62 



will not give true surface temperatures because it will respond to an average 

 temperature of the top few tenths of a millimeter of water. 



THERMAL SENSITIVE DEVICES 



When making a thermometer one should think carefully of how tempera- 

 ture affects various substances and structures and select a primary sensitive 

 element which: 



a. gives a large output per degreeC. This usually should be measured 

 in units of work per degree; 



b. has good thermal stability over the period of time in question; 



c. is not unduly affected by pressure; 



d. is reliable; 



e. has sufficient speed of response; 



f. does not have different time constants for different parts of it; 



g. is not too sensitive to vibration. Here one wants a short natural 

 period. 



The choice of thermal elements is fairly large and a particular applica- 

 tion may dictate a particular choice. 



1. Thermal expansion of a gas. This will permit large work outputs 

 and is convenient except for its very large pressure coefficient. 



2. Thermal expansion of a fluid. This is commonly used and can have 

 good reliability and a large work output. Mercury thermometers 

 and BT's are typical. 



3. Thermal expansion of a solid. This has the least motion and most 

 designs have either a slow speed of response or are sensitive to vi- 

 bration. Bimetallic elements are used in the thermo sonde. 



4. Change in the electrical resistance of a metal or conductor. Resis- 

 tance thermometer bridges are stable, accurate and well understood. 

 Thermistors are certainly coming into their own and while they still 

 lack long time stability they make it possible to build a cheap and 

 sensitive thermal element. Their high resistance makes them very 

 good for inputs to servo amplifiers. 



5. Peltier effect - the emf produced by a thermocouple. The power de- 

 veloped in a thermocouple circuit is very small but it is well adapted 

 for comparing temperatures at several different points. 



6. Vapor pressure thermometers are widely used in industry and they 

 are based on the vapor pressure of a given fluid. 



INSTRUMENT DESIGN 



It is impractical to recommend a specific method without knowing the 

 problem and the techniques available to the builder. Personnel preference, 

 training and ability will usually determine how an instrument is built. The fol- 

 lowing notes may be of value to designers. 



a. Mechanical levers. If the primary measuring element gives a large 

 work output it may be practical to use mechanical levers for ampli- 

 fication. When well done this method is simple and reliable. Such 



systems are sometimes allergic to vibration. If bearings are used 

 corrosion can be a number one problenn. 



b. Optical levers. These have the great advantage of giving high mag- 

 nification and being almost inertialess. Another great advantage is 

 that several traces can be lined up and still overlap. 



c. Servo systems. The use of an electrically operated self-balancing 

 bridge is perhaps the most versatile system as it can be operated 

 with very small inputs and can give a large output. Many manufac- 

 tors produce a wide variety of commercial servo systems. 



