Flow measurement

I have a CompactDAQ with a NI 9201 voltage measurment module. I am using labview to program a VI to give me a flow rate for flow through a pipe. I have a flow meter that outputs a analog signal that pulses between -0.3 and 0.3 volts. The transition rate between the possitive and negative voltages corresponds to the flow rate. I can aquire the signal but I have not been able to convert it to a flow rate. I have tried different express VI's to measure the pulse duration but they keep returning errors saying that the signal doesn't pass the mid point enough times. Does anyone know of examples or have suggestions of how to do this?

Thank-you for your response to my Question. I attached a file showing the signal. I hope this clears it up a bit.
Attachments:
flowmeter signal.JPG ‏52 KB

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I have a Cole-Parmer 18200-00 DAQ that I'm using to measure the flow rate using an ABB Aquamaster S. The flow meter sends out a pulse that is equivalent to the flow rate in the system. I'm having some trouble writing a program that would give me an accurate flow measurement. I tried converting the pulse into a voltage by using a resistor connected into the analog port but instead of getting a fixed or approximate number for the flow rate I get a range of flows. I'm guessing I should connect the flow meter to a counter port but I don't know how ?
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Attachments:
Flow Meter.vi ‏86 KB
Flow Program.docx ‏14 KB

Unless I am confused by your text document, you can simply count pulses.
One pulse means that the pinwheel rotated one time.
They have calibrated one revolution to mean that 1/20th of a Liter has passed.
Therefore, your volume will be:
V = (# pulses) * 1/20
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Q = V / (t2 - t1)
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Vahid,
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I'm Ahmed Mohamed Sheir, Recently graduated from faculty of electric and electronic engineering,
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This forum is for suggesting new ideas to improve LabVIEW. You are asking for help with a specific problem. I've asked the moderator to move it to the LabVIEW forum.
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NI9411 wrong frequency measurement of flow sensors / 24 impulse signals

Hello Ni Forum,
I already searched for similar problems, but haven't found anything helpful. So I'm posting our problem here. I'm apologizing in advance for bad english, it's not my mother tongue
We're encountering some strange behaviour of our NI9411 Module. We're using it in a Compact Rio with some other modules (2x Analog In).
Our goal is to measure the frequency of three flow sensors (impulse high = 24V, 0-22kHz) among with a torque sensor (TTL 5V, 30-60kHz) and a speed sensor (TTL 5V, 0-51,2kHz).
They are wired as follows:
DI0a = torque sensor
DI1a = flow sensor 1
DI2a = speed sensor channel A
DI3a = speed sensor channel B
DI4a = flow sensor 2
DI5a = flow sensor 3
Correspondig ports b are left unconnected, as it is said in the data sheet. All ground ports of sensors are connected to COM port of 9411. The module was connected to a 24V power supply.
Now the torque and speed sensor measurement works fine. But measuring the signals of the flow sensors seems to be difficult.
If an external frequency generator (impulse high about 9V, f=1,000kHz) is connected to port DI1a instead of the sensor signal, LabView shows exactly 1,000kHz. DI5a showed an avarage of 10 Hz, with a range from 10 to 60 Hz. DI4a equaled constant zero. The flickering up to 60Hz of DI5a was gone after disconnecting the 24V supply of the module. This changed nothing on the phenomenon described below.
If the signal high impulse voltage is raised, the measured signal frequency goes up to 1300 Hz @ 24V signal high impulse voltage. The signal additionally raises, if an oscilloscope is connected to DI1a, to 2180 Hz (range 2110 to 2240Hz). However, there is no change of frequency if the osci is connected to DI5a. With the 24V supply connected, DI5a showed a frequency up to 600Hz @ 24V signal high impulse voltage on DI1a, also rising when connecting the osci on DI1a. As mentioned, there was no frequency measured on DI5a after disconnecting the 24V power supply.
The signals of the generator showed some peaks on each edge (rising and falling) with values for example of 11V above high impuls voltage of 9V (20V together). That may causes that wrong frequency measurement.
The signals of the flow sensors do not have such peaks, the LabView-frequency nevertheless does not match the real frequency, as shown below (all values in Hz):
LabView
ideal values
Flow Sensor 1
Flow Sensor 2
Flow Sensor 1
Flow Sensor 2
4600
12200
740,6
6760,0
6400
13500
1481,3
7500,7
9700
16400
2962,6
8982,0
12900
19500
4443,8
10463,2
We're kind of helpless now. How can we get the signals of the flow sensors measured the right way? Are we doing something essentially wrong? Thanks in advance for offered help. I hope, I've given any necessary information. Any missing details will of course be delivered.
Puls-Measure-VI is attached beneath.
Regards,
Johannes Rost
Attachments:
vi.png ‏20 KB

Dear Lynn,
thank you for your reply. The impedance was an issue that our local technical distributor already suggested to check. According to the datasheet of the flow sensor (see attachement) it can drive a current of 25mA. The signal generator we used is a test generator that can be used instead of the flow sensors to check the any evaluation unit the sensor is connected with. So it should generate signals that are equal to those the sensor would have generated. And even with this the frequencies measured with LabVIEW are not as expected.
We are in contact with the sensor manufacturer to exclude mistakes in connection on the sensor side. But the behavior with the signal generator makes us think that is has to do something with the characteristics of the NI9411 that we don't know yet.
Attachments:
block diagram signal amplifier and signal description.png ‏151 KB
technical data.png ‏124 KB

Counting pulses and measuring flow rate per second

Hi everyone
I am using a compactField Point (cFP-2220) controller to measure the flow rate. The flow sensor that i have, gives the output in terms of the pulses. I need to count the number of pulses for a particular interval and divide it by a K-factor( constant value) to get the flow rate. If i select the interval to be of a second, i would want to count number of pulses per second and divide it by K-factor to get flow rate ( per second).
How do i count pulses per second and consequently measure flow rate using this count and K-factor.
Any kind of help would be highly appreciated.
Thanks in advance.

You may be able to use the Edit Tempo function to work this out. With you waveform in Edit view Right Click on the time scale at bottom of waveform and select Bars and Beats display. Select a number of beats by highlighting with the cursor. Then again on the Time bar select Edit Tempo. In that dialogue you can try clicking on Extract to find the Tempo ie. beats per minute. Also you can try adjusting the tempo by entering avalue manually to see how close that gets to your waveform.
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Flow measurement

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