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nthallen / keutsch-hcho

Data Acquisition for Harvard Keutsch Group Formaldehyde Instrument
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Real-time Display of Preliminary HCHO Mixing Ratios #4

Open jdshutter opened 6 years ago

jdshutter commented 6 years ago

This would require using a (1) pre-determined cal factor and (2) power meter calibration after having found the difference counts between the online and offline positions.

Mitchellpkt commented 6 years ago

@jdshutter For preliminary data, I wonder if you could bypass need (2) for power meter calibration, by normalizing to UDT voltage instead of laser power? As long as nobody futzes with the alignment, I'd expect a reasonably-stable "rough-draft cal factor" relating (Diff Counts)/(seconds * mV) to ppb HCHO.

nthallen commented 6 years ago

Remind me: what is UDT voltage?

jdshutter commented 6 years ago

UDT voltage is just the voltage from the power meter. Mitchell is right that we can simply use it as a rough cal factor. I've been normally converting UDT voltage to mW in order to compare everything to Tom's ISAF instrument.

nthallen commented 6 years ago

OK, sure, but you still need a calibration to get the cal factor relative to UDT voltage. As you say, if the alignment changes, all bets are off, whether we're talking volts or mW.

Mitchellpkt commented 6 years ago

Whoops, I think "UDT" is a brand/model identifier. Sorry, I meant to indicate normalization to the instrument's permanent beam power meter (measuring real-time laser power into &/or out of the cell)

The only difference is practical considerations that arose in the field with the old HCHO instruments. Since no disassembly is necessary to acquire the "rough draft" cal factor normalized to voltage, it was convenient to measure between flights in broad daylight while bolted down to the aircraft. However, the extra step to calibrate the power meter (laser wattage <--> power meter voltage) required taking apart the optical box in a dark room. More of a twice-per-campaign task, as opposed to re-measuring the "rough draft" cal factor routinely.

True, both cal factors change after any alignment changes -- with our previous instrument configurations, it was just WAY easier to update the voltage-normalized cal factor on-the-fly.

nthallen commented 6 years ago

Does it make sense to think of (Diff Counts)/(seconds * mV) to ppb HCHO as the primary calibration, with the mV to mW calibration being secondary? Referring to the former as only rough cut doesn't really make sense to me in this context.

Mitchellpkt commented 6 years ago

Yes & agree. I think of the voltage-normalized value as the "raw calibration factor," since the per-mW version is essentially just calculated by unit conversion with the measured mW/mV ratio.

jdshutter commented 6 years ago

The real-time display is working well, Norton! Would it be possible to add this as a variable that could be plotted?