Troubleshooting flow readings

Troubleshooting flow readings for your Alicat mass flow meter or mass flow controller

If none of the troubleshooting questions or resolutions below address your issue, please contact us through the survey at the bottom of this page, live chat (click the icon at the bottom right of your browser window), email ( or phone (+1.888.290.6060). We’ll do all we can to help you resolve your issue.

How can I see flow readings in different units?

You can choose from a number of engineering units for display of flow, pressure and temperature readings.

  • From the main screen (Standard Mode if your Alicat has multiple modes), push the button below the flow reading twice, and then choose Set button eng units. Use the UP/DOWN keys to move the > cursor to the desired unit, and then press SET. This changes engineering units shown on the display, but not the units in the serial data frame.
  • You can change engineering units globally by selecting MENU > BASIC CONFIG > DEVICE UNITS. Changes made in this menu affect both the instrument display and the serial data frame.

My flow readings are negative.

Alicat flow instruments will display negative readings when flow occurs in the opposite direction. If you expect flow in both directions through your instrument, you can ask for a bidirectional flow calibration at your instrument’s annual recalibration.

  • Under conditions of no flow, a negative flow reading indicates that the meter was flowing when it was last tared. Ensure that your instrument has no flow passing through it, and select MENU > TARE to give it a fresh tare.

  • If you have a low pressure drop flow meter (Whisper or FP-25) that is not connected to anything, it may be reading a light breeze that is entering its outlet. Plug one end to see if the flow returns to 0.

Does an Alicat flow meter work if it is laying down or if it is placed on top of a vibrating device? Will its flow readings be accurate?

Yes to both questions! Alicat pressure-based laminar flow instruments are internally compensated for any changes in orientation, so you can use it sideways, on its back, or upside-down—all without affecting the flow readings. It can also be placed on top of a vibrating pump or other device without affecting its accuracy.

The live flow readings won’t settle down.

Alicat flow measurement is very fast, so your instrument can detect subtle variations in flow that may go unnoticed by other flow instruments. If your pressure supply is variable or you are calibrating a pump that is oscillating, try the following to smooth out the reported flow readings:

  • Increase the flow averaging time constant by selecting MENU > ADV SETUP > SENSOR SETUP > FLOW AVG.
  • If you ordered your device with totalizer averaging (TOTAVG in your part number), reset your totalizer. The averaged flow rate updates live, so you can watch it until the value stops changing.
  • If you have an FP-25, you can record a single averaged flow reading over a specified amount of time by pressing START AVG from the Instant screen.

It is also possible that you may have some small debris (such as shredded pieces of Teflon tape) stuck in the flow path. Inspect the inside of the upstream process port to ensure there is no debris.

My flow readings don’t change when the flow changes (readings stay near 0 or full scale).

Flow readings that won’t change regardless of actual flow usually indicate that the flow sensor has been damaged. The most common cause of this is exceeding the maximum allowable instantaneous differential pressure for your device (75 psid for most instruments; 15 psid for Whispers, FP-25).

  • If you have a flow meter, this can happen if a solenoid valve upstream of the meter suddenly snaps open to allow the high-pressure gas through the meter.
  • If you have a flow controller, its valve will normally help to protect the sensor from large pressure differentials. However, a sudden change in pressure can subject the sensor to high pressure drops if the valve does not have enough time to adjust.
  • Please contact Alicat to troubleshoot and possibly set up a repair. Note that sensor damage due to excessive differential pressure is not covered by Alicat’s warranty.

Alicat mass flow meters can be tared by grounding a specified pin (see your pinout diagram). If this pin is continuously grounded, it will cause the flow reading to read 0 at all times.

  • Please check your electrical connections to ensure this pin is not grounded.

The flow readings jump to 0 when flow rates are low.

Your flow instrument is equipped with a programmable deadband that is preset at the factory. Usually, the deadband preset is 0.5%, so any flow reading below this level displays as 0.

  • Reduce your deadband threshold by selecting MENU > ADV SETUP > SENSOR SETUP > ZERO BAND.

Note: The deadband threshold has no effect upon the serial data frame.

My instrument calls out a specific gas in its part number. Can I use it with gases other than my calibration gas?

In most cases, yes. Most Alicat gas mass flow instruments are designed specifically to work with many different gases. Gas Select (MENU > BASIC CONFIG > GAS) includes up to 130 preloaded pure gases and gas mixtures, or you can define your own using COMPOSER. There are some exceptions:

  • If you are using an FP-25 flow standard, this device is designed to flow air only.
  • If you have a BASIS mass flow controller with a helium (He) or hydrogen (H2) calibration, it can flow only that one gas. If it has any other calibration, it is equipped with a limited Gas Select of six gases (selectable over your serial connection): air, argon (Ar), carbon dioxide (CO2), nitrogen (N2), oxygen (O2) and nitrous oxide N2O.

I accidentally selected the wrong gas calibration. What can I do?

Both the mass flow rate and volumetric flow rate depend on the viscosity of the gas that is passing through your flow meter or controller. If a different gas was selected than what actually passed through the device, the measured flow readings will be off by approximately the ratio of the viscosities of the two gases. You can correct your data by multiplying each reading by the ratio of the selected gas’s viscosity to the desired gas’s viscosity.

My volumetric flow readings don’t match another flow calibrator I use.

  • If you are flowing dry gas, the differences in flow readings are likely the result of pressure drop. Every flow meter has some amount of pressure drop, especially those that use differential pressure as the measurement method. For example, volumetric flow readings taken with a standard Alicat flow meter may differ from those taken with one of our low pressure drop Whisper flow meters.
  • If you are using an Alicat flow instrument with a live humidity sensor (FP-25), you will see bigger differences in flow readings the higher the relative humidity or temperature is. This is because other flow meters do not account for the content of water vapor in the air (or gas), which physically changes how air flows.

My mass flow readings don’t match another flow calibrator I use.

  • Check the STP or NTP settings (MENU > BASIC CONFIG > STP/NTP) to ensure that your standard (or normal) temperature and pressure references match those of your other flow calibrator.

  • Ensure that both meters are configured to flow the gas you are currently flowing (MENU > BASIC CONFIG > GAS).