Flowmeter - pulse or voltage?


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Not sure if this is the right section, but...

I need to measure a flow. In looking at paddle-wheel type flowmeters, some output a pulse per fixed quantity. Some more expensive ones will generate a V proportional to flow. Is there a simple way to use my U3 via daqfactory and read a pulse frequency, or a pulse count, or should I spend more for the voltage output type?


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Whenever you have a choice between a device that outputs pulses and the same device that outputs volts for the same measurement you should choose the pulses. This is of course provided you are using one of the LabJack devices (any type), have an available counter or timer, and the frequency of the pulses is less than the max freq of the counter or timer.

The reason is that pulses are digital signals and volts are analog. Like everything else that toutes digital over analog, the reason is that digital signals are less subject to noise. All your device with a voltage output is doing is counting the pulses coming out of the paddle wheel and generating a proportional voltage. Its called a frequency to voltage converter for the simple reason that that's what it does. So, you have a pulse, which is then converted to an analog voltage which you feed into an analog input on a LabJack which converts it to a digital signal. If you use the pulses directly with a counter, you simply have a pulse which the LabJack converts into a digital signal. Notice how "analog" isn't in this method.

Exception to this rule:

If the distance between your sensor and your LabJack is long and you are in a noisy environment, then there is the possibility of getting a noise spike that looks like a pulse. You can easily check for this by simply stopping the process and making sure the counts remain at 0. If they don't, there are two solutions:

1) move the LabJack closer, either by using a UE9 that supports ethernet, or extending your USB using hubs. Note that you cannot run a USB cable more than 6 feet without a hub. Yes they sell USB cables that are longer than 6 feet, but the spec says 6 feet max and if you want reliability you'll follow the spec.

2) you could use a freq-voltage converter and then a voltage to current converter to get a 4-20mA signal and run that to the Labjack, then put a resistor to convert the current back to voltage and read it with an analog input. Current signals are not subject to noise, thus the reason 4-20mA signals are used in industrial environments. Of course this method requires extra hardware, and will generate noise of its own because of all the conversions so its a bit of a toss up. Which is better really depends on environment.

As for how to do it with DAQFactory and U3, go to www.daqexpress.com and you'll find samples that include reading the U3 counter.

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Yes, the pulse output is going to be best in most situations. The U3 has 2 hardware counters plus 2 timers that can be configured as firmware counters. The UE9 has 2 counters and 6 timers. See the following forum topic for good general info:


The noise problem mentioned by AzeoTech is usually handled very easily. Sometimes this can simply be a stronger pull-up resistor, or sometimes a simple RC filter is used. The idea is that if you know a pulse is at least some length of time long, you filter out pulses shorter than that, and transients due to noise are generally very short.

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