Flowmeters (App Note)
Flowmeters are instruments used to measure the flow of liquids and gases. They can have either analog or digital outputs.
Power Supply
This section describes power supply options. Once the power supply is properly connected to the sensor and LabJack, the only other sensor wire left to connect should be the signal output. The sensor signal output should be connected to the appropriate LabJack I/O terminal; see the subsequent sections of this application note.
VS (5V)
If your sensor can be powered with 5 VDC, you could connect the sensor Power+ terminal to any VS terminal on the LabJack for power. Connect GND/Common/Power- on the sensor to LabJack GND.
Higher Voltage Requirements
Flowmeters commonly define a power supply range such as 12-40 VDC. Voltages greater than 5V cannot be sourced from LabJack devices. Fortunately, higher voltages are easy to provide with an external power supply. See our Externally Powered Signals App Note for power supply recommendations and other information such as wiring guidance.
Pulse Output
Pulse outputs are typically the best option available for interfacing flowmeters with LabJack devices.
Signal Connection
Be sure to see the power supply section above for power supply connection information.
A pulse output would typically be connected to a LabJack DIO terminal. For example, the FIO4 terminal.
NPN and PNP outputs are perhaps the most common pulse output types.
NPN outputs can be connected directly to LabJack DIO terminals
PNP outputs would require pull-down resistors.
See the detailed NPN/PNP configuration information in our Open Collector App Note.
There could also be Push-pull (driven) pulse outputs. As long as the output voltage is 5V or less, it should be possible to connect the output directly to a LabJack DIO. Otherwise, a series resistor to limit the voltage at the DIO may be necessary. See the guidance in our Driven Signal App Note.
Software and Raw Signal Measurement
We recommend seeing your device Quickstart Tutorial to get more familiar with your device. The "Moving forward from here..." sections have more information about software options that could be used to measure your flowmeter output.
Pulse output signals can easily be monitored on LabJack DIO terminals using counter or frequency measurement features:
If you have a UD device, also see our Configuring and Reading Counters and Configuring and Reading Timers guides to get started.
If you have a T-series device, also see our Configuring and Reading Frequency and Configuring and Reading a Counter guides to get started.
Measurement Conversions
Your sensor datasheet may have a constant or coefficient that describes the number of pulses expected per unit flow. If you cannot find this specification but there is a range of flow specified, you might expect to see 1 pulse per unit flow and the measured frequency on the LabJack should directly correspond to the flow rate.
Using the information above, you can find the following relationships:
flowRate (units flow/second) = measuredFreq (pulses/second) / pulseCoefficient (pulses/units flow)
totalFlow (units flow) = totalPulsesMeasured (pulses) / pulseCoefficient (pulses/units flow)
For example, say you have a flowmeter that is specified to output 500 pulses per liter:
flowRate (liters/second) = measuredFreq / 500
totalFlow (liters) = totalPulsesMeasured / 500
Troubleshooting
See our Troubleshooting a Digital I/O App Note.
4-20mA Output
Signal Connection
Be sure to see the power supply section above for power supply connection information.
4-20mA outputs are analog signals and would be measured using the device analog inputs (AIN).
4-20mA output requires signal conditioning to be measured with LabJack devices.
The most likely option to condition the signal is to use our LJTick-CurrentShunt.
See our Measuring Current App Note for more information about conditioning options and wiring diagrams.
Some 4-20mA sensors may denote support for the HART protocol. Our devices currently do not support the HART protocol, but you should still be able to measure 4-20mA regularly with a HART compatible sensor. If you are doing procurement, we recommend looking for a sensor without HART support since HART sensors typically cost more than regular 4-20mA output sensors.
Software and Raw Signal Measurement
We recommend seeing your device Quickstart Tutorial. The "Moving forward from here..." sections have more information about software options that could be used to measure your flowmeter output.
Current outputs would be measured by reading the device AIN and converting the voltage measured to flow.
Measurement Conversions
Typically a sensor output of 4 mA will correspond to the no flow condition and 20 mA the maximum or rated flow, with a linear relationship between flow and current output. You should be able to find related specifications in the sensor datasheet.
Since 0 flow usually corresponds to 4mA output, the conversion equation from current to flow needs an offset of -4 mA from the measured current.
The relationship between flow and current should be linear, so you can find a conversion equation slope (unit flow/mA) like the following:
(ratedFlow - 0) / (20mA - 4mA) = ratedFlow / 16 mA
Combining the slope and offset information, you can find a conversion equation:
flow = (currentMeasuredmA - 4) * ratedFlow / 16
LabJack analog inputs can only measure voltage, so another step is required to condition the current output into a voltage measurement and convert the voltage measured to current measured.
LJTick-CurrentShunt
If using the LJTick-CurrentShunt, the conversion equation from voltage measured to current measured is the following:
currentMeasuredmA = 8.475 * voltageMeasured
From there, you can replace currentMeasuredmA in the flow conversion:
flow = (8.475 * voltageMeasured - 4) * ratedFlow / 16
For example, say that you have a flowmeter with a specified maximum flow of 5 GPM (gallon per minute):
flow (GPM) = (8.475 * voltageMeasured - 4) * 5/ 16
Shunt Resistor
Note: If you want to use a shunt resistor, you need to be careful not to exceed the maximum load specified by the sensor.
If using a shunt resistor for the 4-20mA measurement, the conversion is an application of Ohm's law I = V/R:
currentMeasuredmA (mA) = voltageMeasured (V) * 1000 (mA/A) / resistanceShunt(Ω)
From there, you can replace currentMeasuredmA in the flow conversion:
flow = ((voltageMeasured * 1000/resistanceShunt) - 4) * ratedFlow/16
Simplifying:
flow = (voltageMeasured * ratedFlow)/(0.016 * resistanceShunt) - ratedFlow/4
For example, say that you have a flowmeter with a specified maximum flow of 5 GPM (gallon per minute), and using a 100 Ω shunt resistor:
flow (GPM) = (voltageMeasured * 5)/(0.016 * 100) - 5/4 → flow (GPM) = (voltageMeasured * 5 /1.6) - 1.25
Troubleshooting
We always recommend starting by removing all external connections to the LabJack and testing the AIN themselves, then if that looks okay compare your signal AIN measurements with a DMM reading. This should help verify that the LabJack and sensor are operating as expected. See our Troubleshooting Your Analog Input Signal App Note.
Test your conditioning circuit. If using the LJTick-CurrentShunt, see the Troubleshooting section of its datasheet.
Check the output at multiple set points. Check whether your measurement at no flow is equivalent to 4 mA or 0 mA. Take a measurement at some known flow setting. If there is a common offset error in both the no flow and known flow measurement, there may be some problem with your flow conversion equation.
Voltage Output
You may find some flowmeters with high level voltage outputs such as 0-5V or 0-10V output. When using LabJack devices, voltage outputs are typically easier to work with than 4-20mA outputs.
Signal Connection
Be sure to see the power supply section above for power supply connection information.
You should be able to directly connect a voltage output signal to one of the LabJack AIN terminals for measurement.
U3-LV: If you have a U3-LV, all of your analog inputs only have a roughly 0-2.5V input range. You would need an accessory such as our LJTick-Divider-4 to measure a higher voltage output such as 0-10V. All other LabJack models support a 10V analog input range on at least some of the inputs.
T8: If you have a T8, you have an AIN+ and AIN- terminal for each analog input. The signal output should be connected to AIN+ and your sensor GND/Common/Power- should be connected to AIN-.
Software and Raw Signal Measurement
We recommend seeing your device Quickstart Tutorial. The "Moving forward from here..." sections have more information about software options that could be used to measure your flowmeter output. Voltage outputs would be measured by reading the device AIN and converting the voltage measured to flow.
Measurement Conversions
The relationship between voltage output and flow should be linear. This means that you should measure the maximum voltage output when at the sensor "rated flow" and 0V at no flow. You should be able to find specifications for things like the rated/maximum flow in the sensor datasheet. The corresponding conversion equation should be as follows:
flow = voltageMeasured * ratedFlow / maxVoltageOut
For example, if you have a flowmeter with a specified maximum flow of 5 GPM (gallon per minute) and an output of 0-10 V:
flow (GPM) = voltageMeasured * 5 / 10
Troubleshooting
We always recommend starting by removing all external connections to the LabJack and testing the AIN themselves, then if that looks okay compare your signal AIN measurements with a DMM reading. This should help verify that the LabJack and sensor are operating as expected. See our Troubleshooting Your Analog Input Signal App Note.
Check the output at multiple set points. Check whether your measurement at no flow is 0 V. Take a measurement at some known flow setting. If there is a common offset error in both the no flow and known flow measurement, there may be some problem with your flow conversion equation.