Alicat instruments are one of the most accurate differential pressure based laminar flow measurement systems on the market. This technology not only provides an unmatched accuracy but also the ability to monitor a number of different parameters in your process with just a single device. Owing to the way flow is measured in an Alicat, each mass flow device reports gas absolute pressure (gage pressure available if internal barometer is requested), temperature of the gas stream, mass flow rate, and volumetric flow rate.
As both mass and volumetric flow readings are provided by Alicats, it gives you the freedom to choose the parameter that you wish to measure or control depending on what is required in your application.
1. Find the values for mass flow and volumetric flow
On the Alicat display
On the display for any Alicat flow instrument, the parameters shown on the device and the button ascribed to them are:
- Absolute pressure: Top left
- Temperature: Top center
- Set point/Tare: Top right
- Volumetric flow: Bottom left
- Mass flow: Bottom center
In the Alicat Serial Data Frame
When communicating serially with an Alicat flow meter or controller, typing the unit ID and pressing enter will cause your terminal to echo a serial data frame. This data frame provides you with all the parameters of importance including the gas selection and status codes.
Below is an example of an Alicat serial data frame for a mass flow controller which is being used to control flow rate of air and has the totalizer configured on it:
A mass flow meter’s data frame reports all the parameters except the set point as there is no ‘control’ element in play here.
2. Change the displayed/controlled parameter
Changing the displayed reading on any mass flow meter or controller
Changing the displayed value on the main screen is straightforward and can be done by just pressing the button below the readings on the display. So, if volumetric flow is the desired display parameter, pressing the bottom left button will display its value as the highlighted reading on the display in the center. Similarly for mass flow, pushing the bottom center will highlight its reading in the center. By default, Alicat mass flow devices will ship with the mass flow as the largest number. If you would rather use volumetric flow, let us know, and we will set that to the largest number instead free of charge.
Changing the loop variable in a controller
A. Changing the loop variable using the display
Loop variable is the parameter that is being controlled by any Alicat. When set to volumetric flow, mass flow, or pressure the PID loop will work to achieve the commanded setpoint for the selected parameter. How can we change this loop variable in an Alicat flow controller?
Loop variable selection can be accessed through this menu selection sequence:
MENU → CONTROL → ADV CONTROL → LOOP SETUP→ LOOP VAR → VOLU/MASS FLOW
B. Changing the loop variable using serial communication
Alicat devices are made with an aim to provide you with a flexibility in the ways that you can interact with your device. Loop variables can also be changed using serial communication on your Alicat with RS-232/RS-485 communication capabilities.
Once, serial communication has been established, change the value for the register dedicated for this function, which in this case is Register 20. For full information on Register 20 and more information on serial communication, see the Serial Communications Primer.
- If the controller is set to control mass flow and the desired control variable is volumetric flow:
- Read the initial value of Register 20, by using the command AR20 (If the device unit ID is A)
- To change the control variable to volumetric flow, subtract 256 from this value and write the result in this register by using the write command, AW20 = result and press enter.
- If the controller is set to control volumetric flow and the desired control variable is mass flow:
- Read the initial value of register 20, by using the command AR20 (If the device Unit ID is A)
- To change the control variable to volumetric flow, add 256 from this value and write the result in this register by using the write command, AW20 = result and press enter.
All Alicat flow controllers are sent out with the control variable set to mass flow unless otherwise requested. Switching the control variable is a very straightforward process which can also be done using serial communication.
If mass flow device with unit ID A is set to Mass flow. The steps that need to be taken to change the control variable to volumetric flow are:
- Establish Serial Communication
- Read register 20 using the command, AR20. For calculation purposes, assume the value to be 9239
- The new value will be 9239-256 = 8983
This should change the loop variable to volumetric flow. To change the loop variable to mass flow, just add 256 to the value in register 20.