76 



AGRICULTURE HANDBOOK i:{4, U.S. DEPT. OF AGRICULTURE 



PN-4788 



Figure 92. — The liquid-stem industrial thermometer has 

 an open scale that permits calibration marks for each 

 ''4° F. and the temperature of the boiling sirup can be 

 measured precisely. The thermometer is mounted out- 

 side the pan so it is not obscured by steam. It is 

 especially suited when the pan is covered with a tight 

 steam hood. 



ters can be obtained with the stem bent at 

 right angles and protected with metal armor. 



The right-angle thermometer is mounted 

 through the wall of the sirup or finishing pan 

 using a special fitting. This arrangement per- 

 mits the thermometer to be mounted high 

 enough on the sidewall of the evaporator or 

 finishing pan to be above the level of the sirup 

 so that the thermometer can be removed for 

 cleaning without loss of sirup. It also locates 

 the scale of the thermometer at an obtuse 

 angle for easy reading. 



The thermometer is calibrated each day in 

 terms of the boiling point of water. The bulb is 

 immersed in water, the water is brought to a 

 boil, and the temperature is noted. To this 

 observed temperature is added 7°, the tempera- 

 ture elevation required to give the boiling point 

 of standard-density sirup (see table 12). 



Dial Thermomt'tt'r 



The degi'ee lines of the dial thermometer (25), 

 like the target thermometer, refer to the boiling 

 point of water (fig. 93). This thermometer has a 

 bimetallic element in the first 3 or 4 inches of 

 the stem. As the indicator is a needle, the 

 openness of scale is governed by the length of 

 the needle and the accuracy required. The scale 

 is twice as open in a dial thermometer 5 inches 

 in diameter as in the target thermometer. 



The dial thermometer is calibrated by im- 

 mersing the part of its stem that contains the 

 bimetallic element in boiling water or sap the 

 same distance that it is immersed in the sirup; 

 when the indicating needle comes to rest, the 

 dial is rotated by means of an adjusting screw 

 until the zero or water boils line coincides with 

 the pointer. Then the sirup line is located T F. 

 above the zero or water boils line to indicate 

 the boiling temperature of standard-density 

 sirup for that day and place. 



The long straight stem of this thermometer is 

 inserted through the wall of the sirup pan and 

 sirup drawoff box so it will be parallel to the 

 bottom of the pan and entirely immersed in the 

 boiling sirup. The dial of the thermometer is on 

 the outside of the evaporator where it is out of 

 the steam and is easy to read (fig. 93). 



Hydixjnietei'S 



A hydrometer is not the ideal instrument for 

 judging the finishing point of sirup. It is not 

 calibrated for use at the temperature of boiling 

 sirup, and it cannot be used to follow the 

 concentration of the sap continuously. For ac- 

 curacy, the exact temperature of the sirup 

 being tested with the hydrometer must be 

 known so that the necessary corrections can be 

 made. However, the hydrometer and refractom- 

 eter are the only instruments that can be used 

 to measure the density of sirup that is not in an 

 actively boiling state. 



•Hot Test' 



The "hot test" is often used to determine 

 whether the process of evaporating sap to sirup 

 is completed. It is made as follows: 



