel 4) of the GP-HR Logger. This operates in standard Flow mode but the channel  can easily be setup to display windspeed in metres per second, kilometres per hour and knots.    

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Frequency

Frequency mode operates in exactly the same way as Flow mode but does not allow any summing, as this is of no use in frequency monitoring applications. The result can  be displayed easily as cps, Hz or kHz. In Frequency mode the GP-HR can record frequencies up to 64kHz.

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Signal Conditioning for Fast Pulse Digital Inputs

The Fast Pulse Digital input of the GP-HR has the following characteristics 

Maximum Count Per Logging Period 65535
Maximum Frequency 64kHz
Preferred Switch Input Type Normally Open
Preferred Digital Input Type Normally High
Minimum High Period 1uSec
Minimum Low Period 1uSec
Input Impedance 470kOhm
Preferred voltage input  3.6 Volts
Maximum voltage input  5.0 Volts

For switch inputs, a debounce capacitor of 0.1uF is required across the logger input and a 1k ohm resister is required in the input line.

Circuit for GP-HR Fast Pulse Digital input when looking at switch contacts 

R1 is 1K ohm and C1 is 0.1uF

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For clean pulse inputs from electronic circuits we need to:

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Smoothing for Flow, Frequency and Windspeed

The GP-HR logger can smooth Flow, Frequency and Windspeed readings if required. For example, we may wish to monitor average windspeed rather than individual gusts. Normally the logger will record and display the average number of pulses for each second. We can increase this time if required. Before starting the logger, the actual time that the pulses will be counted for can be set. This is set at the bottom left hand corner of the Start Logger Tab of the Logger Control Screen.

Set this to the required average time in seconds. ( This will default to 1 second, which gives no smoothing)

For example, if we set Windspeed average time to 10 seconds, the pulses from the wind speed sensor (anemometer) will be counted for 10 seconds. The pulse count will automatically be divided by 10 in the Omnilog software so point, average, maximum and minimum values will be the average of 10 seconds of readings, not 1 second of readings. This will mean that the Point reading will be the average of the last 10 seconds, the Average reading will be unchanged and the Maximum and Minimum readings will be maximum or minimum windspeed for a 10 second period.

Using this feature reduces the total number of pulses per second that can be read. Maximum pulses per second for a given Average Time are as follows:

Pulse, Flow or Windspeed Average Time Max pulses per Second
1 sec 65535
2 sec 32766
5 sec 13107
10 sec 6552
30 sec 2183
1 min 1091
2 min 545
5 min 217
10 min 108
30 min 36
1 hour 18
2 hour 9
5 hour 3

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