DF Series Hydrostatic Head Level Switch for Precise Tank Pump Control

Name: DF Series Hydrostatic Head Level Switch
Brand: Murphy by Enovation Controls
SKU: DF755

The DF Series hydrostatic head level switch is a diaphragm-operated instrument that automatically starts and stops electric or engine-driven pumps at preset liquid levels. Its nitrile sensing diaphragm and SPDT snap-switch combination delivers ¼ in. (6 mm) level repeatability across atmospheric tank applications — requiring no external power supply, no transmitter, and minimal wiring to integrate into existing pump control circuits.

¼ in. Repeatability

15 A SPDT Rating

25 psi Max Pressure

1 NPT Process Conn.

Key Features

The DF Series hydrostatic head level switch is a straightforward yet highly reliable instrument for automatic pump control in industrial liquid storage and process systems. It senses changes in hydrostatic head pressure through a nitrile diaphragm that activates an SPDT snap-switch wired directly into the pump's electric pilot circuit. The result is accurate, repeatable tank level management — with a control resolution of ¼ in. (6 mm) — without complex instrumentation or dedicated power supplies. Suitable for atmospheric tanks in non-hazardous areas, the DF Series integrates into new or existing installations with minimal modification, making it a practical choice for engineers managing waterflood systems, crude oil tanks, cooling towers, diesel day tanks, and sumps.

Nitrile Diaphragm for Broad Chemical Compatibility: The pressure-sensing element is a nitrile (NBR) diaphragm, resistant to most common industrial liquids including water, saltwater, diesel, and light crude oil. Its flexibility allows the switch to respond to small head pressure changes with a control repeatability of just ¼ in. (6 mm), ensuring consistent pump cycling at configured setpoints without the risk of level hunting or short-cycling.
SPDT Snap-Switch for Direct Pilot Circuit Wiring: At the core of the DF Series is a special alloy leaf SPDT snap-switch rated at 15 A at both 125 VAC and 250 VAC. This rating allows the switch to be wired directly into the electric pilot circuit of most standard pump motor starters without intermediate relays — simplifying installation, reducing component count, and eliminating additional points of failure in the control circuit.
Preset Factory Differential on the DF755: The DF755 ships with the snap-switch factory-set for a 4 in. (102 mm) liquid level differential, operating at approximately 2 in. (51 mm) and 6 in. (152 mm) above the diaphragm mounting point. This eliminates on-site calibration for standard tank applications and allows the unit to be placed into service immediately upon installation.
Durable All-Weather Construction: The housing lid and body are machined from aluminum for structural rigidity and corrosion resistance, while the bottom plate uses glass-filled nylon — selected for its chemical resistance and dimensional stability in wet process environments. Electrical connections are made through a ½ NPT conduit entry, compatible with standard industrial conduit systems and enabling weatherproof wiring terminations.
Versatile Tank Mounting Options: The DF755 supports multiple installation configurations including direct tank wall mounting, bull plug installation, tank wall mounting with a drain cock, direct bottom-of-tank mounting, and riser pipe mounting. This range of options allows the switch to be installed at the exact level setpoint required by the process without additional mounting brackets or gauge instruments in most applications.
Engine Shutdown Compatibility: Beyond motor pump control, the SPDT output of the DF Series can be wired to single or dual magneto ignition systems to shut down engine-driven equipment at a predetermined liquid level. Simple terminal wiring changes accommodate both high-level and low-level shutdown scenarios, making the DF Series applicable to engine-driven pumps as well as electric motor-driven units.
RoHS Compliant Design: The DF Series is manufactured in compliance with the EU Restriction of Hazardous Substances (RoHS) directive, confirming that restricted hazardous materials are not present in the construction — an important consideration for operators supplying equipment to European markets or working under environmental material compliance frameworks that require documented substance declarations.

Applications

Waterflood Systems: Multiple DF Series switches can be deployed across raw water and clear water tanks to manage supply, transfer, and injection pumps at individually configured level setpoints — enabling fully automatic multi-tank level control with a single, standardised switch type across the entire waterflood installation.
Saltwater Disposal Systems: Produced saltwater disposal facilities require reliable level switching to prevent tank overflow and pump dry-run conditions. The nitrile diaphragm's resistance to saline liquids makes the DF Series a proven choice for these demanding produced-water handling environments across upstream oil and gas operations.
Cooling Towers: Maintaining the correct water level in cooling tower sumps is essential for efficient heat exchange and pump protection. The DF Series provides a cost-effective method of automatically controlling makeup water pumps to keep sump levels within the operating band required by the cooling system design.
Crude Oil Tanks: Applied to atmospheric crude oil storage tanks to control transfer pumps and prevent overfill conditions. Its compatibility with light petroleum liquids and standard 1 NPT tank nozzle connections makes it a field-proven choice across upstream and midstream oil operations at both attended and unattended sites.
Diesel Day Tanks: Engine-driven generators and prime movers rely on diesel day tanks maintained at the correct fuel level. The DF Series provides automatic fuel transfer pump control to keep day tanks adequately supplied without manual intervention, reducing operator workload particularly on remote or unmanned installations.
Sumps: Industrial facility sumps that accumulate process liquids, rainwater, or spill containment fluids are managed efficiently with the DF Series — automatically activating a sump pump at a preset high level and deactivating it at a safe low level, preventing overflows and dry-running without requiring continuous operator monitoring.

Technical Specifications

The DF755 is the currently available model in the DF Series, factory-configured for atmospheric tank applications in non-hazardous areas. All key parameters are listed below for engineering reference and procurement use.

Parameter	Specification
Switch Type	SPDT Snap-Switch (Special Alloy Leaf)
Switch Contact Rating	15 A @ 125 VAC; 15 A @ 250 VAC
Sensing Element	Nitrile (NBR) Diaphragm
Level Repeatability	¼ in. (6 mm)
DF755 Switch Differential (Factory Set)	4 in. (102 mm) — operates at approx. 2 in. (51 mm) and 6 in. (152 mm) above diaphragm
Maximum Pressure Rating	25 psi (172 kPa / 1.72 Bar)
Process Connection	1 NPT
Conduit Connection (Electrical)	½ NPT
Body / Lid Material	Aluminum
Bottom Plate Material	Glass-Filled Nylon
Suitable Tank Type	Atmospheric (Non-Hazardous Area — DF755)
Compliance	RoHS Compliant
Shipping Weight	3 lb. (1.4 kg)
Shipping Dimensions	6¼ × 6¼ × 6¼ in. (159 × 159 × 159 mm)
Overall Height	6-5/32 in. (156 mm)
Overall Width	5-1/2 in. (140 mm)

How the DF Series Hydrostatic Head Level Switch Works

As liquid rises in a tank, the column of fluid above the diaphragm creates an increasing hydrostatic head pressure at the process connection point. This pressure acts directly on the nitrile sensing diaphragm, causing it to flex upward. As the diaphragm deflects, it drives an internal actuator arm that — once sufficient liquid head pressure is reached — snaps the SPDT switch contacts from one position to the other, sending a start or stop signal to the connected pump control circuit.

On the DF755 model, the snap-switch is factory-configured to change state at approximately 2 in. (51 mm) and 6 in. (152 mm) of liquid head above the diaphragm mounting level. This built-in 4 in. (102 mm) operating differential prevents short-cycling by ensuring the pump runs long enough to move a meaningful volume of liquid before the switch changes state again. As the tank level drops back below the lower setpoint, the reduced head pressure allows the diaphragm to relax, returning the actuator arm and resetting the switch to its original position ready for the next cycle.

When wired into a pump motor starter circuit, the two switch states start and stop the pump automatically — providing complete level control with no additional logic, controllers, or dedicated power supply required. The same SPDT output can also interface with single or dual magneto ignition systems for engine shutdown applications at preset high or low level conditions, or feed a supervisory alarm circuit for remote monitoring of tank levels.

Technical FAQ

Common Questions Answered

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A hydrostatic head level switch senses liquid level by measuring the pressure exerted by the column of liquid above its diaphragm — with no moving parts in contact with the fluid surface. A float switch uses a buoyant element that physically moves with the liquid level. The diaphragm approach offers greater repeatability, fewer mechanical parts to foul or jam in service, and compatibility with a wider range of liquids including produced water and light hydrocarbons.

Yes. Using two DF Series switches positioned at the desired high-level and low-level setpoints, one switch starts the pump and the other stops it — maintaining tank levels automatically within the selected operating range. Standard wiring configurations support motor starters equipped with Hand-Off-Automatic (HOA) switches for both start-at-low and start-at-high level control arrangements.

The DF755 uses a nitrile sensing diaphragm that is resistant to most common industrial liquids, including diesel fuel and light crude oil, as well as water and saltwater. It is advisable to verify compatibility with the specific fluid being handled — particularly if it contains aggressive chemical additives or solvents — before committing to installation in any hydrocarbon or specialty chemical application.

The DF755 supports several tank mounting configurations: direct tank wall mounting, bull plug installation at the tank bottom, tank wall mounting with a drain cock, direct bottom-of-tank mounting, and riser pipe mounting. This flexibility allows the switch to be installed at the precise level setpoint required by the process without needing additional gauge instruments or custom mounting brackets in most standard applications.

Yes. The SPDT snap-switch output can be wired directly to single or dual magneto ignition systems to shut down engine-driven equipment when liquid reaches a preset level. Simple terminal wiring changes accommodate both high-level and low-level engine shutdown configurations. Connection to dual-magneto engines is accomplished using a compatible magnetic switch accessory in series with the DF Series output.

Genuine service parts available for the DF755 include a complete diaphragm repair kit (part 15000123), a snap switch and insulator assembly (15000121), and a snap switch, insulator, and movement assembly (15000122). These cover the most commonly serviced internal components, allowing field-level repair and restoration of full switch function without replacing the entire unit body.