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.
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.
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.
Accuracy and response time are closely related. The purpose of the analyzer system is to produce an alarm before the solvent concentration can increase to an unsafe level. This implies that the alarm is given in sufficient time to take effective corrective action.
Each drying process has a unique rate of solvent increase in the normal and upset conditions. The potential maximum rate of solvent concentration increase should be estimated, so that the analyzer has time to generate an alarm, and the method of corrective action has time to reduce the solvent concentration, before a flammable or explosive limit is reached in the dryer.
The response time of the analyzer system, including all components in the final installation, is so critical that it should be given careful attention.
One useful method of testing the system time response of the analyzer is to inject test gas directly into the end of the probe inside the process, and to obtain the time that it takes for the alarm to sound. The test gas concentration should be equivalent to at least 10% LFL above the high alarm point.