The upper portion of the valve head is similar to a standard pressure regulating head. On the lower portion of the head another diaphragm is connected to the main diaphragm by a push rod. As the thermal bulb warms, the liquid in it expands, pushing up on the rod and opening the regulator. Because this is accomplished by an outside power source, the pressure drop through the head is reduced considerably. The valve head will function in connection with the regulator on a 1/2- to 3/4-lb overall pressure drop. The point at which the modulation or compensation takes place may be adjusted by turning the adjusting stem. By turning the stem in, the product temperature is increased. By turning the stem out, the product temperature is decreased. The back-pressure valve will remain wide open, taking advantage of the line-suction pressure until the product being cooled approaches the temperature at which modulation is to begin. The valve head will hold the temperature of the product to within ±1/2°F (0.28°C) of the desired temperature. In the case of failure of the thermal element, the valve head can be used as a straight back-pressure valve by readjusting it to the predetermined suction pressure at which the system is desired to operate (see Fig. 10-25).