Questions about monitors always seem to be common. There is so much that could be considered the “basics” of monitoring devices and monitoring practices that a few posts will be dedicated toward addressing various aspects of monitoring basics. This first part addresses the two main types of monitors in regards to some terminology and application issues that have come up.
Diffusion Versus Pump
There are two basic styles of portable air monitors: Diffusion (or passive) monitors and pump (or active) monitors. Both have benefits and drawbacks. It is important to evaluate your application and use to determine what style or styles are appropriate for the work you are performing.
Diffusion monitors, also known as passive or personal monitors, are typically much lighter and cheaper than their pump style counterparts. The primary application for diffusion monitors is for personal monitoring (employees clipping the monitor to their clothes to monitor gas exposure throughout the shift). The light weight is ideal for this purpose so employees are not bogged down with heavy equipment or have monitors pulling their clothing. The lower cost is also desirable for this application because these monitors usually get dirtier and dinged up more being attached to the person’s clothes.
The main drawback with these monitors is the delay in the readout. Since these monitors rely on diffusion to collect samples, it takes longer to obtain accurate readings on the monitor. Diffusion is the migration of particles in the air due to air movement and pressure exerted between particles. Basically as gasses float around the air they eventually work their way into the monitor and into the sensors. This process can take around 30 seconds between the time a change in the atmosphere occurs and the time the monitor accurately reflects those changes.
Diffusion monitors would not be appropriate in situations where a rapid readout would be necessary, such as in rescue scenarios. They also do not offer the ability to offer remote monitoring by the use of hoses or wands. This makes diffusion style monitors inappropriate for monitoring confined spaces from the outside. Entrants can use diffusion monitors for continuous air monitoring within the space, but attendants cannot use diffusion monitors for pre-entry monitoring or for other non-entry monitoring.
Pump style monitors are often referred to as portable monitors (though diffusion monitors are also portable) or sniffers. These monitors are typically heavier and more expensive than diffusion monitors, and are usually identified by their annoying humming while active. The pump in these monitors is the reason for the extra cost and weight, and creates a vibration and hum that often annoys users, especially if they try to use them as a personal monitor. These monitors are required for confined space entry (unless fixed monitors are in place within the space) and are ideal for rescue situations. They are also a good choice for hazardous environments, such as flammable atmospheres to ensure employees know exactly when the danger becomes unacceptable.
One thing to remember with pump style monitors is that the hose and/or wand delay the readout of the monitor. A good rule of thumb is that for every foot of hose and/or wand the readout is delayed about 1-2 seconds.
*Unless specific citations are shown, all answers are based on interpretations provided by authorized officials. As such, all information is deemed reliable, but not guaranteed.