flow measurment using labview

[vnarang]

Hi thanks Lynn and Enzian for the reply.I have not tried wt u have suggested,I am going to tryit todya am waiting fro the resistor/I have few questions in mind.Putting resistor the way u guiys suggested do u think this will sove my frequenxy problem i am getting now.I am getting frequency in the range of 65K Hz but I am suppoosed to get max og 100 HZ.I am not able to undertsnd  how this is going to fix the problem

please advise

Thanks a lot

I will update u as soon as I am done with resistor

[johnsold]

The resistor will not change the frequency.  However, it is possible that you may have been missing some of the pulses without it. 

Are you sure that you have a flow rate which will produce 100 kHz? Some cavitation or even the flow resistance of the meter might be enough to reduce your flow so that it is either not what you expect or not measured correctly.  Flow can be easy to estimate or to measure approximately and difficult to measure accurately.

Lynn

[vnarang]

Hi Lynn

Thanks for the reply.Idea u gave I am actually able to acquire data in Oscilloscope very well and when I am varying the flow I can see change in frequency in Oscciloscope.The maximum frequency generated by Flow meter is 100 Hz. not 100 KHZ

But when I am connecting my flow meter to NI DAQ,I am having the same proble,frequency observed is not correct.i am still getting frequency is 60 KHz and I am not supposed to get more yhan 10 HZ.

I am using NI 9401.Am I using the write module for this kinda signal

On NI chasis there are two pin holes for trigger 1 and trigger 2.I knw its a dumb question bt do I have to connect something tothese trigger 1 and trigger 2.

I used 3 types of resistance 500 Ohms,1 K ohm and 10 Kohm

Please suggest me something.I will be greatfull to you,I am seriosuly stuck

Regards

Vikas

[johnsold]

Vikas,

You are making some progress now.  With the resistor and oscilloscope you now know that you have a usable signal and the correct frequency.

Next, look at the signal on the oscilloscope while it is connected to the NI 9401.  Do you still have a clean logic level signal?  It is possible that the sensor switches too slowly for the digital inputs and they may be recording multiple counts for each pulse.  Look closely with the oscilloscope at the slope of the pulse as it passes approximately 1.4 V.  What is that slope?  You may need to put a Schmidt trigger device between the sensor and the DAQ device to clean up the signal.

The 9401 appears to be a digital input device, not a counter. How are you getting your frequency measurements?

Lynn

[vnarang]

Hi  Lynn

Thanks a lot. when I am trying to measure the frequency I am still getting frequency in thousands,but I when I use counter to measure the counts I am getting rite flow and when I am varying my flow physically i can see the change in the flow in lab view and I think its showing rite.

U saying NI 9401 cant measure counters,Please advise

Secondly, when I connect -V of my DC to the ground screw at the bak of NI 9178 i dont get rite counters and when i remove it I am getting rite counters.

I have attcahed the VI which I am using to get my flow.Please have a luk and comment.

Your suggestions helping me a lot.

Regards

Vikas

[johnsold]

Vikas,

1. I do not have the DAQ Assistant so I cannot tell what it is doing.

2. The NI 9401 data sheet says it is a digital I/O device.  It does not count pulses.  If you are reading the flow sensor output as a time series of digital inputs, you could count them.  Is that what the DAQ Assistant is doing? Or is that what the detflow new.vi does?  You have not posted that Vi.

3. It is beginning to sound like you have some kind of wiring problem.  Are different parts of your system connected to different grounds?

You do not need the local variables.  Wire directly or through shift registers to accomplish the same things.

Please post the subVI.  Also run your program to show some typical data, then Make Current Values Default, and save.  Post the VI with saved data.  That way we can see what you have without your hardware.

Lynn

[vnarang]

Hi Thanks Lynn

Grounding: I have connected the grounds as shown in the figure attached

I have attached the flow meters vi with detflow as u suggested.

when I am connecting _v of dc suppy with NI chasis I am getting very high counts which I am sure are wrong.when I donot connect ground with chasis i get frequency of around 92 Hz.I think thi is rite.as pipe to which flow meter is connected is 3/4 inch and maximum flow can be 16 gpm.

In labview i am using counter to measure the counts and when i use frequency as u suggested till i am getting 67000 HZ and more and more.

But in osciloscope if i dont connect - V (DC) to ground /common on oscciloscope i donot get signals.and when I connect it i get frequeny oof 25 Hz.

I am still not able to understand y d difference.

Please suggest.

regards

Vikas

[dkfire]

Lynn:

It is probably the chassis that has the counter. The module it self can't count, but the chassis can if you setup DAQmx/ DAQ Assistant.

Vikas: Wich chassis are you using?

Something like the NI-9178 ? http://sine.ni.com/nips/cds/view/p/lang/en/nid/207534

EDIT: And after looking at your diagram, I can see you are using the NI-9178.

[vnarang]

Hi Lynn

yes i am using NI 9178 and chasis NI 9178 has the counter built in.

I have attached the picture showing the grounding screw in the chasis

Please tell me y there is a big difference in the counts.in osccope and Ni daq

[dkfire]

How have you connected the ports on the NI 9401 to your flowmeter?

And I don't you need to ground the chassis that way you are doing. It should be ok to only connect to the NI 9401, but in the right way.