MEASUREMENTS OF GAS EXCHANGE 



133 



side. As the experiment progresses, the pressure inside the vessel becomes 

 greater than, or less than, that outside. 



A reservoir of manometer fluid in a short piece of rubber tubing at 

 the bottom of the U-tube permits the 

 adjustment of the fluid level and allows 

 us to bring the internal volume back to 

 its original level. If we allow the vol- 

 ume to increase, or decrease, it becomes 

 very difficult to calculate just how 

 much gas has been produced or used. 

 Imagine the complexity of calculation 

 in the equation above if P, V, and n 

 all change rapidly. At each reading, the 

 fluid level is returned to the starting 

 point on the closed arm, and the pres- 

 sure diff^erence is read from the open 

 arm of the U-tube. 



The manometer and vessel are con- 

 veniently mounted on a supporting rack. 

 The temperature in the vessel is main- 

 tained at a constant value by immers- 

 ing the vessel in a thermostatically con- 

 trolled water bath. The water bath is 

 usually the most expensive part of the 

 entire apparatus. A sensitive mercury 

 "thermoregulator" operates through re- 

 lays to turn on heaters if the tempera- 

 ture falls. Often a refrigeration system 

 is included. The cooling system of 

 course is essential for experiments con- 

 ducted below room temperature. The 

 thermoregulator balances the heaters 

 against the cooling system to keep the 

 temperature constant. A stirring device 

 agitates the water vigorously so that the 

 temperature is uniform throughout the 

 bath. Water baths diff^er in detail (see 



Fig. 10-2), but all of them maintain the temperature within a very 

 narrow range, perhaps at ±0.05° C. The water bath system also includes 

 a means of holding the manometer racks and a means of shaking the 



Fig. 10-1. Manometer and vessel. 

 A flexible reservoir of fluid is at- 

 tached at the bottom of the U-tube. 



