Teledyne 4020 THC analyzer is a great hydrocarbon analyzer but has one flaw! This flaw only shows up as an annoyance when you are monitoring a process AND controlling off of the 4020’s output signal. The flaw is in the software; the hardware is fine.
Here is the scenario: your process is looking good with about 11ppm of THCs. Then a small upset occurs in your process and the hydrocarbon readings start to climb (they can climb slow or fast, it doesn’t matter). At some point, the analyzer has to change its gain on the Preamplifier Board (a.k.a., preamp board) to avoid signal saturation; this is “normal” when the process readings change. Depending on the configuration of the preamp board (there are about eight different configurations) will determine when it actually changes gain. For the sake of this example, let’s say that it changes gain at 20 ppm … at that magic moment of gain change, the analyzer blips, hiccups, or spikes, whatever you want to call it. The spike can be significant maybe up to 60, 80, or even 100ppm. When your DAS or PLC sees that (errant) spike, your process may end up getting tripped on (falsely) high hydrocarbons.
Solution: ensure that your PLC doesn’t enact a trip on a spike but is coded to perform signal averaging of the process signal (value).
The “real” solution: the OEM needs to improve their code such that the output is held for a few seconds during the gain change. The analyzer knows when it’s going to do the change so it can certainly “hold last known value” until the spike dissipates (only a few seconds is necessary).