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The Gemini 501A Smoke Detector Analyzer

Technical Data


Tests all detectors                                                                                        Portable, fast, easy to use


Meets all NFPA 72 requirements                                                                    Harmless to detectors


Tests all detectors in place: ceiling, side wall, underfloor and up to 27 ft high with high reach kit



The National Fire Protection Association (NFPA) code 72  requires periodic inspection and testing of smoke detectors.The required test includes a calibrated sensitivity test to assure that each detector is within its listed sensitivity range and each detector would initiate an alarm when exposed to smoke.Local codes and other accrediting associations require similar sensitivity testing of smoke detectors. 

The Gemini 501 Tester

The Gemini 501A Smoke Detector Tester is a calibrated test  instrument. It produces an adjustable, stable smoke from a special FDA grade natural mineral oil. The smoke is created by a nebulizer, not by heat or flame.The smoke particle sizes and size distribution generated by the Gemini 501A resemble the smoke from the UL-268 Standard Smoke Box.  Because of the small particle sizes, low concentration, and short exposure time, this smoke is not harmful to the detectors or operator. The Gemini 501 A is approved by leading smoke detector manufactures.  

Smoke Detectors

All types of smoke detectors are designed to alarm when exposed to low concentrations of smoke to assure early detection of fire. The concentration of smoke is measured in per cent of obscuration per foot (%/ ft obs), a standard used by Underwriters Laboratories.

After installation, the sensitivity of smoke detectors can shift due to dirt, adverse conditions, or aging. If the smoke detectors lose their sensitivity, the advantage of early fire detection is lost. In some cases, they may not function at any concentration. If  too sensitive, the detectors may cause false alarms. 

Portable, easy to use

The Gemini 501 A Smoke Detector Tester was specifically designed for testing installed smoke detectors by service personnel. It is portable and is easy to use.  

The Gemini 501A tests all smoke detectors: all types, all brands; no extra modules, meters or connectors. One easy to learn procedure tests all detectors. It comes with extension wands to test detectors on ceilings up to 20 feet high. With optional wands, detectors on ceilings as high as 30 feet can be tested without a ladder or lift. Using readily available fittings, the wand can be modified to test hard to reach detectors. 

Principles of Operation

The Gemini 501A Smoke Detector Tester uses a nebulizer to produce a spray of particles. An impactor removes the large particles, leaving micro, smoke sized particles. The smoke resembles in size the smoke in the UL 268  Smoke Box. The built in compressor- vacuum pump supplies air pressure for the nebulizer and an adjustable vacuum to remove excess concentration of smoke. A blower supplies room air to dilute the smoke and transport it through the hose and wands to the detector. 


In production, each instrument is individually calibrated with a Particle Mass Monitor and Standard Reference Detectors.  The instrument output is presented as mass concentration in mg/m3 and smoke obscuation in %/ft.

Test Procedure

After setting up of the Gemini 501A tester, it is adjusted to produce the same per cent obscuration smoke as the listed rating of the detector. The detector should alarm within 30 seconds. A second test may be conducted to determine if the smoke detector sensitivity is within an acceptable range. 

This test is both a senstivity and a functional test. A Gemini 501A  sensitivity test adds a minimal amount of time to a standard smoke detector cleaning and testing procedure. 

Quality construction

Since 1982, the Geminis have been supplied to hospitals, nuclear facilities and fire alarm service companies. Each Gemini is carefully built using quality, rugged components assuring durability in the field. Prior to shipment each Gemini 501A is thoroughly inspected, tested  and individually calibrated with a Mass Particle Monitor and Standard Reference Detectors. The instrument output is presented as both mass concentration in mg/m3 and smoke obscuration in %/ft.

For detailed specifications see:   Gemini 501A brochure.pdf

For prices see: Gemini Price List