Back to the Basics: Accuracy Requirements for Steady-State Conditions

Common practice is to assume complete accuracy in the LFL values from whatever authority is recognized at the time. But, as we have seen, one can reasonably assign an uncertainty of 10% LFL to the initial LFL values, as indicated by the precision of the published LFL values and the amount of agreement between the various competent authorities. Temperature effects can account for perhaps an additional 10% LFL, if one chooses to use the lesser amount of correction and in a particular case, the greater amount of correction is appropriate.

Back to the Basics: Accuracy Requirements for Transient Conditions

For transient conditions the response time of the analyzer is critical.

Upsets can cause the solvent concentration to reach 100% LFL within seconds. In the US, NFPA-86 indicates that five seconds’ response time could be required of the analyzer in order to make an effective alarm. Experience has shown that process upsets can produce solvent increases of 10% LFL per second or more.

The time lag in an analyzer’s response causes “dynamic error,” which is the instantaneous difference between the actual solvent concentration and the analyzer reading.

Back to the Basics: Alarm Actions

An analyzer system should be reliable. At minimum alarm actions include an annunciator to notify the operator with a loud, unique sounding, horn or buzzer at the "warning" level, and relays to initiate a shut-down at the “danger" level.

The shut-down should include (as appropriate):

  1. Stopping solvent input (stop conveyer, stop coater)
  2. Heating (which slows vaporization and possibly prevents a source of ignition)
  3. Increasing ventilation to a maximum


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