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09 May 2001

Sensors sing in harmony but not same key

A sensor whose response calls for operator intervention may not meet the requirements for a sensor that automatically returns the process to a safe state.

Compare the difference between a slowly (PV1) and rapidly (PV2) changing process variable (PV) monitored in this figure. Ultimately, without successful operator intervention, an automated shutdown sequence will initiate, as represented by the dotted lines for both PV curves.

The sensor selected for detection of this upper threshold may be the same for either PV1 or PV2. However, the sensor used to detect the lower threshold may be quite different, depending on the type of operator intervention required.

The operator may be able to compare PV1 with other indicated process variable states to determine the specific action to take, requiring a continuous transmitter output.

Slow vs. rapid PV change

In response to PV2, the operator may need to take immediate, predetermined measures to safely control the process, requiring the fast response of a discrete switch.

Know process variances

Assume that intermittent spikes of the PV beyond an upper threshold value will happen during normal, safe operation of the process and that the selected sensor is capable of capturing the actual PV profile.

Frequency, amplitude, and duration of PV

Recognizing the difference between safe and unsafe conditions may require knowledge of either the frequency (t₁), amplitude (PV), duration (t₂), or combinations of all three values to eliminate spurious trips.

At the other extreme, a sensor may either ignore or average the effects of these same PV spikes. As a logic solver is capable of processing only the inputs it receives, the algorithms would need to be quite different for the two sensors described.