When personal monitoring pumps first appeared in the 1950s, they were simple devices, designed for the rugged task of airborne dust monitoring in underground coal mines. By the mid-1970s, designs had become more complex, with flow control systems to maintain the flow rate more accurately. By the end of the decade, two types of flow control systems emerged aimed at two distinctly different goals. These control systems are known generically as constant flow control and constant pressure control. Understanding the differences between these two basic control formats allows us to access the full potential of todayâ€™s personal sampling pumps.
Constant flow controlMost personal monitoring pumps today utilize constant flow control in high flow personal sampling applications (1 to 4 LPM) as in the sampling protocols for total and respirable dust, asbestos fibers and fumes of heavy metals like lead and chromium. This control format maintains the set flow rate within a small tolerance (usually 3 percent to 5 percent) in the event that the pump sees a change in back pressure, as with dust sampling and filter build-up in very dusty conditions.
Constant flow control in a personal monitoring pump works like the cruise control feature in a car. When the pump senses that the flow rate is beginning to fall off (such as the sample filter loading with dust), it will automatically speed up the motor in order to keep the flow rate constant. This is similar to how the cruise control on a car speeds up the engine when the car starts an uphill climb.
Pump manufacturers have taken different approaches in monitoring and controlling a constant air flow rate. Some monitor motor speed, while others monitor the flow rate directly with pressure or flow measurements, but they all accomplish the same task in maintaining a constant air flow.
Constant pressure controlA second control format, generically called constant pressure control, maintains the flow rate by holding a constant low pressure level inside the tubing between the pump and the sample holder. This control design requires that the sample holder include a needle valve in order to adjust the flow rate. This flow control system is usually applied to low flow pumps and sorbent tube sampling (such as activated charcoal tubes), and it allows the sample to be split so that multiple sorbent tubes may be sampled simultaneously using a manifold. Up to four simultaneous samples may be taken with available sampling heads, and because of this feature, constant pressure control is sometimes called multi-flow control.
How does constant pressure control allow multiple samples? Think of it this way: A constant flow control pump would never allow a split sample, because adjusting the flow on one side would affect the other. For example, if you tried to slow down the flow rate on one side, the pump would sense the slower total flow rate, and it would speed up the motor to compensate. The flow rates on both sides of the split sample would change when that happened. But with a constant pressure control system, the low-pressure level inside the tubing between the pump and sample holder is held constant. The force driving the flow rate for each side of the split remains the same. Theoretically, one side could be shut off completely, and the other side would still maintain its set flow rate.
Whatâ€™s the difference?To understand the differences between these control systems, we can look at a simple water spigot. (See Figure 1.) To set the desired flow rate of water from the spigot, we open the spigot valve gradually until the water flow is throttled to the desired level. To measure and control this flow rate, we can either monitor the water flow directly or we can monitor the pressure in the pipe that drives the water flow. Under given conditions, if one is constant, so is the other. By monitoring the water flow, we are applying constant flow control. By monitoring the water pressure, we are applying constant pressure control. Either method can produce the desired flow rate.
So when do we choose one of these systems over the other?
Constant flow control systems are very important when the flow rate must be controlled very tightly, as with respirable dust sampling using a cyclone. If the flow rate changes (as older pumps would tend to do in dusty conditions) then the particle size cut point curve of the cyclone changes also. Plus, constant flow control provides an industrial hygienist with a sense of security that the flow rate is reliable throughout the entire sample, no matter what kind of sampling is taking place.
Constant pressure control works great in low flow applications with sorbent tubes, where high back pressure shifts are not expected. It further allows multiple sampling, which could include duplicate sampling of the same type (like two benzene samples on charcoal tubes), or two completely different samples run simultaneously, such as benzene sampling with a charcoal tube and hydrochloric acid sampling with a washed silica gel tube.
Sidebar: Key pointsRemember the following points about these two control systems:
1 Constant flow control maintains the air flow rate automatically, like the cruise control feature on a car. If the back pressure increases, the motor will speed up to compensate.
2 Constant pressure control maintains the low pressure force inside the connection tubing between the pump and the sample head.
3 Constant pressure sampling requires a needle valve on the sampling head to set the flow rate, and multiple simultaneous samples will require manifolds with multiple needle valves.
4 Constant flow control sampling is very important in flow rate sensitive sampling methods, such as respirable dust sampling with cyclones.
5 Constant pressure control sampling allows split samples to be collected reliably.