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NMEA-2000 Instrumentation: Fluid Tank Level Monitoring

by James W. Hebert

A primer on fluid tank level monitoring with modern electronics

On small boats with two-cycle outboard engines there are generally two tanks whose fluid level are monitored: the gasoline fuel tank and the engine oil reservoir tank. The fuel tank level can be monitored directly by use of a variable resistor sender in the tank that provides an electrical signal proportional to the fuel tank level. Oil reservoir tanks can be monitored for level with similar proportional senders.

Standard Resistive Sender

Historically in the USA a standard has come into use for the fluid tank level sender to provide a variable resistance in a range from 33-Ohms when the tank is full to 240-Ohms when the tank in empty. Using a 12-Volt current source and a suitably calibrated milliAmpere meter, a remote electrical tank level gauge can be created very simply. As the tank level increases, the resistance in the circuit is reduced, more current flows, and the dial pointer of the milliAmpere meter moves upscale toward the FULL calibration. Many small boat fuel tanks have such senders installed in their fuel tanks and have remote electrical fuel tank gauges on their instrument panels. The operator is provided with a continuous indication of the tank level. These tank level resistive senders are made for specific tank dimensions, typically made for a particular depth of tank. Because fuel tank dimensions vary, there is no one single sender that can fit every tank.

Oil reservoir tanks typically do not have senders that show the tank fluid level proportionally, but they usually have a float switch sender which signals an alarm if the fluid level in the tank drops below a preset threshold. The float switch provides only an electrical contact which is used to control an aural or visual alarm indicator, alerting the operator that the tank level has fallen to a low level. Such a switch or on-off action sender cannot be adapated to show a continuous indication of tank level, but there is no reason why a proportional resistive sender, similar to ones used in fuel tank, cannot be adapted to an oil tank. Indeed, at least one manufacturer, Evinrude, provides for such a resistive adaptor for their oil tanks.

NMEA-2000 Instrumentation

A more modern form of boat instrumentation uses the NMEA-2000 digital data network and protocol. NMEA-2000 networking and protocol creates a digital data network on a boat, and to the data network are attached various sensors. The sensors transmit data onto the network. Also on the network are various multi-function gauges or display, which listen for data from the network. The data received by the gauge or display is then converted into some form of presentation for the operator. The presentation can take many forms. Gauges that appear to be traditional dial pointer gauges can be used, or various ingenious display screen representations that imitate dial pointer gauges or other forms of measurement device might be employed.

The simplest method of converting existing tank fluid monitoring systems into modern NMEA-2000 instrument systems will employ the existing resistive fuel level senders and connect them to NMEA-2000 network sensor or convertor devices. The convertor device reads the resistive input from the sender, and converts this signal into digital data it transmits to the NMEA-2000 network, using specific datagram known as PNG 127505, Fluid Levels.

NMEA-2000 Fuel Tank Level

A typical NMEA-2000 fluid level sensor is the Lowrance Fluid Level Sensor. (The Fluid Level Sensor is a new device that has replaced the now obsolete EP-65R device that performed the same function.) The Fluid Level Sensor provides two wires for connection to the resistive tank level sender. The Fluid Level Sensor will provide its own current to the resistive tank level sender, so any previous electrical connections to the tank level sender must be removed. This will result in any existing remote electrical fuel tank level gauge becoming inoperative. One work-around is to install a double-pole double-throw switch that switches the resistive sender between the original gauge and a NMEA-2000 adaptor like the Fluid Level Sensor.

The Fluid Level Sensor is intended for use with a resistive sender with a 33- to 240-Ohm range. The Fluid Level Sensor comes pre-calibrated for conversion of the resistance into tank level. This calibration can be refined for an individual tank by use of a calibration procedure. Because the Fluid Level Sensor itself has no user interface, the calibration procedure is accomplished through a companion Lowrance gauge or display which must have the appropriate firmware installed to perform the calibration. This generally limits calibration of the Fluid Level Sensor to be done only by other Lowrance devices, such as the LMF series of small gauge displays or by the HDS series of chart plotter displays. The details of such calibration procedures are outside the scope of this article, but they are explained at length in the sensor set up guide for the Fluid Level Sensor and its suitable companion gauges and displays. An Fluid Level Sensor will send NMEA-2000 PGN-127505, FLUID LEVEL, to the network. This data may be shown on any display that can handle that PGN.

GARMIN makes the GFL 10 NMEA-2000 fluid level adaptor. It is designed to be configured and calibrated by GARMIN multi-function displays, and costs about twice as much as the FLuid Level Sensor from Lowrance. This sensor is frequently used on boats with GARMIN instrumentation. There is a significant advantage to the GFL 10 sensor: it has a flexible input circuit. The input circuit can be used in two ways. It can be connected directly to a resistive tank level sender; or it can be connected to an existing fuel tank level gauge. This allows the GFL 10 to be used in conjunction with an existing gauge. See the Garmin GFL-10 manual for details.

MARETRON makes a model TLA100 tank level adaptor. It is suitable for connecting to resistive senders. It supports both the common American 240 to 33-Ohm sender and the European standard 10 to 180-Ohm sender, and can be calibrated to work with any resistive sender between 0 to 300-Ohms. This adaptor is not as popular on small boat installations due to its cost (about three-times the cost of the EP-65R) and the need to have the companion MARETRON DSM200 display to make any adjustments of calibration of the adaptor. Like the Garmin, the MARETRON TLA100 can be connected to either an existing tank level gauge or to a resistive sender, allowing it to be used in parallel with an existing gauge. Again, see the TLA100 manual for details.

MARETRON has recently introduced new fluid tank level devices, the TLM100 and TLM150 tank level monitors. These devices employ a novel ultrasonic depth measuring technique to determine the fluid level in a tank, and they produce a NMEA-2000 output directly. These tank level monitor devices replace the normal and widely used resistive-type tank level sender, and must be fitted directly to the tank.

NMEA-2000 Oil Tank Level

For electronic monitoring of oil reservoir tank levels, it will generally be necessary to install a resistive-type tank level sender into the oil reservoir tank. The existing electrical sender in the oil tank is usually only a float switch. A switch is either open or closed, and its electrical signal cannot be converted into a proportional indication of tank fluid level.

Some manufacturers of two-cycle outboard engines have made provisions on their oil tank assemblies for installation of a second sender to be installed for the purpose of providing a resistive tank level sender. When such a resistive sender has been installed in an oil reservoir tank, it can be connected to a NMEA-2000 tank level convertor such as the EP-65R. The NMEA-2000 tank level convertor then works exactly as it does with fuel tanks, converting the resistive input into a stream of digital data and sending it to the NMEA-2000 network.

Evinrude Oil Tanks

Notably, Evinrude has made provision on its 1.8-gallon and 3-gallon oil reservoir tanks for the addition of a resistive fluid level sender to the tank, which does not replace the existing float level sender that is part of the oil pickup tube assembly. The plastic tank housing has to modified by cutting a new access hole for the new sender. Because the physical dimensions of the oil reservoir tanks are different, a specific size resistive sender is required for each tank size. Evinrude also offers their own branded version of the EP-65R fluid level sensor with pre-configuration of the device instance and device name; this facilitates installation of the EP-65R without having to set the instance and assign a name to the device. The tank fluid level sender and the pre-configured EP-65R are sold in a bundle. These can be ordered from Evinrude via their on-line part website or through a dealer. Kits are available for one, two, and three engines, and for 1.8 and 3.0-gallong tanks (thus six kits in total), or unbundled the individual tank level senders and EP-65R convertors can be purchased. The convertors can also be purchase un-configured or pre-configured for particular instance settings. See the Evinrude part website for further details.

NMEA-2000 Other Tank Levels

In a similar manner, fluid tank level sensors can be installed in multiple fuel tanks, multiple oil tanks, or into other tanks on the boat, such as waste water holding tanks, and those tank levels monitored via NMEA-2000 instrumentation. The same EP-65R or similar NMEA-2000 sensor is used, but each sensor is configured electronically by software to identify itself in a distinct manner. Again, since the EP-65R has no user interface itself, such configuration must be done with a companion gauge or display. The details of such configuration is outside the scope of this article, but it is explained at length in the installation instructions for the EP-65R and its suitable companion gauges and displays.

In NMEA-2000 networking and protocols there are provisions for multiple instances of the same device to appear on a network. The devices are distinguished by setting a parameter called INSTANCE to a unique value for each device.

Evinrude Legacy E-TEC Engines and Oil Tanks

The standard Evinrude external oil reservoir tank used with E-TEC engines does not contain an electronic level sender. It just contains a float switch that closes when the oil tank level goes below threshold. That switch is connected to an input on the E-TEC engine management module (EMM) to signal to the supervisory software that an alarm should be sounded to let the operator know the tank level has fallen below the low-level threshold.

If a tank level proportional resistive sender and NMEA-2000 electronic adaptor are installed on an E-TEC oil tank, they do not send data to the E-TEC engine management module. They send data to their NMEA-2000 network. This sender is added to the tank, while the original float level switch remains in place. On the molded top surface of the tank there are already indentations to mark where the added sender should be installed. To install, you must cut away some of the plastic tank material.

While the original float level switch connects to the E-TEC EMM, the added level sender connects to a NMEA-2000 network. On the NMEA-2000 network you can configure a multi-function gauge to display oil tank level. If you have an ICON Pro RPM gauge you can also use an ICON Accessory Series OIL LEVEL gauge as a dedicated display of oil tank level.

The Evinrude E-TEC EMM does not monitor the oil level data on the NMEA-2000 network; it relies on the float switch to signal when the level in the oil reservoir is low.

Evinrude E-TEC G2 Engines

Evinrude E-TEC G2 engines which have an under-cowling oil reservoir will monitor the oil level in the reservoir and send data to the NMEA-2000 network.


In NMEA-2000 communication, a device possesses a parameter called its INSTANCE. The default INSTANCE for most devices is INSTANCE = 000. If there are more than one device of the same type on a NMEA-2000 network, the identical devices must be distiguished by changing them to have unique INSTANCE settings, in a range from 000 to 255.

In the specific case of a fluid tank level sender, the possiblility of having multiple identical devices is high. There are many types of tanks on boats that need tank level monitoring. Tank types include fuel, oil, freshwater, gray waste water, black waste water, ballast water, and so on. There can also be multiple tanks of the same type, for example, more than one fuel tank. For this reason the setting of the fluid tank level sender INSTANCE may become very complicated. Some vendors sell fluid tank level senders with pre-configured INSTANCE set to something other than the default 000 setting. Some display manufacturers have non-changeable pre-set configurations for which tank type can use which INSTANCE settings. A careful consideration of fluid tank level instance setting is necessary to create a working NMEA-2000 network system of senders and display devices.

Further Reading

For readers unfamiliar with NMEA-2000 instrumentation, I recommend reading a companion article, NMEA-2000 Instrumentation for Modern Outboard Engines. For more details about the NMEA-2000 datagram used for tank levels, PGN 127505, Fluid Levels, see my article on NMEA-2000 PGN's. If using a fluid tank level sender with Evinrude branded displays, I strongly recommend reading a separate article about the pre-configured settings needed for fluid tank level senders. And in particular if using an Evinrude NAUTILUS 3.5 Color display, check the version of firmware; in earlier firmware there was a problem for the end user to set the fuel tank volume.

Copyright © 2014 by James W. Hebert. Unauthorized reproduction prohibited!

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