FAQ: Using The Photo-Tach

Should I Use A Photo-Tach Or Mag Pickup?

The Photo-Tach/Reflective Tape and Mag Pickup/Interrupter methods serve exactly the same purpose for balancing and rotor speed measurement. With only a couple of exceptions, one can usually be substituted for the other.

• When signal cables interface directly to a MicroVib or MicroBalancer (most fixed wing applications) you should always use a Photo-Tach since input signal conditioning circuitry of these two units cannot process Mag Pickup signals.
• When signal cables interface directly to a MicroVib II (most fixed wing applications) or when signal cables interface to a Signal MUX (most rotary wing applications) you can use either a Photo-Tach or Mag Pickup.

When either a Photo-Tach or Mag Pickup will do the job, your choice will be based on what is easier to install. If your aircraft is already pre-wired for a particular sensor, then by all means, use the sensor intended for use with your aircraft. If you find it is easier to install one or the other, use the sensor that is easiest to use.

The LOW TACH message indicates that the MicroBalancer is receiving incoming tach pulses but that some of the pulses are spaced too far apart, that is to say that the tach pulse frequency is to low. LOW TACH can either be caused by running the rotor below the minimum input speed for the Balancer (usually 150-180 RPM) or an installation that produces poor signals and skips pulses from time to time.

The HIGH TACH reading indicates that the incoming tach pulses are spaced too close together, in other words, the tach pulse frequency is too high. HIGH TACH can either be caused by running the rotor above the maximum input speed for the balancer or an installation that produces poor signals which produce "double" pulses from time to time.

If your RPM reading is varying that can also be a symptom of poor quality tach signal. The poor quality tach signal may be missing pulses from time to time which causes the displayed rotor speed to vary.

Using old or degraded reflective tape: Reflective tape loses its special optical properties over time, particularly if it is exposed to engine exhaust, high temperatures, rain, dirt/dust or long term sunlight.

Using reflective tape not supplied by DSS: We have found a number of cases where customers attempt to use other brands of reflective tape which have different properties than those of genuine DSS reflective tape and the performance suffers considerably.

Photo-Tach too close to reflective tape: The minimum distance from photo-tach to reflective tape is about 4 inches. The transmitter and receiver in the photo-tach are mounted side by side,  and when tape is too close, the receiver will not "see" the location where the transmitter beam is striking the reflective tape.

Photo-Tach too far from reflective tape: Maximum distance to tape is about 32 inches, although this value varies from one Photo-tach to another. If you need a longer distance from Photo-Tach to Reflective Tape, you may want to look into using our Laser-Tach which can be positioned many times farther.

Photo-Tach pointing at sun or other very bright light: When the photo-tach is pointed at a very bright light sources, that light can overload the sensitive electronics and cause poor performance.

Blade pitch changes move tape away from beam: Move tape or place a larger piece of tape so that reflective tape is in the photo-tach beam at all times.

Reflective tape velocity too high: Reflective tape must be in the photo-tach beam for a minimum of 300us. Sometimes operators mistakenly place the reflective tape at the mid-point of the propeller blade which causes the tape to pass through the photo-tach beam very quickly due to the very long radius. A clue to this problem is being able to obtain proper tach speed at low RPM's but when the engine is run at a higher RPM, the tach signal degrades.

The solution is to reduce the distance from center of rotation to the reflective tape. This can be accomplished by placing the tape at the root of the blade, next to the spinner or even on the spinner backing plate. If you cannot place the tape at a short radius, then you can lengthen the tape along the chord axis of the blade so that it activates the photo-tach electronics for a longer period of time.

Photo-Tach has been exposed to water: If a photo-tach has been exposed to a wet environment, it may have condensed water on the inside of the lens assembly. In this case, you can carefully remove the two screws which hold the lens on the photo-tach body and let the parts air dry before reassembly.

Photo-Tach and Accelerometer cables swapped: If the photo-tach and vibration sensor cables are swapped, the signal processing circuitry of the Balancer will try to find a tach pulse in the vibration signal which will have very poor quality. Insure that you have the cables connected properly.

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