What is the difference between the LM500 and LM500-TF?

The LM500-TF includes a built-in test button that allows the operator to verify that both the float mechanism and the snap-switch are functioning correctly with a single press — without draining oil or disassembling the unit. The standard LM500 does not include this test feature.

How does the LM500 thumb valve work?

As the crankcase oil level drops, the stainless steel float descends and mechanically opens the Viton thumb valve, allowing oil to flow from the supply tank into the crankcase. Once the oil reaches the correct operating level, the rising float closes the thumb valve, stopping oil flow. The thumb valve is non-clogging and provides a positive, leak-free seal during normal operation.

What does the Form C (three-wire) switch enable on the LM500?

The SPDT Form C contact provides both a normally open and a normally closed output. This allows the LM500 to be wired into a closed-loop controller or annunciator circuit, creating a fault-sensitive system that can detect broken wires or switch failures — rather than simply relying on a single wire break or make to signal a problem.

What mounting options are available for the LM500?

The LM500 can be mounted directly to the engine or equipment, or installed using either a pipe bracket kit (fits 7/8 in. diameter pipe) or a universal bracket kit that accommodates various mounting configurations. Optional accessories include vent fitting kits, hose kits, bubble lens kits, and fittings kits.

What is the maximum supply tank height for the LM500?

The supply tank should be positioned between a minimum of 2 ft. (0.6 m) and a maximum of 25 ft. (7.7 m) above the LM500 unit to maintain the required inlet head pressure within the rated range of 9.50 psi / 25 ft. of oil head.

LM500 lube level maintainer for automatic crankcase oil level control on engines and compressors

LM500 Series Oil Level Control

Viton Thumb Valve | SPDT Form C Switch | 250°F Max Ambient

Engines · Pumps · Compressors

LM500 Lube Level Maintainer for Automatic Crankcase Oil Level Control

Name: LM500 Lube Level Maintainer
Brand: Murphy by Enovation Controls
SKU: LM500

The LM500 lube level maintainer is a float-operated instrument that continuously monitors and replenishes crankcase oil on engines, pumps, and compressors of any size. Its Viton thumb valve draws oil from a connected supply tank whenever the running oil level drops, while an integrated SPDT low-level snap-switch triggers an alarm or equipment shutdown if the oil supply is exhausted — protecting machinery without operator intervention.

10 A SPDT Rating

15 psi Max Case Press.

250°F Max Ambient

½ NPTF Inlet Connection

Key Features

The LM500 series lube level maintainer is a purpose-built instrument for continuous, unattended crankcase oil level management across engines, reciprocating pumps, and compressors. Using a stainless steel float to drive a non-clogging Viton thumb valve, it draws clean oil from a remote supply tank whenever the running level falls below the setpoint — and shuts off flow the moment that level is restored. The integrated SPDT Form C low-level switch adds a critical layer of protection: if the supply tank runs dry and crankcase oil continues to fall, the switch activates an alarm or initiates an equipment shutdown before internal damage can occur. With a die cast aluminum housing, a clear large-diameter viewing lens, and connections compatible with standard industrial pipe fittings, the LM500 series installs quickly into new or existing lubrication systems with minimal modification.

Non-Clogging Viton Thumb Valve for Reliable Oil Flow Control: At the heart of the LM500 is a uniquely designed thumb valve constructed from Viton — a fluoroelastomer recognised for its resistance to oil, heat, and chemical degradation. The valve opens and closes through the direct mechanical action of the stainless steel float, requiring no solenoids, pilots, or external power to regulate oil flow. Its non-clogging geometry prevents particulate debris from lodging in the seat, ensuring consistent, leak-free operation over long service intervals even in environments where oil quality may vary.
SPDT Form C Switch for Closed-Loop Fault-Sensitive Circuits: The LM500 incorporates a three-wire SPDT (Form C) snap-switch rated at 10 A at 125 VAC and 10 A at 30 VDC. Unlike simpler single-wire switching arrangements, the Form C contact provides both a normally open and a normally closed output simultaneously. This enables the switch to be wired into a controller or annunciator as a closed-loop circuit — meaning a broken wire, failed switch, or disconnected terminal is detected as a fault condition rather than going unnoticed, significantly improving system reliability in unmanned or remote installations.
304 Stainless Steel Float for Long-Term Durability: The float assembly is fabricated from 304 stainless steel, offering excellent corrosion resistance in the oil-laden crankcase environment and resistance to deformation under sustained thermal and mechanical load. The stainless steel construction ensures the float maintains its buoyancy and operating geometry over years of continuous service, without the degradation risk associated with plastic or coated metal floats in high-temperature crankcase conditions.
Clear Frog Eye Viewing Lens for Instant Level Confirmation: A large-diameter clear polycarbonate lens — UV-stabilised, heat-resistant, and rated for high-impact service — provides a direct visual view of the internal float position and the oil level dial. The dial features a high-visibility white background with a solid green band marking the correct running level range. Operators can confirm oil level status at a glance during routine walk-arounds without connecting test equipment or removing any covers, reducing inspection time on multi-engine installations.
LM500-TF Built-In Test Feature: The LM500-TF variant adds a manually operated test button that allows the operator to verify correct float and switch operation with a single press — without draining the crankcase, interrupting the oil supply, or disassembling any part of the unit. This on-demand function test is particularly valuable during commissioning, after maintenance, or as part of a periodic safety verification programme, providing documented confidence that the low-level protection circuit will respond correctly in a real depletion event.
High-Temperature Rating Suitable for Engine Environments: The LM500 is rated for a maximum ambient temperature of 250°F (121°C), making it suitable for installation directly on or adjacent to operating diesel engines, gas engines, and industrial compressors where radiant and convective heat from machinery can significantly elevate local ambient temperatures. The combination of die cast aluminum housing and UV/heat-stabilised polycarbonate lens maintains structural and optical integrity at these elevated service temperatures without deformation or sealing failure.
Multiple Mounting Options for Installation Flexibility: Beyond direct equipment mounting, the LM500 is compatible with two dedicated mounting bracket accessories: a pipe bracket that clamps to a 7/8 in. (22 mm) diameter pipe, and a universal bracket that supports a range of custom mounting configurations where direct mounting is not practical. Multiple outlet connections — two 3/4 NPTF side ports and one 3/4 NPTF bottom port — allow the crankcase outlet to be routed in whichever direction best suits the installation geometry.

Applications

Industrial Diesel and Gas Engines: Any reciprocating engine operating continuously in field, plant, or marine service benefits from automatic crankcase oil replenishment. The LM500 removes the dependency on manual oil checks between service intervals, maintaining the correct running oil level and triggering a protective shutdown before oil starvation can cause bearing or cylinder damage on attended or unmanned engines.
Reciprocating Pumps and Compressors: Reciprocating pumps and gas compressors rely on precisely maintained crankcase oil levels to lubricate crankshafts, connecting rods, and crosshead assemblies. The LM500 maintains oil at the correct running level without over-filling — which can be equally damaging — and provides low-level alarm output to site control systems when oil supply is interrupted.
Natural Gas Production and Gathering: Wellhead engines and field compressors in natural gas production and gathering operations often run unattended for extended periods. Automatic oil level maintenance with low-level shutdown protection reduces the risk of catastrophic equipment failure between scheduled operator visits, lowering both maintenance cost and unplanned downtime across multi-well pad installations.
Generator Sets and Standby Power Systems: Generator sets in critical power backup roles must be in full operational readiness at all times. The LM500 ensures generator engine crankcases remain properly filled during standby periods, and the Form C switch output can be integrated with generator management panels to raise a pre-start fault alarm if oil level is outside the normal operating band before a start sequence is initiated.
Marine Engine Rooms: Auxiliary engines and main propulsion engines in marine applications benefit from continuous oil level management in environments where manual checking frequency may be limited by sea conditions or watch schedules. The LM500's compact footprint, robust aluminum housing, and high ambient temperature rating suit the space and thermal constraints of shipboard engine room installations.
Oil and Gas Pipeline Compressor Stations: Pipeline compressor stations operate large-bore engines and compressors around the clock. The LM500 supports continuous operation by maintaining crankcase oil autonomously and feeding low-level fault signals to the station control system, enabling operators to respond to oil supply depletion before equipment protection limits are reached.

Technical Specifications

Specifications apply to both the LM500 (standard) and LM500-TF (with test feature) unless otherwise noted. All key parameters are listed below for engineering reference and procurement use.

Parameter	Specification
Crankcase Balance Vent Connection	1/2 NPTF (top)
Inlet Connection	1/2 NPTF with removable screen (side)
Outlet Connections	2 × 3/4 NPTF (side, plugged) + 1 × 3/4 NPTF (bottom) — one connected, two plugged
Thumb Valve Material	Viton
Switch Type	SPDT (Form C) Snap-Switch
Switch Contact Rating	10 A @ 125 VAC; 0.5 A @ 125 VDC; 10 A @ 30 VDC
Wire Leads	18 AWG × 14 in. ± 2 in. (355 mm)
Conduit Connection (Electrical)	1/2 in. conduit, female (top)
Case Material	Die Cast Aluminum
Viewing Lens	Clear Frog Eye polycarbonate — non-staining, high-impact, UV & heat stabilised
Dial	High-visibility white background with solid green band for normal level indication
Float Material	304 Stainless Steel
Maximum Inlet Pressure	9.50 psi / 25 ft. oil (head pressure)
Maximum Case Pressure	15 psi (103 kPa)
Maximum Differential	2 in. (51 mm) between running and stopped oil levels
Maximum Ambient Temperature	250°F (121°C)
Overall Dimensions (LM500)	6-9/16 in. (H) × 6-3/16 in. (W) × 3-1/2 in. (D)
Overall Dimensions (LM500-TF)	6-35/64 in. (H) × 6-3/16 in. (W) × 3-1/2 in. (D)
Shipping Dimensions	9.5 in. (W) × 7.5 in. (D) × 11 in. (H) [241.3 × 190 × 279.4 mm]
Shipping Weight	3 lbs. 7 oz (1.57 kg)
Pipe Bracket Compatibility	7/8 in. (22 mm) diameter pipe
Supply Tank Height (Recommended)	2 ft. (0.6 m) minimum to 25 ft. (7.7 m) maximum above LM500

Technical FAQ

Common Questions Answered

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The LM500 is designed to automatically maintain the crankcase oil level on engines, pumps, and compressors of any size. It replenishes oil from a connected supply tank as the crankcase level drops during operation, and activates a low-level alarm or equipment shutdown via its SPDT snap-switch if the supply is exhausted — providing continuous protection without requiring manual checks between service intervals.

The LM500-TF includes a built-in test button that allows the operator to confirm both the float mechanism and the SPDT snap-switch are functioning correctly with a single press — without draining oil, interrupting the supply, or disassembling any part of the unit. The standard LM500 does not include this test feature. Both models share the same core specifications, dimensions, and connection arrangement.

Unlike a simple two-wire switch that only indicates open or closed, the SPDT Form C contact provides both a normally open and a normally closed output simultaneously. When wired into a controller or annunciator as a closed-loop circuit, a broken wire, disconnected terminal, or failed switch is detected as a fault — rather than going unnoticed as it would in a single-wire open-circuit arrangement. This fault-sensitive configuration is particularly important for unmanned and remote installations where the consequences of an undetected wiring fault could be severe.

The oil supply tank should be positioned between a minimum of 2 ft. (0.6 m) and a maximum of 25 ft. (7.7 m) above the LM500 unit. This height range maintains the inlet head pressure within the unit's rated maximum of 9.50 psi / 25 ft. of oil. For sealed crankcase systems where venting to atmosphere is not possible, the vent connection must be piped back to the crankcase above the oil level rather than open to atmosphere.

The LM500 supports direct equipment mounting as well as installation via two optional bracket accessories: a pipe bracket kit (part 15000518) that clamps to a 7/8 in. (22 mm) diameter pipe, and a universal flange kit (part 15000519) that accommodates a variety of mounting configurations where direct or pipe mounting is not practical. Optional accessories also include vent fitting kits, hose kits, bubble lens kits, and fittings kits to complete the installation.

Yes, with a modification to the vent connection. On vented crankcases, the 1/2 NPTF balance vent at the top of the LM500 can use the included plastic vent plug or an optional vent tube kit (part 15000954) to vent to atmosphere. On sealed or pressurised crankcase systems, the vent connection must be piped back to the crankcase at a point above the oil level to equalise pressure between the LM500 body and the crankcase, ensuring the float operates correctly and oil flows reliably under all operating conditions.