the pulping digester schematically illustrated in fig1 is a much simplified depiction of a pulping digester manufactured by kamyr corporation of glens falls , n . y . this digester is of the continuous operation type and can be utilized with several of the currently used commercial pulping processes including the sulfite process , the kraft process , and the soda - oxygen process . the digester , generally designated 10 , is a relatively tall pressure vessel having a cylindrically shaped outer wall 12 , a bottom 14 and a hemispherically shaped top portion 16 . a frustoconically shaped baffle 17 is positioned in the top of the digester and extends downwardly and outwardly from a feed screw or auger 18 positioned centrally in the top of the digester . chips and liquor ( digesting medium ) are fed into an opening in the top of the digester and are forced into the interior of the digester by screw 18 . one of ordinary skill in the art will understand that the fittings and piping associated with the top and bottom of the vessel are sealed so that the interior of the pulping digester can be maintained at relatively high pressures , normally on the order of 165 psig . as the pulp and liquor are withdrawn through a conduit 20 , the chips fed to the top of the reactor will travel downwardly and be progressively delignified as they reach the bottom of the reactor . the pulp and liquor withdrawn from the bottom of the digester through the outlet conduit 20 are normally discharged into a blow tank ( not shown ) in which the pulp is separated from the liquor . the liquor is then subjected to intermediate processing and recycled into the top of the digester . under normal operating conditions , an adequate amount of chips and makeup liquor are added to the top of the digester to compensate for the pulp and liquor withdrawn from the bottom . the liquor is normally supplied to the digester in an amount sufficient to maintain the level of the liquor at the top of the digester . the amount of chips fed to the digester is normally sufficient to maintain the chip level , indicated by dashed line 24 , at a relatively constant location in the digester . to maintain optimum conversion efficiency , that is , to maximize impregnation of the chips with liquor , it is desirable to maintain the chip level 24 at some location near the top of the digester but at or perferably slightly below the bottom of the feed screw 18 . at four locations in the digester , a portion of the liquor is separated from the combined mass of liquor and chips and reintroduced into the digester at a location above that at which it was withdrawn . for example , a first portion of the liquor is withdrawn through an annular separator 26 positioned adjacent the outside wall of the digester , circulated through an upper heater or heat exchanger 28 located outside the digester and reintroduced into the digester . at a location below the upper separator 26 , additional liquor is withdrawn through a second separator 30 , passed through a lower heater or heat exchanger 32 , and reintroduced into the digester . the liquor that passes through both the upper heater 28 and the lower heater 32 is reintroduced into a pair of concentric pipes 34 and 36 . the outer pipe 34 terminates above the upper separator 26 while the inner pipe 36 terminates above the lower separator 30 and below the lower terminus of the outer pipe 34 . th heated liquor from the upper heater 28 is introduced into the annular channel between the outer and inner pipes while the liquor from the lower heater 32 is introduced into the inner pipe 36 . additional liquor is withdrawn in an extraction zone 38 and recirculated as a quench flow via pipe 40 into the central portion of the digester at an elevation below the lower separator 30 but above the extraction zone 38 . an additional separator 42 is positioned adjacent the bottom of the digester . additional liquor is withdrawn through the lower separator 42 , passes through a wash heater or heat exchanger 44 and is reintroduced via pipe 46 as a wash flow above the lower separator 42 . for purposes of simplicity both pipes 40 and 46 are shown as entering the digester slightly above the extraction zone 38 and the separator 42 . in an actual kamyr digester , these pipes are constructed and mounted concentrically within inner pipe 36 and extend downwardly through the digester with their outlets respectively above the extraction zone 38 and separator 42 . in accordance with the present invention , a thermocouple 50 , or other suitable temperature sensing means , is positioned within the digester adjacent the outer surface of the outer pipe 34 through which the liquor from the upper heater is reintroduced into the digester . preferably , the thermocouple 50 is placed in close proximity to but not in intimate thermal contact with the outer surface of the outer pipe 34 . this location allows the thermocouple to sense the liquor temperature immediately adjacent the outside of the outer pipe 34 , but not to sense the temperature of the pipe itself . the thermocouple is vertically positioned adjacent the outer pipe 34 at a location below the level of the liquor ( since the digester is full of liquor ) and also preferably above the desired operating height for the chip level 24 . the thermocouple 50 is connected by a signal transmitting lead 52 of conventional construction to a temperature indicator 54 . the temperature indicator 54 can be any of a variety of conventional devices available commercially for visually displaying a temperature or otherwise indicating a value corresponding to the sensed temperature . the indicator is placed at the digester operator &# 39 ; s station and can be used to visually indicate the temperature sensed at the location of the thermocouple 50 . referring to fig2 a characteristic temperature profile 56 is produced by plotting the temperature sensed by the thermocouple 50 against the chip height . the temperature profile 56 is somewhat parabolically shaped , with the location of the thermocouple indicated by the dotted vertical line 58 . the portion of the profile 56 to the left of the thermocouple location line 58 is the corresponding temperature profile for a chip level as it varies below the thermocouple location . the portion of the profile 56 to the right of the thermocouple location line 58 illustrates the corresponding temperature profile for a chip level as it varies above the thermocouple location . when the chip level is below the thermocouple location and rises , that is , advances toward the thermocouple location , the indicated temperature will drop , reaching its lowest point when the chip level is even with the thermocouple location . as the chip level rises above the thermocouple location , the sensed temperature rises rapidly . vice versa , when the chip level is above the thermocouple and is lowered to below the thermocouple location , the temperature rapidly drops , again reaching its lowest point when the chip level is slightly below the thermocouple location . as the chip level drops away from the thermocouple , the sensed temperature again begins to rise as indicated . thus by observing temperature changes over time , that is , by taking a temperature reading , adjusting the chip level in the digester and by then taking another temperature reading , the operator of the digester can ascertain not only whether the chip level is above or below the location of the thermocouple but can also determine the chip height , that is , a measurement of the location of chip level in the column relative to the thermocouple location . it is preferable for the operator to maintain the chip level in the region adjacent the left hand side of the thermocouple location line 58 on the graph of fig2 . that is , it is preferable to maintain a chip height slightly below the location of the thermocouple in the digester since a more accurate determination of chip height can be derived from the sensed temperature because of the relatively gradual change of temperature with a change in chip height on the left hand side of the thermocouple location line 58 on the graph . once the chip level rises above the location of the thermocouple , the sensed temperature rises very rapidly and then levels off at a temperature near that of the outer pipe 34 independently of a variation in the chip level . the optimum vertical location in the digester for the thermocouple 50 will vary in practice depending upon the construction of the particular digester with which the invention is to be associated . normally , however , the thermocouple will be located below the bottom of the feed screw 18 . the characteristic temperature profile illustrated in fig2 is made possible by the unique heat balance existing in the upper portion of the pulping digester . briefly summarizing the heat balance , heat is being transferred to the liquor in the region adjacent the location of the thermocouple 50 from the heated liquor passing through the concentric pipes 34 and 36 . in addition , heat is being lost outwardly from the liquor adjacent the thermocouple through the outer wall 12 of the digester . additional heat is being transferred into the liquor adjacent the thermocouple from the chip - liquor mixture below the chip level 24 . as the chip level rises toward the thermocouple from below , the chips serve as an insulating layer having an increasing vertical depth , reducing the heat transfer from the chip liquor mixture to the region adjacent the thermocouple 50 . thus as the chip level rises toward the thermocouple , more heat is lost than is added to the region adjacent the thermocouple , causing a lowering in the temperature of the liquor surrounding the thermocouple . as the chip level reaches and exceeds the location of the thermocouple , the chips in the mixture still tend to form an insulator but at this height prevent heat loss from the liquor and chips surrounding the thermocouple . thus there is a net heat addition to the liquor surrounding the thermocouple causing a concomitant temperature rise . although the sensed temperature is affected by variations in the input flow of liquor and chips to the digester , this variation is small under normal operating conditions and will not significantly affect the results of the present invention . the present invention takes advantage of this heat balance condition by monitoring the temperature of the liquor , displaying that temperature , and providing an operator with operating history of the digester via a graph similar to that in fig2 . thus by observing the changes in temperature and knowing whether or not there has been a net addition or subtraction of chips from the reactor , the operator can very closely determine the chip height in the digester and thus not have to rely on prior art level indicating devices , which only provided an indication of whether the chip level was above or below a given location in the digester . referring now to fig3 a second embodiment of the invention is depicted . in this embodiment , the digester structure is somewhat different . the concentric pipes are indicated by the flow lines 34 &# 39 ; and 36 &# 39 ;. the altered digester 10 &# 39 ; contains an annular shell 60 that extends downwardly from the frustoconically shaped baffle 17 &# 39 ; through a location normally below the average chip height in the digester . the shell 60 is concentrically mounted about the concentric pipes 34 &# 39 ; and 36 &# 39 ; and is spaced radially outwardly from these pipes and radially inwardly from the outer wall 12 &# 39 ; of the digester . the chips are fed from the top of the digester via the screw 18 &# 39 ; into the interior of the annular shell 60 . the chips travel by gravity downwardly through and exit from the bottom of the annular shell 60 , which is located normally below the upper separator 26 &# 39 ;. the chip level 24 &# 39 ; to be controlled is contained within the shell while the region surrounding the shell 60 , the annular cavity between the shell 60 and the outer wall 12 &# 39 ; of the digester , is relatively free of chips but contains liquor . the liquor level 22 &# 39 ; is maintained at approximately the same location as in the previous embodiment . in this embodiment the thermocouple 50 &# 39 ; is positioned adjacent the interior wall of the shell 60 and is spaced from the outer concentric pipe 34 &# 39 ;. preferably , the thermocouple is placed in close proximity to the inner surface of the wall of the shell 60 so that is senses the liquor temperature adjacent the shell . the vertical positioning of the thermocouple in this embodiment of the invention is determined by the same considerations as in the previous embodiment , that is , it is normally located above the desired operating chip level 24 &# 39 ; and below the feed screw 18 &# 39 ;. the same characteristic temperature profile dependent upon chip level as that shown in fig2 is derived in this embodiment although the placement of the thermocouple is slightly different . by reviewing the heat balance in this embodiment , it can be seen that there is a net heat input to the liquor from the heated liquor passing through the concentric pipes 34 &# 39 ; and 36 &# 39 ;. there is also a net heat input into the liquor inside the shell 60 from the liquor surrounding the shell 60 . in addition there is a heat input by conduction from the mixture of chips and liquor below the chip level . again , as the chip level rises toward the thermocouple location from the bottom , there is a net reduction in heat input to the liquor surrounding the thermocouple , resulting in a temperature drop . when the chip level reaches and exceeds the location of the thermocouple , there will be a net heat addition to the liquor surrounding the thermocouple location , thus causing a temperature rise quite similar to that illustrated in fig2 for the first embodiment . the remaining considerations and operating parameters for the second embodiment of the invention are the same as those for the first embodiment . thus the operator can determine the chip level within the shell 60 by observing the changes in temperature over time with known net addition to or withdrawal of chips from the digester . after reading the foregoing specification , one of ordinary skill will be able to effect various alterations , substitutions of equivalents and other changes without departing from the broad concept disclosed . for example , a plurality of thermocouples can be positioned in the digester at spaced vertical locations . each thermocouple and its associated indicator would provide a relative indication of chip height . by comparing indicated temperatures from each of the plurality of sensing locations , an operator would be able to determine the exact chip height relative to each of the sensing locations . it is therefore intended that the scope of protection granted by letters patent hereon be limited only by the definition contained in the appended claims and equivalents thereof .