COMPASS for Flow - Proper K Factor and Voltage device selections
COMPASS by default also applies the Measurement K Factor to the Control voltage (or current). You will see this when you go to Run a typical COMPASS Test as there are two screens where the K Factors are shown. The first one presented is for flow Measurements and the second is for the Control voltage. There are some limited cases where it is desired to also apply the K Factor to the Control voltage (when using a dedicated MFC for control only), but in most cases the K Factor needs to be reset to 1 so that no correction is being applied to the Control Voltage. This needs to be done manually during the Test initialization.
If the Control K Factor is not reset the only real effect is the setpoints that are being sent will not be the expected value as the K Factor is being applied to them also. For example, if your Test has a setpoint of 100slm defined but you apply a Control K Factor of 1.41, you would see the actual setpoint produce about 141slm.
It can also be confusing which device to select as the Voltage device so will cover that in this article also. Below is how to make the proper selections as go through the Run Test initialization screens.
The first screen shows the Measurement K Factor and uses the value from your DUT setup. In this case we want a measurement K Factor of 1.41. No changes need to be made to the K Factor in this first screen. For the Voltage device make sure to select your proper Molbox or other voltage measurement source that has “Voltage Measurement” listed at the end of it. You DO NOT want the one that says supply voltage measurement on this screen.
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Once you get to the second MFC initialization screen, it initially puts you in the “Output 2/2” display screen. DO NOT click on the blank Voltage drop down yet as that can change it to a different display screen. This is where the Control K Factor is set and you will see by default it uses the same measurement K Factor as before, but we typically do not want this K Factor:
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So, make sure to manually change the K Factor to 1.00 in this display screen:
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Next, click on the Voltage drop-down and select your proper Molbox or other device that has “Voltage Supply” at the end of it. Once this selection is completed you will see it automatically changes to the “Output 1/2” display screen. Make sure to realize it has changed the display screen so now it will show the measurement K Factor of 1.41 and that is still correct. DO NOT reset this K Factor by accident as it was correctly defined on the first MFC measurement K Factor screen a few screens back.
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It is recommended to then use the slider bar in the above window to scroll between Output 1/2 and Output 2/2 to verify everything still looks good for both before clicking the Next button. Output 1/2 should have your K Factor (say 1.41 in this example) and a “Voltage Measurement” device selected in the Voltage drop down. Output 2/2 should have the K Factor set as 1.00 and a “Voltage Supply” device selected in that dropdown.
COMPASS by default also applies the Measurement K Factor to the Control voltage (or current). You will see this when you go to Run a typical COMPASS Test as there are two screens where the K Factors are shown. The first one presented is for flow Measurements and the second is for the Control voltage. There are some limited cases where it is desired to also apply the K Factor to the Control voltage (when using a dedicated MFC for control only), but in most cases the K Factor needs to be reset to 1 so that no correction is being applied to the Control Voltage. This needs to be done manually during the Test initialization.
If the Control K Factor is not reset the only real effect is the setpoints that are being sent will not be the expected value as the K Factor is being applied to them also. For example, if your Test has a setpoint of 100slm defined but you apply a Control K Factor of 1.41, you would see the actual setpoint produce about 141slm.
It can also be confusing which device to select as the Voltage device so will cover that in this article also. Below is how to make the proper selections as go through the Run Test initialization screens.
The first screen shows the Measurement K Factor and uses the value from your DUT setup. In this case we want a measurement K Factor of 1.41. No changes need to be made to the K Factor in this first screen. For the Voltage device make sure to select your proper Molbox or other voltage measurement source that has “Voltage Measurement” listed at the end of it. You DO NOT want the one that says supply voltage measurement on this screen.
Once you get to the second MFC initialization screen, it initially puts you in the “Output 2/2” display screen. DO NOT click on the blank Voltage drop down yet as that can change it to a different display screen. This is where the Control K Factor is set and you will see by default it uses the same measurement K Factor as before, but we typically do not want this K Factor:
So, make sure to manually change the K Factor to 1.00 in this display screen:
Next, click on the Voltage drop-down and select your proper Molbox or other device that has “Voltage Supply” at the end of it. Once this selection is completed you will see it automatically changes to the “Output 1/2” display screen. Make sure to realize it has changed the display screen so now it will show the measurement K Factor of 1.41 and that is still correct. DO NOT reset this K Factor by accident as it was correctly defined on the first MFC measurement K Factor screen a few screens back.
It is recommended to then use the slider bar in the above window to scroll between Output 1/2 and Output 2/2 to verify everything still looks good for both before clicking the Next button. Output 1/2 should have your K Factor (say 1.41 in this example) and a “Voltage Measurement” device selected in the Voltage drop down. Output 2/2 should have the K Factor set as 1.00 and a “Voltage Supply” device selected in that dropdown.