Maintaining adequate hydration is an important part of maintaining health, especially under extreme operational and environmental conditions where medical assistance might not be readily available. One approach to measuring the amount of fluid consumed is through the use of Fluid Intake Monitor (FIM) technologies to monitor fluid consumption. These devices can be used to ensure that warfighters consume recommended/ appropriate amounts of water, and to tailor logistical support to actual water needs, as water is expensive to transport.

Figure 1. Bladder with Fluid Intake Monitor (FIM) attached to hose of a flexible back-worn hydration system (left), with close-up view of the FIM. Fluid consumed through the FIM unit is measured in 0.075-ml increments.
Twenty-seven prototype FIM units were evaluated in a bench test, and 15 prototype FIM units were tested in a free-living field test. The FIM is comprised of two small gears that rotate in response to fluid movement through the drink line; the FIM can be incorporated into modified, flexible, back-worn hydration systems already in use.

Figure 2. Schematic diagram of the Fluid Intake Monitor (FIM). Fluid is measured by the amount of movement of the gears as water passes over the teeth of the gears.
Bench testing demonstrated that the current FIM prototype accurately detected each sip event 100% of the time. The ability to reliably detect drinking events, particularly when combined with other measures, could provide useful insights into the health status of a sick or injured warfighter being monitored on the battlefield or in training from a remote location.

Bench test results show that with the exception of the first sip, average measurement error was small. There was good consistency in the measurements from sip to sip after sip 1. While the overall reliability of the sample of FIM units showed relatively good consistency across sips, for sips 2 through 10, the accuracy of the individual units varied considerably.

The fact that, after the first sip, there is generally good consistency across sips suggests the fundamental gear design was acceptable. The relatively high error rate or overestimation on the first sip can be attributed to an initial presence of air in the hose between the water-filled bladder and the FIM. When a drinking action caused the air in the hose to move over the gears, the gears spun at high speed, resulting in an artificially high reading on the first sip.

The FIM incorporates a check valve to prevent water backflow into the drinking bladder. However, the valve is not completely airtight, allowing air to enter the hose when the system was not in active use. After the first sip, once the system was primed and water filled the entire system, this source of error was not present. In the broad scheme, the relatively small amount of error associated with the first sip, when compared to a whole bladder’s worth of water consumption, is inconsequential when monitoring warfighter water consumption.

This work was done by William J. Tharion, Anthony J. Karis, and Reed W. Hoyt of U.S. Army Research Institute of Environmental Medicine. For more information, download the Technical Support Package (free white paper) at  under the Bio-Medical category. ARL-0074

This Brief includes a Technical Support Package (TSP).
Reliability and Validity of a Prototype Fluid Intake Monitor

(reference ARL-0074) is currently available for download from the TSP library.

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