A20 & A25 Series Temperature Swichgage for Engine Protection

The A20 and A25 Series Temperature Swichgages are compact, vapor/pressure-actuated temperature gauges with integrated electrical switching for automatic alarm and shutdown activation. Designed for engines operating in demanding environments, these sealed instruments provide both visual temperature indication and reliable circuit control — protecting engines from overheating without the need for separate sensors or external controllers.

32–440°F Temp Range

2 / 2½in Dial Sizes

SPDT Snap Switch

IP Sealed Housing

Key Features

The A20 Series (51 mm dial) and A25 Series (64 mm dial) Temperature Swichgages are diaphragm-actuated instruments that combine real-time temperature display with built-in electrical limit switching. Engineered for engines in oil field, marine, construction, irrigation, and trucking sectors, these gauges monitor coolant temperature, crankcase oil, and transmission oil. Fully sealed against environmental exposure and available across a wide range of temperature scales, they deliver dependable protection with minimal installation complexity — making them an industry-standard choice for engine monitoring and automatic shutdown systems.

Diaphragm-Actuated Sensing Mechanism: These gauges use a vapor/pressure actuation system. Heat causes liquid in the sensing bulb to vaporize, creating pressure that moves a beryllium copper diaphragm to drive the mechanical gauge pointer — no external power is needed for temperature indication.
Wide Temperature Range Coverage: Available in eight factory-set temperature ranges spanning 32°F (0°C) up to 440°F (227°C), these instruments cover engine coolant, crankcase oil, and transmission oil monitoring requirements for a broad range of engine types and operating conditions.
Built-In Limit Switching for Alarm & Shutdown: The A20T and A25T models use the gauge pointer itself as one switch pole, completing a pilot-duty circuit when the adjustable limit contact is reached. This integrated design eliminates additional temperature switch hardware and simplifies wiring.
Internal SPDT Snap-Action Switch (TE Models): The A20TE and A25TE variants incorporate a snap-acting SPDT switch internally, rated at 3 A @ 30 VDC inductive and 4 A @ 125 VAC inductive. The switch sets on rising temperature and automatically resets as temperature decreases — ideal for applications requiring snap-action reliability.
Pre-Alarm Capability (TABS Models): The A20TABS and A25TABS models add a second SPDT snap-switch for staged protection — an alarm is triggered at a preset lower threshold, while a second pointer-contact circuit completes the shutdown when a higher limit is reached. This two-stage approach gives operators advance warning before shutdown occurs.
Fully Sealed Environmental Protection: Each instrument is sealed using a polycarbonate case and lens combined with a polished stainless steel bezel and O-ring seals. This construction protects internal components from dust, moisture, and corrosive environments encountered in field and marine applications.
Adjustable Limit Contact Over Full Scale: On A20T and A25T models, the shutdown setpoint is adjustable across the entire scale using a standard 1/16-inch hex wrench, allowing field technicians to set precise alarm or shutdown thresholds without specialized tools.
Flexible Capillary and Sensing Bulb Options: Standard models include a 4 ft (1.2 m) PVC-armored copper capillary. Optional stainless steel armor, extended capillary lengths, and multiple sensing bulb styles allow configuration for different engine types, mounting locations, and adapter thread sizes.
Dual-Scale Dial for Universal Readability: The standard U.S.A. dial features both °F and °C scales on a white-on-black layout with a tempered nickel silver pointer and red tip, ensuring clear readability in varied lighting conditions on job sites, vessels, and engine rooms.
Hazardous Area Certification Available: The A25TE-EX model is CSA listed for Class I, Division 1, Groups C & D hazardous locations, making it suitable for environments with flammable gases or vapors — such as fuel handling areas and certain oil field installations.

Applications

Marine Engine Rooms: Monitors coolant and oil temperatures on main propulsion and auxiliary engines aboard vessels, providing automatic shutdown capability to prevent overheating in unmanned or remote spaces.
Oil Field Engines & Pumping Units: Used on prime movers driving rod pumps, compressors, and production equipment in oil and gas fields where continuous unattended operation demands reliable high-temperature shutdown protection.
Agricultural & Irrigation Engines: Protects engines running irrigation pumps over long daily cycles, triggering alarms or shutdowns before overheating can cause damage — critical where engine replacement and downtime costs are high.
Construction Equipment: Installed on heavy plant and construction machinery to monitor engine thermal performance during intensive load cycles, helping prevent heat-related engine failures on remote sites.
Trucking & Transport Vehicles: Monitors transmission oil and coolant temperatures on heavy-duty commercial vehicles, providing early warning of rising temperatures under towing, grading, or loaded-haul conditions.
Air-Cooled Engine Applications (TH Models): The A20TH and A25TH variants are specifically designed for air-cooled engines, offering cylinder head temperature monitoring for engines commonly found in generator sets and agricultural equipment.

Technical Specifications

Diaphragm-actuated temperature-indicating gauges with integrated switching, designed for engine protection in marine, oil field, irrigation, and industrial applications. Available in 2-inch (A20) and 2-1/2-inch (A25) dial configurations.

Parameter	Specification
Dial Size — A20 Series	2 inch (51 mm)
Dial Size — A25 Series	2-1/2 inch (64 mm)
Dial Face	White on black; dual scale °F/°C standard (USA); other scales available
Temperature Ranges Available	32–120°F (0–49°C) · 32–160°F (0–71°C) · 130–220°F (54–104°C) · 130–250°F (54–121°C) · 140–300°F (60–149°C) · 160–320°F (71–160°C) · 180–350°F (82–177°C) · 300–440°F (149–227°C)
Operating Principle	Vapor/pressure actuation via sealed capillary tube and sensing bulb
Sensing Element	Beryllium copper diaphragm
Sensing Bulb Material	Copper (standard); multiple bulb styles available for different engine adapters
Capillary (Standard)	4 ft (1.2 m), PVC-armored copper; stainless steel armor optional
Capillary Length Options	Up to 20 ft in 2 ft increments; up to 34 m in metric configurations
Maximum Ambient Temperature	-40°F (-40°C) to 150°F (66°C)
Case Material	Glass-filled polycarbonate, corrosion-resistant; steel mounting clamp included
Bezel	Polished stainless steel (standard); alternative finishes available
Lens	Polycarbonate, high-impact
Pointer	Tempered nickel silver with red tip
Sealing	Fully sealed with O-ring seals; environmental protection for field and marine use
Limit Contact (A20T / A25T)	SPST; pilot duty, 2 A @ 30 VAC/VDC resistive; isolated from case ground; gold-flashed silver contacts; normally open, closes on high-limit
Contact Adjustment	Adjustable over 100% of scale via 1/16 in. hex wrench
Snap-Switch (A20TE / A25TE)	Internal SPDT; 3 A @ 30 VDC inductive; 4 A @ 125 VAC inductive; sets on rise, resets on fall
Wiring Terminals — A20 Series	No. 4 screw terminals
Wiring Terminals — A25 Series	No. 6 screw terminals
Hazardous Area (A25TE-EX)	CSA listed — Class I, Division 1, Groups C & D
Unit Weight — A20 Series	11.9 oz (0.370 kg)
Unit Weight — A25 Series	13.3 oz (0.413 kg)
Unit Dimensions — A20 Series	4-3/4 × 4-3/4 × 3 in (121 × 121 × 76 mm)
Unit Dimensions — A25 Series	4-3/4 × 4-3/4 × 2-3/4 in (121 × 121 × 70 mm)
Adapter Nut Thread Options	1/8 to 3/4 NPT, BSPT, NPSF, UNF, UNC, and metric M10–M24 (×1.5) threads available
EMC Compliance	Complies with EMC Council Directive 89/336/EEC (except as noted)

Technical FAQ

Common Questions Answered

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The A20T uses the gauge pointer itself as a switch contact — when the pointer reaches the adjustable high-limit setting, it completes a pilot-duty circuit for alarm or shutdown. The A20TE features an internal SPDT snap-action switch instead, which offers a higher switching capacity (3 A @ 30 VDC inductive) and a reset-on-cooling behavior suitable for applications requiring more reliable contact performance.

Yes. On A20T and A25T models, the adjustable limit contact can be set anywhere across 100% of the temperature scale using a standard 1/16-inch hex wrench — no special tools or calibration equipment required. For snap-switch models (TE), the trip point is adjustable over the upper half of the scale and must allow for the switch's reset differential.

The A20 and A25 Series are suitable for monitoring engine coolant temperature, crankcase oil temperature, and transmission oil temperature. Temperature ranges start from as low as 32°F (0°C) and extend up to 440°F (227°C), with eight standard factory ranges available to match the specific operating profile of your engine.

The pointer-type contacts are rated for pilot duty only (2 A @ 30 VAC/VDC resistive) and are intended to operate low-current control devices. For inductive loads, high-current loads, or direct engine shutdown circuits, a compatible magnetic switch should be wired in series to protect the Swichgage contacts from damage.

The A20TABS and A25TABS models provide two-stage protection: a factory-preset internal SPDT snap-switch triggers a pre-alarm at a lower temperature threshold, while a second face-adjustable pointer contact activates the shutdown circuit at a higher limit. This staged approach is recommended for critical engines where operators benefit from advance warning before an automatic shutdown occurs.

Yes. The A25TE-EX model is CSA listed for use in Class I, Division 1, Groups C and D hazardous locations — covering environments where flammable gases such as propane or natural gas may be present. It is the recommended choice for installations in fuel-handling areas, gas compressor stations, and similar oil and gas facilities.