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These notes, from
RAE
Systems, provide advice on using your PID.
Question: PIDs cannot measure diesel
fuel or kerosene, right?
Answer: Wrong.
This was an old wive’s tale that was true for older PIDs but does not
apply to RAE PIDs. Photoionization detectors are sensitive to diesel fuel,
jet fuels, kerosene and other heavy hydrocarbons. In fact, in general, the
heavier the compound, the more sensitive the RAE PID is.
The source of this rumor probably stems from
the fact that heavier hydrocarbons absorb easily into tubing and filters.
Therefore, if several feet of Tygon tubing are used to sample for jet
fuel, there may be no response because all the fuel is absorbed by the
sample line and never reaches the PID. The same tubing could be used with
no problem to measure isobutylene, which is very volatile. Thus, the
impression could be gained that the PID is working, but will not respond
to heavy compounds. Never use Tygon or other soft rubber tubing to draw a
sample into a PID — the use of metal or Teflon tubing is recommended. For
very heavy or reactive compounds (such as methylhydrazine) it is even
recommended to remove all filters on the inlet.
RAE PIDs, particularly the ppbRAE and MiniRAE
2000, are designed for rapid flow through the sensor and have only Teflon,
metal, or glass parts in contact with the sample-upstream of the sensor.
Volatile compounds (including diesel fuel) have a response time (t90) of
2-3 seconds. Heavier compounds may have a slower response and take up to a
minute to stabilize. Even heavier compounds with boiling points above
300ºC can be detected but often do not give a reproducible or complete
response. Therefore we use a rule of thumb of a 300ºC boiling point as the
upper limit for compounds to be quantified by a PID. According to this
rule, most fuels are measurable, but lubricating oils, fuel oils, PCBs,
polycyclic aromatic hydrocarbons (PAHs) and MDI fall into the category of
“detectable, but not quantifiable”compounds. Any instrument, including
FIDs and infrared monitors, will have slow or weak response to heavy
hydrocarbons if the sample pump is placed upstream of the sensor or if
rubber tubing is used to draw in remote samples.
For more information, please refer to RAE
Technical Note TN-106 on the
RAE
website. |
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