Temperature: Knowledge of the temperature of a fluid or the atmosphere in which it works can be very important. Two styles of temperature gauge are shown in Figure 18-4. When pneumatically operated machines are in atmospheres of 32° F or less, the condensed moisture in them may freeze. When hydraulic circuits operate much above 140° F they can leak or slow down and the fluid in them starts to break down.

It is best to keep hydraulic systems between 75° and 130° F. Temperatures above 130° F can vaporize important additives and cause excessive bypass due to reduced fluid viscosity. Fluid temperatures below 75° F can result in sluggish performance.

Flow meters: The cross-sectional view in Figure 18-5 shows a typical inline flow meter that indicates flow in cubic feet per minute (cfm), gallons per minute (gpm), or liters per minute (lpm). This style of meter is made of aluminum or non-magnetic stainless steel to allow the magnet-powered notched steel ring to function.

Fluid entering from the left passes through flow holes and against a spring-returned piston fitted with magnets. This piston wraps around a tapered metering cone and has a sharp-edged orifice in contact with it. The only way for fluid to get through is to push the spring-returned piston with magnets to the right. When the piston moves far enough up the tapered metering cone to allow the present rate of fluid to pass, it stops and holds. The magnets in the piston draw the notched steel ring along and the notch reads the flow amount on the clear-plastic cover with flow scales.

This type flow meter is not completely accurate but gives a clear enough indication of flow to meet most troubleshooting needs. Other designs are more accurate but less tolerant of the harsh interaction of a high flow system.

The upper symbol on the right in Figure 18-5 is for a device that only shows whether flow is taking place in the line or not. The middle symbol represents the cross-sectioned device. It indicates both the presence of flow and the flow rate. The lower symbol represents a device that shows the flow rate and keeps a running total of the amount that has passed through it.

Shuttle valves: The circuits in Figure 18-6 illustrate one reason for using shuttle valves. The spring-return cylinder in the upper circuit must be controlled from three locations. This circuit uses pipe tees to interconnect the three normally closed, palm-button-operated, 3-way directional control valves with the cylinder. The only problem is this circuit will not work. When any of the 3-way valves are actuated, input air can flow directly to atmosphere through the other 3-way valves, bypassing the cylinder.

The lower circuit uses shuttle valves in place of the pipe tees. Air from any of the 3-way directional control valves can only go to the cylinder. The floating ball in the shuttle valve blocks air to the other directional control valves. Exhausting air can go to atmosphere through the valve it entered, go out the opposite valve, or exhaust through both valves. If each 3-way directional control valve has a different pressure at its inlet (as indicated), the cylinder always gets the highest pressure of the valves actuated. The ball in the shuttle valve always moves away from the highest inlet pressure.

Other circuits use shuttle valves to send more than one pilot signal to a directional control valve, read feedback signals from more than one source, or send signals from multiple actuators to a load-sensing pump. Any time multiple inputs are necessary, a shuttle valve will separate them, allow for return flow, and pass the highest input pressure. (Check valves can serve two of these functions but will not allow back flow.)