Troubleshooting an Alicat device

When your Alicat instrument isn’t working as expected, we want to help identify and resolve the issues as quickly as possible. Fortunately, most problems can be fixed without having to return the device to the factory. This article covers common issues with Alicat devices and outlines solutions.

Resolving error codes

When an error occurs with your Alicat device, you will receive an error code. Here are common error codes and troubleshooting measures you can take to resolve the issue.


This error is indicative on an analog to digital conversion error. Typically this will occur when there is an internal hardware issue. This is caused by the main board not being able to talk to the processor chip.

  • Unit must be sent into Alicat for repairs.

This is an error indicating that the mass flow has gone over range of the device (128% of full scale). This also shows up if the setpoint is too far over the custom range of the device. Custom ranges increase accuracy by decreasing the full scale.

  • Reduce flow and recheck operating conditions.
  • Before using, verify readings on the device at atmospheric conditions
  • Compare these readings to a known state.
  • If unit has a custom down-scale range, and a higher range is desired, unit must be sent in for reconfiguration and re-calibration.
  • If sensor damage is suspected, check registers 8 and 10, and compare them to the values originally shipped on the device.

The instantaneous (with no averaging) pressure has exceeded the designated over pressure limit. This causes the valve on the device to close.

  • The setpoint on the device must be adjusted digitally or physically on the front screen.
  • A power cycle will also clear the OPL error.

Similar to MOV and VOV, this error indicates that the pressure has gone over the specified range of the device (128% of full scale).

  • Reduce pressure and recheck operating conditions.
  • Before using, verify readings on the device at atmospheric conditions
  • Compare these readings to a known state.
  • If sensor damage is suspected, check registers 8 and 10, and compare them to the values originally shipped on the device. You may also check the PSIA reading of your device at ambient conditions. If your value is non-reasonable number, you may have a damaged sensor.

This error means the temperature is outside of the bounds specified within our devices. When the temperature is outside specified bounds, flow readings are no longer reliable. On older non-rechargeable battery units, this would happen roughly 10-60 minutes before the unit would lose all charge.

  • Check power supply output. If a power supply dips in voltage, the temperature readings are typically the first to show signs.
    • On older non-rechargeable battery units, this would happen roughly 10-60 minutes before the unit would lose all charge.
  • Replace battery (older battery units)
  • Verify there is no sensor damage
  • Compare readings in a known atmospheric condition.

Similar to MOV and POV, this error indicates a volumetric flow reading that is outside the range of the device.

  • Reduce flow and recheck operating conditions.
  • Before using, verify readings on the device at atmospheric conditions
  • Compare these readings to a known state.
  • If sensor damage is suspected, check registers 8 and 10, and compare them to the values originally shipped on the device. You may also check the PSIA reading of your device at ambient conditions. If your value is non-reasonable number, you may have a damaged sensor.

Power and communication issues

Display won't turn on, or is dim/weak
  • Check the power and ground connections to your instrument.
  • Check the display contrast settings by selecting MENU > ADV SETUP > DISP SETUP > LCD CONTRAST. The default contrast level set at the factory is 10.
Buttons on display do not respond and 'LCK' appears on the screen

Alicat devices come with a display locking option to prevent inadvertent changes to setpoints or settings. If the error code ‘LCK’ appears on the screen, your device display has been locked.

  • If your device was manufactured with firmware version 6v or later, you can unlock the display by pressing and holding the buttons located at the four corners of the display until ‘LCK’ disappears.
  • All devices equipped with our standard serial communications can be unlocked by issuing the following serial command: [Unit ID]$$U<CR>. (Replace [Unit ID] with your device’s single letter Unit ID.)
Analog output indicates values lower than what appears on instrument display

Analog signal voltage degrades over long distances. You can minimize this effect by using wires with a heavier gauge, especially in the ground wire.

Connecting an Alicat to a computer serially

Nearly all Alicat instruments are manufactured with digital serial communications. Follow the steps below to establish a connection to your Alicat from your Windows computer:

  1. Download our Serial Terminal program.
  2. If you are connecting with a USB cable, install the USB driver. This creates a virtual COM port on your computer.
  3. Open your computer’s Control Panel, and then open Device Manager.
  4. Under “Ports (COM & LPT)”, find the COM port number that is connected to your Alicat. If you are using a USB cable, the virtual COM port will be called “USB Serial Port (COM#)”. Take note of the COM port number.
  5. Open the Serial Terminal program.
  6. Serial Terminal will ask for the COM port and baud rate. Enter the COM port you found above and the default baud rate of 19200.
  7. Type the single-letter Unit ID for your Alicat instrument, and press enter <CR>. If you do not know your device’s Unit ID, you can find it by selecting MENU > ADV SETUP > COMM SETUP on the display.
  8. If your computer returns a line of data after Step 7 above, you have successfully established serial communications.
Computer cannot communicate with the Alicat over a serial connection
  • Check that your unit is powered on and connected to your computer properly. See above instructions to connect your instrument.
  • Be sure that the COM port on the computer to which the Alicat is connected is active. Open Device Manager to confirm that the COM port is visible.
  • When connecting multiple devices to a single COM port (or virtual COM port if using USB), be sure to give each Alicat device a unique Unit ID. You can change the Unit ID by selecting MENU > ADV SETUP > COMM SETUP > UNIT ID.
  • Alicat’s RS-232 output uses a 0-5 Vdc signal. See this document for details: RS-232 Output Specifications.
Computer cannot find BB3 or BB9 multi-drop breakout boxes
  • If your BB3 or BB9 multi-drop box is showing up under “other” in your device manager instead of “Ports (COM & LPT)”, the most likely cause is a missing driver. While this should install automatically when plugging into your computer, occasionally security settings can prohibit it from installing. You can download and install the driver manually from our Software & Drivers page.
  • If you are using a USB port with Windows 7 and receive the error, “One of the USB devices attached to this computer has malfunctioned and Windows does not recognize it”, the USB port itself is not responding properly. Microsoft has released a solution on its forum.
  • If you are using the 9-pin serial connection on your computer and your COM port does not appear in Device Manager, the COM port on the computer or the cable may have stopped functioning. Please try connecting to the computer using a different cable or different port (USB), and check Device Manager again.

Incorrect flow readings

Negative flow readings
  • If gas is flowing through your device, it may be flowing in the wrong direction. If you expect flow in both directions, you can ask for a bidirectional flow calibration during 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. To correct this, tare your device.
  • If you have a low pressure drop flow meter (Whisper or FP-25) that is not connected to anything, the device may be reading a light breeze that is entering its outlet. Plug one end to see if the flow returns to 0.
Changing flow reading units

Flow, pressure, and temperature readings can be displayed using a wide range of engineering units.

Live flow readings won't settle

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.
  • If you ordered your device with totalizer averaging (TOTAVG in your part number), reset your totalizer. The averaged flow rate updates in real time, 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.
Readings don't change when flow changes

Scenario 1:

Flow readings that won’t change regardless of actual flow usually indicate a damaged flow sensor. This is usually the result of exceeding the maximum allowable instantaneous differential pressure for your device (75 PSID for most instruments; 15 PSID for Whisper and FP-25).

For meters, this may happen when an upstream solenoid valve suddenly snaps open to allow the high-pressure gas through the meter. For controllers, the valve usually protects the sensor from large pressure differentials. If the pressure changes suddenly, the valve may not have time to adjust and the sensor may be subjected to high pressure drops.

Note: Sensor damage due to excessive differential pressure is not covered by Alicat’s warranty.

Scenario 2:

Alicat mass flow meters can be tared by grounding a specified pin (pinout diagrams). 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.
Readings jump to 0 at low flow rates

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.

Incorrect gas selected

Alicat devices can be used to flow over 98 gases and gas mixes, but it is important to verify that the correct gas is selected prior to flowing. Having the wrong gas selected will not cause any damage to your device, but all flow readings will be off by approximately the ratio of the viscosities of the two gases.

This is an easy issue to fix. All you have to do is multiply the readings by the ratio of the selected gas viscosity to the desired gas viscosity. To prevent the problem from occurring again, update the gas selection in the Gas Select™ menu.

For more details, check out this flow data correction tutorial

Volumetric flow readings don't match another flow calibrator
  • 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.
Mass flow readings don't match another flow calibrator
  • 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).

Liquid ingress into a gas flow instrument

Gas flow controllers and meters are designed to flow only gases, but accidental liquid ingress into a gas flow device can happen in most environments.

Tuning issues

When an Alicat unit leaves the factory, the valve is configured to operate at certain standard conditions.  Sometimes these valve characterization conditions do not match the actual operating conditions. When this happens, it is possible the device won’t control with the speed or stability typical to an Alicat controller.

Don’t panic! With Alicat devices, it is possible to change gains in the control algorithm using the front panel or serial communication. More often than not, what seems to be a non-functional unit is just a controller configured for the incorrect application.

Downloading device registers

When troubleshooting an Alicat device, it can be helpful to first download all of the device registers. An applications engineer can use these registers to help you isolate the problem and find a solution.

FastTrack Ordering

M/MC mass flow meters and controllers that ship in just 3-5 business days.
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Analog, RS-232, RS-485, or Modbus RTU
MD8, Locking Industrial, DB9M, DB15


2 SCCM - 500SLPM


2 SCCM - 100SLPM

Service and Support

Whether it is time for your instrument’s annual recalibration or your instrument needs a repair or upgrade,
you can fill out the Service Request Form below, email us, call us, or start a live chat session to get the service process started.

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