Trying to decide whether you need a photoionization detector (PID) or a flame ionization detector (FID) for your project? Both sensitive low-range gas and vapor instruments are used to detect the presence of a variety of gases — mainly volatile organic compounds (VOCs) — but they are markedly different, so it’s important to choose the right one. When you have a thorough understanding of how each device operates, you’re better equipped to choose which one to use for your unique situation.

PIDs

A PID is a portable vapor and gas detector that is used to identify a variety of organic compounds. This instrument contains an ultraviolet lamp that releases photons absorbed by the compound in an ionization chamber. PIDs generate results almost immediately, allowing you to get a quick and easy reading. Despite requiring cleaning, they’re typically considered reliable, cost-effective and long-lasting. They’re very accurate, fast and offer good sensitivity for the detection of VOCs.

Similar to FIDs, PID devices measure organic vapors and gases and some inorganic gases. Many are liquid solvents and other types of gases that are difficult to use for calibration. Consequently, using isobutylene as the calibration gas is recommended. Readings for other gases are acquired by multiplying the reading by a correction factor. Manufacturers usually provide a list of correction factors with PIDs. One important limitation of PIDs is that they cannot detect methane.

It’s also important to note that PIDs can sometimes display false-positive readings in conditions of extremely high humidity. Modern PIDs such as the MiniRAE 3000 have built-in humidity compensation which can dramatically reduce this effect. Keeping the sensor clean and dry and the use of a clean filter on the inlet also helps eliminate the issue.

FIDs

An FID is larger and heavier than a PID and it uses a hydrogen-air flame to ionize a sample gas and determine its concentration. The device is a sensitive detector for hydrocarbons, but it destroys samples so afterwards they can no longer be used for further analysis — unlike the non-destructive PIDs. However, FIDs are generally not impacted by humidity, so they may be a better choice than PIDs for this type of climate.

This type of device also measures organic vapors and gases and a few inorganic gases. However, this instrument is costly and less reliable than a PID, as the flame can sometimes go out and the hydrogen cylinder needs to be refilled on a regular basis. FIDs produce very different readings than PIDs, as they respond better to carbon chain length than functional groups. They’re also a better choice when measuring natural gas, as they’re frequently calibrated with methane, which is the main component of natural gas.

If you want to obtain the most accurate reading possible, you need the assistance of the correct device. Now that you know the main differences between PIDs and FIDs, you can make an informed decision on which type of instrument best fits your needs.

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