Here’s a little background (with respect to mixed fuel analyzers only):
- When the analyzer is in pristine (new) condition, the zero potentiometer is almost at 10 turns (100%) on the duodial (turns counter). This is where the electronic compensation (for contamination) is at a MINIMUM. Rosemount designed it this was so that turning the pot CCW also caused a decrease in the display value (it’s a little easier for the human brain to comprehend, that is, going CCW causes something to decrease).
- We release repaired analyzers with the zero pot anywhere from 5 turns (50%) to 9.8 turns (98%); we strive to get the highest reading possible (which implies minimal contamination)
- RIGAS usually upgrades the 400A Preamp board by replacing the unshielded burner signal wire with a coax (shielded) wire. This helps reduce noise but also makes the signal so clean that we have to install a special potentiometer to inject a small biasing signal. [link to R20 modification]
Here’s a little background (with respect to pure fuel analyzers only):
- When the analyzer is in pristine (new) condition, the zero potentiometer is anywhere between 5 turns (50%) and 8 turns (80%) on the duodial (turns counter). The pure fuel analyzer is WAY more sensitive so any minor contamination easily shows up and needs some zero pot to compensate. 10 turns (100%) is where the electronic compensation (for contamination) is at a MINIMUM. Rosemount designed it this was so that turning the pot CCW also caused a decrease in the display value (it’s a little easier for the human brain to comprehend, that is, going CCW causes something to decrease).
- We release repaired analyzers with the zero pot anywhere from 4 turns (40%) on up; we strive to get the highest reading possible (which implies minimal contamination)
- RIGAS usually upgrades the 400A Preamp board by replacing the unshielded burner signal wire with a coax (shielded) wire. This helps reduce noise but also makes the signal so clean that we have to install a special potentiometer to inject a small biasing signal. [link to R20 modification]
When a 400A (or any hydrocarbon analyzer) won’t zero, do these checks:
- Read flame temperature. It should be around 100mVDC.
- if it’s <50mVDC then you're probably running hotter than you should be. Decrease fuel pressure and watch for the change in zero reading.
- if it’s <5mVDC then you're probably running VERY HOT and in danger of melting the Teflon burner jet. Decrease fuel pressure ASAP.
- Alter fuel pressure to see if a few pounds pressure changes the reading (there will be a change as talked about above but it should be slight).
- if the change is significant, you may have contaminated fuel.
- if the change is significant, you may have the WRONG FUEL. A cylinder of 40% hydrogen / 60% Nitrogen (or Helium) is NOT THE SAME as FID ION FUEL (40% hydrogen / 60% Nitrogen (or Helium)). FID ION FUEL has a cylinder assay where the hydrocarbon contamination is VERY low, maybe even listed as “undetectable”.
- Alter burner air pressure to see if a few pounds pressure changes the reading (there should only be a MINIMAL change, if any). This statement is null and void if your analyzer is running right at the stoichiometric point such that decreasing pressure affects the actual flame due to oxygen starvation.
- This problem will sneak up on you with an aged analyzer as the air restrictor clogs up over many years of service; you’ll set the air pressure to 15# (per the OEM manual) but the burner won’t actually be getting the prescribed 450 cc/min of burner support air.
- An analyzer running at stoichiometric will be hard to zero and hard to span.
- Alter sample pressure when flowing zero gas. You should expect some variation in the readings but nothing significant. If the reading does change by a huge amount then sample line contamination is almost guaranteed.
- If your process contains a ‘sticky’ hydrocarbon molecule such as Toluene, then expect significant delay times to achieve a good zero as it will take time for the Toluene molecules to detach from the sample transport tubing walls.
Keywords: Rosemount 400A troubleshooting