While each type of sensor uses a different method to detect gas, all gas detecting sensors produce an electrical output that is proportional to the amount of gas detected. In general, the greater the output, the higher the reading. However, the electrical output is not as simple as “gas in = signal out”. The changes in current fl ow produced by the sensor must be interpreted by the instrument before the readings can be displayed. A very important point is that even if the sensor is not detecting gas there is still a current fl owing through the sensor. In other words, even when the sensor is located in an atmosphere with “zero contaminants” the sensor is still producing a signal.


WHAT DOES IT MEAN WHEN A GAS DETECTOR DISPLAYS NEGATIVE READINGS?


Whenever you “Auto Zero” or fresh air adjust a sensor you are telling the instrument that the electrical signal at the time the sensor is adjusted is what the instrument should expect the sensor to produce while it is located in fresh air. The output signal in fresh air is used as the point of comparison for subsequent readings. A higher output signal is interpreted by the instrument as indicating a higher concentration of gas. However, if the signal drops below the output at the time the sensor was last fresh air adjusted, the comparison point is now negative with respect to the actual readings.


• Do all instruments display negative readings?


Most instruments eventually display negative readings or sound a “downscale” or “negative alarm” if the negative difference with the fresh air output value becomes signifi cantly large.


However, different manufacturers have different ideas about what constitutes a “signifi cant” negative reading. For instance, to avoid raising concerns with their users because of “minor” fl uctuations in readings near zero, manufacturers often include a “dead band” in the instrument’s programming. Until the signal exceeds the limits of the dead band, readings remain locked on zero.


In some designs the dead-band can be quite substantial. For instance, some brands of instruments equipped with miniaturised pellistor type LEL sensors have a dead-band that stretches all the way from -3% to +3% LEL. In other words, in the presence of a rising concentration of gas, the fi rst reading displayed is 4% LEL. At 3% LEL the instrument reading is still”locked” on zero. Similarly, until the output signal reaches a value of -4% LEL, the display shows a reading of zero.


Although it is physically impossible for the atmosphere to contain a negative quantity of a substance, it is not uncommon for a gas detector to display a negative reading on the LEL, PID or toxic measurement channels.


Figure 1: Always allow all of the sensors installed in your instrument to stabilise completely in fresh air BEFORE making a fresh air zero adjustment.


Sometimes the manufacturer may simply make the readings a littlemore “sticky”close to zero, to proportionally reduce fl uctuation as the signal gets closer and closer to zero. Whether or not to include a dead-band is a manufacturer decision based on the instrument design and the stability and resolution of the senso.


Dead-bands may reduce user concerns because of trivial fl uctutions in the instrument readings, but they may also leave the user unaware of changes as the instrument initially begins to respond to increasing concentrations of gas. Even worse, when the instrument starts out from a negative output level, it takes that much more gas before the instrument reaches the alarm concentration. For instance, if the LEL alarm is set at 10% LEL, when instrument starts out at -3% LEL it will take 13% LEL gas to cause the instrument to go into alarm. When the display is digitally locked on zero the user is unaware of the need to fresh air zero adjust the instrument.


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