Bleeding your liquid device tutorial

Alicat differential pressure based liquid flow meters and controllers should only be used to flow liquids. Sometimes, however, a liquid contains trapped air or gas bubbles. It is critical that the device only measures the pressure of the gas-free liquid flowing through the device (no bubbles), or readings will be incorrect.

Each liquid device is equipped with two bleed ports on the front, used to remove any trapped air or gas in the pockets near the sensor ports. Bleeding a port is the process of loosening the nylon screw slightly so the flowing liquid leaks and allows bubbles to escape.

Below are instructions on how to properly use the bleed ports on an Alicat device. Before you begin, ensure that the meter/controller installation is secure, and add line pressure to the system. It is a good idea to have a towel handy to soak up any dripping liquid.

Video tutorial

Video transcript

Hi, my name is Marina and I’m an app engineer at Alicat Scientific. Today, I’m going to show you how to use the bleed ports on your liquid device. Let’s get started.  

Using the bleed ports on your Alicat liquid devices is important to remove any air bubbles that are in your liquid stream that could affect your flow readings. To do this, let’s first start by either giving it a zero setpoint in the case of a controller, or plugging the downstream port in the case of a meter. We have a controller, so let’s start by giving it a zero setpoint. 

From the front panel, go to set point (top right button), click clear (bottom middle button), and set (bottom right button).  

Now let’s apply some pressure to the inlet, around 20-30 PSIG for the best results. 

Now let’s loosen that upstream bleed port. We want to do this slowly in order to prevent any liquid from spraying in unexpected directions. In addition, we’re only loosening the port, not removing it completely.  

Now, let’s give the device at least a 50% setpoint. We’ll go to setpoint (top right button). You can see that our full scale is 1 CCM, so we want to give a setpoint of at least 0.5 CCM. In this case, let’s go ahead and just give it a 100% setpoint.  

Now, let it bleed for 15-20 seconds. While this is happening, tap the top of the device with the end of the screwdriver and wipe off any water that leaks out, like so.  

Now, after that 15-20 seconds, let’s close the bleed port. When you do this be careful to not over-tighten, as you can cause permanent damage if you do.  

Now, let’s give the device a zero setpoint (top right button).  

Now, we’re essentially going to repeat those exact same steps but for the downstream port. We have our zero setpoint, and we still have our incoming inlet pressure of 20-30 PSIG.  

Let’s open the downstream bleed port, and give the device at least a 50% setpoint (top right button). Like before, let’s just give a 100% setpoint.  

Again let’s let this bleed for 15-20 seconds while tapping the top of the device with the end of the screwdriver and wiping away any liquid that leaks out. Now close the bleed port and give a zero setpoint (top right button).  

At this point, we have completed bleeding out the liquid device to remove any air bubbles, so the last thing we need to do is give your device a fresh tare. To do this, remove that inlet pressure we applied earlier so that the Alicat unit is in atmospheric conditions with no flow.  

Go to our menu (bottom right button) and tares (top right button). Click ‘tare flow’ (top right button). Now, you’ll see a message telling you to ensure no flow before pressing tare. Now you can click tare. You’ll see a warning message which is essentially saying what I just told you and click ‘set’ (bottom right button) 

There you go, and that is how you use the bleed ports on your liquid device.” 

Step-by-step instructions

  1. For controllers, give the device >50% setpoint.  For meters, make sure line pressure is applied.
  2. Loosen the upstream bleed port (if the device is mounted horizontally and facing towards you, this will be on the left-hand side) by turning the screw 1-2 turns slowly. If you loosen the screw too quickly, you can get a stream of water rush out of the device that can spray in unpredictable directions.
  3. Tap the flow body of the device gently to add some manual force in removing any bubbles.
  4. Allow the water to drip for about 15 seconds.
  5. Tighten the screw until the bleeding stops. Be careful to avoid over-tightening the screw, which can damage the tip of the nylon screw.
  6. Repeat these steps with the second bleed port.

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