• The SAF Series
• Analyzing your Airflow Sensing Applicatons

Description

The advanced SAF Series Airflow Switch provides early detection and protection from overheating in mainframe computers and peripherals, large power supplies; HVAC applications, medical diagnostic equipment, and other complex electronic systems requiring forced air cooling.

Its patented (U.S. Patent No. 4,686,450) design is based on a heated thermistor which monitors the airstream and detects loss of reduction of airflow due to fan failure, clogged screens or filters and obstruction of air inlets/outlets. The thermistor temperature and, therefore, its resistance, is affected by changes in air velocity. The SAF thermistor is part of a sensing bridge which compares its own resistance against a reference circuit and determines the air velocity at which the device must trigger the output.

This highly innovative, low profile airflow sensor is also easily mountable directly on the most densely populated circuit boards, on equipment racks with as little as .625 inch spacing between boards, on sockets or bracket mount in plenums. All SAF models are complete, self contained sensor/alarm devices - no additional circuitry is required.

All SAF Series Sensors are designed to be used in conjunction with typical logic circuitry. They operate on a +5 VDC supply, and their output provides an open collector NPN transistor with its emitter connected to ground (Ov).

This type of versatile output allows driving logic circuits, indicating incandescent or LED lights, or even a magnetic relay, from DC Sources varying from 5V to 30V.

Advantages

• A solid-state airflow sensor switch with improved features.
• Drives logic circuits, audible/light alarms, magnetic relays
• Exclusive dust-proof & sealed versions
• Easy, versatile mounting, to PCB, plug-in socket, or hard wired
• Wide setting range - 50 to 1500 linear feet per minute (FPM)
• Not affected by ambient temperatures
• 5VDC design with low power dissipation
• No signal conditioning required

Improved Ambient Temperature Compensation

Warren G-V design criteria for the SAF Solid-State Airflow Sensor incorporates ambient temperature compensation techniques which provide close operating point tolerances over the ambient range of +10° to +60°C.

Indicates an "alarm condition" on low or no airflow by turning output transistor "on"; output is similar to a closed mechanical contact between output and ground when in alarm condition. A round version, SAF 1006 without a flange is available for less board space utilization.

The Fail Safe Series/SAF 1025

An alternative to above-described series, a device failure indicates an "alarm condition" by turning the output "off"; similar to open contacts in alarm condition. It will operate with a wider hysteresis.

The Hermetically Sealed Series

Will withstand all board washing methods; more suitable for applications with hostile environments. Sealed versions of all the above units are available.

When planning, there are several key considerations to be evaluated by the applications engineer regarding known conditions and desired settings or alarm points. The following are the most common areas for review:

What is the normal, safe airflow level in the system?

• The best approach to this primary consideration is to measure the normal safe flow at the point where the SAF air sensor is to be installed. To obtain accurate results it may be necessary to provide an access hole and insert an anemometer probe into the closed system cabinet. Measurements of airflow levels without the enclosure will reflect false readings. If actual measurement is not possible, an estimate is needed. In this case, try samples of different settings until the optimum choice is made.

Selecting a Setting for your Application

• The setting selection is largely a judgement of the applications engineer. If the object of the sensor is to simply report complete loss of airflow, then an arbitrary setting between "normal" flow and no-flow may be made. In such a case, it's desirable to select on the lower side of the available settings (which range from 50 to 1500 FPM).
• A low setting limits the possibility of false alarms created by a mis-assessment of "normal" or other application conditions such as turbulence. If the objective is to detect a level of filter clogging or some other arbitrary setting, such as 50% of normal flow, care must be taken to determine the normal safe flow. Anemometer tests might be made of a typical clogged filter. Be sure to take measurements of air velocity at extreme operating conditions, including input voltage and temperatures that could affect fan or blower performance.

Other Conditions and Environments

• The SAF Sensor is temperature compensated and is designed for use in an environment of +10° to +60°C, under which conditions the characteristics and tolerances of the SAF specifications would apply. If a mildly corrosive application is encountered, consider our Hermetically Sealed SAF Series.

Turbulence

• It's difficult to prevent turbulence, but you can avoid this common condition by careful location of the airflow sensor. The use of an anemometer is helpful for detecting the desired areas of low turbulence. Airflow sensors placed in highly turbulent locations are exposed to many simultaneous air velocities and will attempt to average them - resulting in incorrect values. This could lead to false operations and intermittent switching.

Positioning, placement and mounting

• The optimum positioning of the SAF sensor is on a plane parallel to the unit base or header, with the airflow perpendicular to the thermistor sensor assembly. This is how all Warren G-V units are factory calibrated. The SAF sensor may be mounted on a TO-3 socket (Warren G-V provides such a socket with leads, p/n 4-1-260) or on a bracket, or directly soldered to a circuit board. For printed circuitry, Warren G-V suggests the round base.
• For multiple fan applications, use one sensor for each fan or blower. Using one sensor to monitor more than one fan can result in false readings; either from a single fan failure due to air turbulence changes, or from airflow volume changes, with velocity still remaining above the setting of the sensor.

Other Considerations

• Response Time: Defined as the time elapsed between the point at which airflow ceases and the point the output collector indicates an alarm. Usually under 10 seconds. A more accurate assessment of response time depends on the given application and the rate of change in air velocity.