Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/147079
Thermistor Flow Switches A thermistor flow switch monitors lubricant flow based on the change of temperature at the sensing element. Certain thermistor flow switches require a controller downstream to evaluate the signal from the lubricant flow. In air/oil lubrication systems, the flow is relatively low, so there's only a thin layer of oil passing through the pipe or feed-line tubing. Detecting the flow of such a thin layer requires a sensitive device. Thermistor flow switches are a viable option due to the faster heat-exchange rate brought by the mixed flow of air and oil. installed as close to the lube point as possible. Ideally, the device should be directly installed at the lubricant inlet. • The sensitivity of most flow switches will be affected by the lubricant's viscosity, yet flow meters are less sensitive to viscosity. Different models may need to be selected to accommodate the lubricant's viscosity. • The temperature of the environment and the flow volume are other important factors to consider when choosing a flow confirmation solution. Magnetic-Field Flow Switches The operation mechanism of a magneticfield flow switch is quite similar to an inductive flow switch. Instead of monitoring check-ball movement with an inductive sensor, magneticfield flow switches utilize a magnetic-field sensor to detect the movement of a magnetic check ball through non-ferrous materials such as stainless steel, brass and aluminum. This sensing technology provides better sensor body material strength, making high-pressure application (5,000 psi) possible. The sensitivity of magnetic-field flow switches has been improved by adding a magnetic-field aligning plunger, which also offers more flow restriction for better sensing of small flow volume and less viscous applications. Selecting Flow Confirmation Sensors With so many variations in the systems and sensors, selecting the right flow confirmation sensor can be challenging. The following factors should be taken into consideration: • Flow continuity (intermittent or continuous flow) • Lubricant (grease, oil, oil mist or air/oil) • Flow volume or flow rate • Operating temperature range • System pressure • Viscosity or apparent viscosity • Sensor location • IP protection rating • Sensor power requirement and output The table on page 28 summarizes the different sensors and their application. Remember, the reliability of centralized lubrication systems can be improved by confirming the lubricant flow. However, implementation of such confirmation should follow certain guidelines, including: • The flow confirmation sensor should be www.machinerylubrication.com | July - August 2013 29