Please view the FMK Get Started Guide video for your model of Apex (2016 or Classic) before installing your new flow sensors!

This sticky serves to emphasize several points made in the videos, as well as provide additional tips and other info to help you properly install your flow sensors:

  • All 4 sizes of flow sensors have an arrow molded in the body of the flow sensor - you'll find this arrow on the back side. This arrow indicates the direction that the water must flow through the sensor. It is essential that you orient the flow sensor properly; a flow sensor installed backwards (arrow pointing opposite the direction of water flow) will impede flow, will read incorrectly, may give erratic readings, and might become damaged over time.

  • All flow sensors come with a cable which is about 16 feet (5 meters) long. This should be ample for most installations. If that length is insufficient, position the FMM closer to the flow sensors, and use a longer Aquabus cable as needed to connect the FMM to the Apex system. Neptune Systems does not recommend attempting to extend the flow sensor cables in any way.

  • The 1" and 2" flow sensors with their unions installed are fairly heavy. Handle carefully, and do not lift them by the cable.

  • Flow sensors are water-resistant, and will withstand the occasional splash or drip. But they are not waterproof. For the best resistance to such splashes and drips, when installing, orient them so that the point where the cable enters the body of the flow sensor is pointing down if possible.

  • The 1/2", 1", and 2" flow sensors come with suitable fittings. For each 1/2" sensor, two female threaded x female slip adapters are included. For the 1" and 2" sensors, two unions are provided. Because many installations of these larger sensors will have rigid, fixed plumbing, they ship with unions to make it easy to remove and clean the sensor as needed. The 1/2" sensors will usually be installed in a way that allows for removal without the use of unions.

  • Like many pumps and plumbing products meant for a global market, the flow sensors use British Standard Pipe (BSP) threads, not US NPT (National Pipe Thread). The fittings/unions provided are BSP as well; use them; do not attempt to use NPT fittings on the 1" or 2" flow sensors.

  • US NPT-threaded fittings will not screw properly onto 1" and 2" sensors. There will not be enough thread engagement, and if you over-tighten it enough to try get a seal, the threads may become damaged and the female fitting will be over-stressed and prone to cracking or splitting. Here is an example of a FS-100 which was incorrectly assembled with thick PTFE tape and two different NTP female fittings. It was tightened as tight as possible, but still leaked. Note that because of the mismatch between the male BSP and female NTP threads, the fittings did not screw onto the flow sensor very far:

    Don't do this!

  • 1/2" NPT-threaded PVC fittings will fit the 1/2" sensor quite loosely and may be difficult to seal.

  • For 1/4" sensors, you can use semi-rigid polyethylene tubing ("RO tubing") or polyurethane tubing. Do not use soft vinyl or silicone tubing. Cut the end of the tubing clean and square with a sharp knife. To install tubing, remove the threaded compression nut from the flow sensor, insert the tubing into the outside hole of the compression nut, and push it through until the end of the tubing protrudes no more than 3/8" / 10mm from the ferrule inside the nut. If the end of the tubing is pushed too far through, chances are that it will just be pushed back to fit when you install and tighten the nut, but it could crush and obstruct the flow. Thread the nut back over the male threaded part of the the flow sensor, and tighten finger-tight. Do not over-tighten. Here is a FS-25 with the tubing pushed the right distance through the compression nut:


  • For all sizes flow sensors, it is best to install the sensors where there is a straight run of pipe (or tubing) at least 5x the diameter of the flow sensor on both the inlet and outlet of the sensor. For example, for a 1" flow sensor, there should be 5 or more inches of straight pipe length both before and after the sensor. And for a 2" sensor, there should be at least 10" of straight piping both before and after the sensor. It is more important to have a straight run of pipe of suitable length leading into the sensor than it is on the output side of the sensor. The goal here is to have a smooth, non-turbulent flow of water entering and exiting the sensor, and therefore give you stable and accurate flow measurements. Avoid installing a flow sensor adjacent to a 90-degree bend if possible. This is not a hard, absolute requirement, but you should follow this guideline if possible.

  • If you need to step up and step down the piping size to install a 1" sensor (such as if your main piping is 3/4"), you can use step-up/step-down adapters directly in the provided unions if that is what's needed to fit the flow sensor and attached unions in the available space. However, it is better to use a short length of 1" pipe on both side of the flow sensor/union assembly, then use a coupling and reducer as necessary; the short 1" pipe length should be at least 5", preferably 10" long, as described in the previous bullet. Use the same concept for 2" sensors - if you are stepping up and stepping down from 1-1/4" or 1-1/2", use 10-20" of 2" pipe on either side of the sensor. If you do not have room to do the recommended short runs of larger pipe sides on both sides of the sensor, do the best you can, and if you must choose between using same-size piping on the input or output side of the sensor, put it on the inlet side.

  • If you install flow sensors with stepped up and stepped down piping, install where the piping runs vertically if possible. If the stepped-up/stepped-down flow sensor is installed in a horizontal run; air bubbles trapped can become trapped inside the larger diameter section, and may cause flow measurements to be less stable.

  • Some aquarists will want to install a flow sensor in a drain line between the overflow and the sump. If the drain line normally runs a full siphon, that is fine. But if the drain line is oversized, water will be mixed with air, and flow readings may be erratic. However, this is not all that important if you are installing a flow sensor in an emergency drain line, and you are primarily interested only if water is flowing through that drain or not rather than how much water is flowing through that normally-dry drain.