Title:
PROCESS CONTROLLER
United States Patent 3770608


Abstract:
A process control apparatus detachably packaged in two integral sections. The lower section contains a compartment for a reference solution and another compartment for a process solution; the two compartments being in thermal equilibrium and having an electrical bridge therebetween. A pair of specific ion sensors extend downwardly from the upper section and reach into the two solution compartments when the controller is assembled. In operation a process solution flows through the controller and the two specific ion sensors generate a potential difference signal for operation of a solenoid valve which controls addition of a concentration correcting fluid to the process stream. There is disclosed an application of such a controller to a three stage washing system for papermaking pulp.



Inventors:
Kelch, David J. (Dayton, OH)
Kraus, Walter R. (Centerville, OH)
Clark, Jack D. (Cedarville, OH)
Application Number:
05/235116
Publication Date:
11/06/1973
Filing Date:
03/16/1972
Assignee:
MEAD CORP,US
Primary Class:
Other Classes:
137/93, 204/409, 204/416
International Classes:
G01N27/416; (IPC1-7): G01N27/46
Field of Search:
204/1T,195R,195G,195M
View Patent Images:
US Patent References:
3696019N/A1972-10-03Arrington et al.
3563874N/A1971-02-16Ross et al.
3539455MEMBRANE POLAROGRAPHIC ELECTRODE SYSTEM AND METHOD WITH ELECTROCHEMICAL COMPENSATION1970-11-10Clark
3479255ELECTROCHEMICAL TRANSDUCER1969-11-18Arthur
3434953ELECTROCHEMICAL ELECTRODE ASSEMBLY1969-03-25Porter et al.
3306837Glass electrode apparatus and process for making same1967-02-28Riseman et al.
2288180Apparatus for measuring ph1942-06-30Brengman et al.
2183531ph meter1939-12-19Allison



Primary Examiner:
Tung T.
Claims:
What is claimed is

1. A process controller for regulation of the concentration of specific ions in a stream of process fluid comprising:

2. a container closed against the atmosphere,

3. means for conducting said stream through said container,

4. a vessel within said container for holding a sample of said process fluid,

5. means for continuous delivery of a small portion of said stream to said vessel,

6. means for maintaining the process fluid within said vessel at a predetermined level and continuously returning excess process fluid to said stream,

7. a compartment within said vessel for holding a reference solution in thermal equilibrium with the surrounding process fluid within said vessel,

8. means for providing an electrical bridge between said process fluid and said reference solution,

9. a pair of identical probes each provided with a solid state membrane sensitive to said specific ions; one probe extending into said vessel for contact with said process fluid sample and the other probe extending into said vessel for contact with said reference solution, and

10. means connected to said probes for generation of a control signal related to the difference between the ion concentrations sensed by said membranes.

11. A process controller comprising an upper section and a lower section releasably joined with a pressure seal therebetween, said upper section comprising:

12. an enclosed housing,

13. an electronics package mounted within said housing and operative to generate a chemical solution correction signal in response to a pair of reference and measured input potentials, and

14. a pair of substantially identical specific ion sensors detachably mounted exteriorly below said housing and in electrical connection with said electronics package;

15. a first compartment for holding a reference solution,

16. a second compartment for holding a sample of a process solution requiring concentration correction in variable amount as determined by said correction signal,

17. means for providing an electrical bridge between ssid first and second compartments,

18. input and output connections for providing a continuous flow of said process solution to said second compartment, and

19. means for dividing the process solution flowing into said input connection whereby a first portion thereof flows directly to said output connection and a second portion thereof flows through said second compartment;

20. A process controller according to claim 2 wherein upper section further comprises a ground probe mounted exteriorly below said housing, said ground probe reaching upwardly for grounding connection with said electronics package and extending downwardly into said second compartment for grounding connection with said process solution.

21. A process controller according to claim 3 wherein said specific ion sensors are of identical construction including at their lower ends a pair of solid state membranes sensitive to the presence of said specific ion in reference and process solutions contained within said first and second compartments.

22. A process controller according to claim 4 wherein said process controller is provided with means for upwardly admitting said process solution to said second compartment at a point directly below the specific ion sensor reaching into said compartment.

23. A process controller according to claim 4 wherein said specific ion sensors are mounted to said housing by means of threaded connections and electrically connected to said electronics package by means of pin jacks.

24. A process controller according to claim 2 wherein said first compartment is a cylindrical tube.

25. A process controller according to claim 7 wherein said means for providing an electrical bridge comprises a tapered circular wall in the base of said lower section and a rough ground taper at the base of said cylindrical tube.

26. A process controller according to claim 8 wherein said cylindrical tube is provided with a plurality of apertures in the region of said rough ground taper for reduction of electrical resistance across said electrical bridge.

27. A process controller according to claim 7 wherein said upper section is provided with a circular groove for reception of the upper end of said cylindrical tube and creation thereby of a condensate collection chamber.

28. Apparatus according to claim 2 wherein said means for dividing the process solution is such that the said first portion of the process solution is a major portion thereof and said lower section of the process controller further comprising means for re-combining said second portion of the process solution with said first portion prior to exit thereof through the output connection.

29. A process controller according to claim 11 further comprising means for measuring the rate of flow of said second portion of process solution and means for regulating said rate of flow.

30. A process controller for regulation of the concentration of specific ions in a process solution comprising:

31. a vessel for holding a sample of said process solution,

32. input and output connections for providing a continuous flow of said process solution through said vessel,

33. a compartment within said vessel for holding a reference solution in thermal equilibrium with said process solution,

34. means for providing an electrical bridge between siad process solution and said reference solution,

35. a first probe extending into said compartment and operative to generate a first electrical signal related to the concentration of said specific ions in said reference solution,

36. a second probe substantially identical to said first probe extending into said vessel and operative to generate a second electrical signal related to the concentration of said specific ions in said process solution,

37. means connected to said first and second probes for generating a third electrical signal related to the difference between said first and second electrical signals,

38. means for generating a concentration correction signal when said third electrical signal exceeds a first predetermined magnitude, and

39. means for interrupting generation of said concentration correction signal when said third electrical signal falls below a second predetermined magnitude.

40. Process control apparatus comprising:

41. a vessel for holding a sample of a process solution,

42. input and output connections for providing a continuous flow of said process solution through said vessel,

43. a compartment for holding a reference solution in electrical communication with the process solution in said vessel,

44. grounding means for establishing the potential of said process solution as a ground potential,

45. a measuring probe comprising an ion sensing membrane and an output connection, said membrane being positioned for contact with process solution within said vessel and said probe being operative to generate between said output connection and said membrane an electrical potential related to the concentration of a predetermined specific ion within said process solution,

46. a reference probe substantially identical to said measuring probe comprising an ion sensing membrane and an output connection, said membrane being positioned for contact with reference solution within said compartment and said probe being operative to generate between said output connection and said membrane an electrical potential related to the concentration of a predetermined specific ion within said reference solution,

47. first amplifying means for amplifying the potential between said grounding means and the output connection of said measuring probe,

48. second amplifying means for amplifying the potential between said grounding means and the output connection of said reference probe,

49. third amplifying means for amplifying the difference between the outputs of said first and second amplifying means, and

50. means responsive to said third amplifying means for generating a process control signal for said process solution.

Description:
CROSS REFERENCE TO RELATED APPLICATION

This application relates to U. S. Pat. Application Ser. No. 235,055, now abandoned, filed on even date herewith.

BACKGROUND OF THE INVENTION

This invention relates to the field of automatic process control. More particularly it concerns the continuous replenishment of chemical solutions employed in a wide variety of processes. Typical processes wherein the invention may find application are wood pulp bleaching, vegetable washing and sterilizing, photographic developing, electroplating, and coating. In each of these cases there is employed a working solution which may become depleted of a particular chemical over a period of time. If the process is to proceed without interruption, then it becomes necessary to make continuous measurements of solution concentration and replenish the solution with the depleted chemical in response thereto.

Other applications of the invention are in processes wherein there may not be a working solution as such, but wherein there is nevertheless a solution requiring continuous chemical monitoring and control. Water chlorination and sewage treatment are examples of applications of this type. Some typical prior art devices for accomplishing continuous automatic replenishment control are disclosed in Russell U. S. Pat. No. 3,195,551, Ladd U. S. Pat. No. 3,273,580, Cardeiro U. S. Pat. No. 3,440,525 and Schumacher U. S. Pat. No. 3,529,529. Other earlier devices related to the apparatus of this invention are disclosed in Data Corporation report DTR-70-2.

Prior art devices of the above mentioned type are impractical for many applications because of limitations in accuracy, bulky and awkward physical configuration, limited dynamic stability margin, or excessive maintenance problems. Moreover, there has not been a compact, high accuracy controller capable of quick conversion from one process control application to another.

SUMMARY OF THE INVENTION

This invention overcomes the inherent limitations of prior art process controllers by providing a compact easily assembled device having a pair of specific ion sensors extending into two chambers; one of which contains a reference solution and the other a process solution. Provision is made for continuous flow of the process stream through the controller, and for continuous generation of a concentration correction signal based upon the voltage output difference between the two specific ion sensors. The correction signal preferably operates a solenoid valve which admits concentration correcting fluid into the process stream at a point upstream from the controller. In preferred embodiment the controller is made in two detachable sections and is provided with means for adjusting the response time of the controller to match the response time of the process being controlled.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation in crossection of a preferred embodiment of a process controller made in accordance with this invention.

FIG. 2 is a left frontal view of an assembled process controller.

FIG. 3 is a right rear view of an assembled process controller.

FIG. 4 is a view of an assembled upper section for a process controller.

FIG. 5 is a partially cut away plan view of an assembled lower section for a process controller.

FIG. 6 is a crossection taken along line 6--6 of FIG. 5.

FIG. 7 is a plan view of a partially cut away bottom plate for the process controller of FIGS. 1 through 6.

FIG. 8 is a schematic representation of the fluid flow system for the process controller of FIGS. 1 through 6.

FIG. 9 is a schematic drawing of a three stage pulp washing system employing a process controller in accordance with this invention.

FIG. 10 is a schematic diagram of the process controller electronics.

FIG. 11 is a schematic representation of a process controller employing this invention in alternate embodiment.

FIG. 12 shows another alternate embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred embodiment of the invention is illustrated schematically in FIG. 1 wherein is shown a process controller 1 comprising a lower section 2 and an upper section 3. A process solution 9 flows through the base of the lower section 2 and a portion thereof is admitted upwardly into a measuring chamber via an admission channel 11. Process solution 9 rises within the measuring chamber until the level thereof reaches an opening at the top of stand pipe 8. From there it runs downwardly to join the main stream. Exit channel 12 is provided for this purpose.

A reference tube 10 fits in a well at the base of lower section 2 and extends upwardly to upper section 3. Reference tube 10 is tapered by a rough grind around the lower end thereof for mating reception by tapered well wall 13. This provides a capillary junction for a purpose as hereinafter explained. The upper end of reference tube 10 fits within groove 14 of upper section 3. A reference solution 7 is contained within reference tube 10. Reference solution 7 is in electrical contact with process solution 9 by capillary contact along the interface of tapered wall 13 with the tapered lower end of reference tube 10. A reference probe 4 extends downwardly from upper section 3 into electrical contact with reference solution 7. Also extending downwardly from upper section 3 are a signal probe 5 and a ground rod 6. Signal probe 5 and ground rod 6 are both in electrical contact with process solution 9 in the chamber surrounding reference tube 10, Electrical resistance at the capillary junction may be reduced by providing apertures 77 (FIG. 6) at the lower end of tube 10.

Reference solution 7 is preferably a sample of process solution obtained under ideal concentration conditions. Signal probe 5 and reference probe 4 contain specific ion sensors of identical construction. Each probe generates a voltage related to the concentration of a particular ion present in the respective surrounding solution. The voltage outputs from probes 4 and 5 are delivered to a difference amplifier packaged within upper section 3. The output from this difference amplifier controls the opening and closing of a solenoid valve which regulates the flow of a replenishment solution into the process stream.

The ion sensitive element within probes 4 and 5 is a solid state membrane of composition depending upon tye type of ion to be sensed. For instance, if it is desired to sense the presence of chloride ions, the membrane may be silver sulfide and silver chloride composition. The construction and operation of such probes is well known as shown for instance in Ross et al U. S. Pat. No. 3,563,874. The above mentioned Ser. No. 235,055 provides another example of a probe well suited for such use. It is to be noted that while specific ion sensing probes are commonly used to make electrochemical measurements, they are ordinarily used in combination with an accurately calibrated reference electrode such as a calomel probe. This creates a requirement for corrections to eliminate a number of measuring errors well known in the electrochemical art. In contrast thereto, this invention relies upon a null point potentiometry principle; no absolute concentration measurement being made. By using a symmetrical measuring configuration most significant sources of error are removed. It has been found, however, that certain other unexpected errors may arise in devices of this type.

One of these errors is due to evaporation from the reference solution. Even in a tightly sealed controller, water may evaporate from the reference solution over a long period of time, and condense on the under surface of upper section 3. Thereafter it may drip down into the process solution surrounding reference tube 10 and leave the reference solution with an increased ion concentration. This source of error is removed by upward extension of reference tube 10 into channel 14 on the lower surface of upper section 3. This restricts the above mentioned evaporation and condensation process to the confines of the reference tube. Water which evaporates from the reference solution 7 tends to collect on that portion of the under surface of upper section 3 which is directly above reference tube 10. Consequently the vapor which thereafter condenses drips back inside reference tube 10. It is to be noted, however, that reference tube 10 does not fit tightly against upper section 3. There is sufficient clearance around groove 14 to permit equalization of air pressure on both sides of the reference tube.

The other unexpected error is due to air bubbles which tend to collect on the lower surface of signal probe 5. These air bubbles interfere with the migration of ions across the solid state membrane thereby producing an erroneous output voltage. It has been found that such bubble collection may be avoided by placing admission channel 11 directly below signal probe 5.

Apparatus for practicing this invention may be conveniently packaged as shown generally in FIG. 2. Again, the numeral 1 denotes the process controller, numeral 2 the lower section thereof, and numeral 3 the upper section thereof. Upper section 3 is releasably joined to lower section 2 by means of a pair of clips 15, one on each side. A rubber ring 33 (see FIGS. 4 and 6) creates a pressure seal between the upper and lower sections. Process solution enters at the rear of the controller (see FIG. 3) via fluid connector 22 and leaves via fluid connector 23.

FIG. 4 shows the general exterior arrangement of upper section 3. It will be noted that the section fits quickly and easily into lower section 2 and clamps securely in place. Probes 4 and 5 are threaded at their upper ends for easy attachment to cap 42. Probes 4 and 5 each also have a pin connector at their upper ends for detachable connection to circuit boards mounted within upper section 3.

It will be appreciated that the solid state membrane 39 installed at the end of each probe is adapted for sensing of only one specific ion. If it is desired to employ the process controller for replenishment of a variety of process solutions, then it is necessary only to provide a set of probes for each type of solution. Upon completion of a process control operation, clips 15 are extended and upper section 3 is pulled away from lower section 2. Probes 4 and 5 are unscrewed from their seats and a new set of probes screwed into place. Then the lower section is drained, and a sample of the new solution at the appropriate concentration is poured into reference tube 10. Following this, the upper section 3 is again lowered into lower section 2 and clamped in place with rubber ring 33 providing a pressure seal as mentioned above. The lower extension of cap 42 fits snugly inside top plate 43 as shown in FIG. 6.

FIG. 5 is a partially cut away plan view of a fully assembled lower section. The locations for reception of probes 4 and 5 and ground rod 6 are shown in phantom lines thereon.

FIG. 6 shows a crossection of a fully assembled process controller taken along line 6--6 of FIG. 5. All major structures are made of polyvinyl chloride or other hard plastic material and are bonded together with resin cement. Two boards of electrical components are mounted inside upper section 3 as shown in FIG. 6. These are an amplifier board 25 and a power supply board 26. Receptacles 27 are provided on amplifier board 25 for reception of the ground rod and the reference and signal probe pins.

FIG. 8 provides a schematic representation of fluid flow routing from connector 22 to connector 23. The major portion of the fluid is admitted by a valve 24 into a bypass channel 37 and flows directly out of the controller. A relatively small portion of the process solution is admitted into a sampling channel 34 and flows forwardly to flow meter 16.

Flow meter 16 is a flow device of the common cone and ball type. Accordingly ball 17 sits at the lower end of meter 16 and is urged upwardly in accordance with the rate of fluid flow. Needle valve 21 provides fine adjustment of the upward fluid flow rate through flow meter 16, and post 18 (FIG. 2) protects the meter against breakage. The purpose of flow rate adjustment is to match the response time of the process controller to that of the process being controlled. Such response time matching is necessary for achieving dynamic stability in some applications.

After the process fluid sample reaches the top of flow meter 16 it is directed laterally by channel 35 to connect with tube 19. Tube 19 carries the process solution sample downwardly to channel 36 which then leads rearwardly to aperture 38. The fluid flows upwardly through aperture 38 filling cylindrical section 20 and overflowing downwardly through stand pipe 8. Stand pipe 8 empties into bypass channel 37.

FIGS. 5 and 7 show the details of the above described fluid passages. As illustrated in phantom lines on FIG. 7, the flow dividing action of valve 24 is accomplished by adjusting knob 78 to position ball 79 relative to valve seat 32. Channels 34, 36, and 37 are merely drilled passages in bottom plate 44. Aperture 28 is provided to enable fluid passage from channel 34 to flow meter 16. Aperture 29 enables fluid passage from tube 19 to channel 36, and aperture 30 enables fluid passage from stand pipe 8 to channel 37. A groove 31 is cut into the upper surface of bottom plate 44 to receive cylindrical section 20.

FIG. 10 presents an electrical schematic for the process controller. As shown therein, a ±15 volt DC power supply 46, a solid state relay 45, and associated resistors, filter capacitors, and indicator lamps are mounted on circuit board 26. Circuit board 25 contains operational amplifiers 53, 54, and 55 and driver amplifier 56. Other associated components are a power switch 52 mounted on the front face of top section 3, a null control 47 also mounted on the front face of top section 3, and a fuse 51 and connector 48 mounted at the rear of top section 3. In operation output voltages from probes 4 and 5 are delivered via their associated connectors 27 to amplifiers 53 and 54. Amplifier 55 operates as a differential amplifier, and produces an output signal which is the amplified difference between the output signals of amplifiers 53 and 54. Capacitors 68 and 69 function as a broad band filter to reduce noise which otherwise would be present at the output of amplifier 55. The output from amplifier 55 is fed to transistor 56 which operates as a driver amplifier for solid state relay 45. Relay 45 contains an SCR which is fired by current flow on line 66.

When relay 45 closes, current from power supply line 69 is routed to relay output line 68, and then through switch 52 and connector 48 to a solenoid valve such as valve 111 of FIG. 9. This initiates a flow of makeup or correcting solution, and also completes a circuit for activating a neon lamp 49, the ON indicator. The controller is designed to operate on an ON/OFF basis, and neon lamp 50 indicates the OFF condition. When relay 45 is closed, indicator lamp 50 is short circuited, but when relay 45 opens, a small current flows from power supply line 69 through lamp 50, and thence through the solenoid valve and back to power return line 71. This activates lamp 50. At the same time the operating voltage to lamp 49 is reduced to a level below the illumination threshold. Lamps 49 and 50 are mounted on circuit board 26, and may be observed through small windows in the front of upper section 3.

Feeding back from the collector of transistor 56 is a positive feedback signal through high resistance resistor 57. The purpose of this feedback is to provide a deadband for ON/OFF operation. A threshold for conduction of transistor 56 is initially set by adjustment of variable resistor 58 in null control 47. Once transistor 56 conducts, the feedback signal through resistor 57 is summed in with the signals provided by amplifiers 53 and 54 to difference amplifier 55. This increases the output of amplifier 55 and maintains transistor 56 above the operating threshold until the difference between the outputs of amplifiers 53 and 54 is reduced to a voltage less than some predetermined value. A resistance of about 20 megohms in resistor 57 produces a deadband of about 0.6 to 0.9 millivolts as seen at the output of probes 4 and 5. Capacitor 59 acts as a filter to slow down fast ON/OFF cycling rates. Resistors 60 and 61 and capacitors 62 and 63 provides a decoupling network between power supply 46 and the above mentioned amplifiers.

Circuit boards 25 and 26 are interconnected by 5 wires. These are the plus and minus 15 volt power lines and lines 64, 65 and 66. Line 64 is a ground line which is connected to ground rod 6 and also the ground terminal of power supply 46.

Switch 52 is a three position device with ON, OFF, and CONTROL positions. Actuation of switch 52 actuates two switches 52a and 52b tied together as shown in FIG. 10. For normal operation switch 52 is in the CONTROL position. Selection of the ON position results in the removal of current from the solenoid valve and the substitution of resistor 67 for the solenoid in the relay output circuit. Power supply 46 is a widely available common device and preferably should be able to deliver about 25 milliamperes of regulated DC current at plus and minus 15 volts as shown.

An example of an application of this invention to a papermaking process is illustrated in FIG. 9 wherein digested pulp is passed successively through three wash tanks 85, 86 and 87 for removal of black liquor residues. Thereafter the pulp will be bleached and then refined for papermaking. The residues which are washed from the pulp comprise a sodium sulfide component, other sodium compounds, organic acids and sugars. It is desirable from an economic viewpoint to recover a large portion of the sodium sulfide, and this is accomplished by taking the effluent from the first wash tank and passing it thourgh an evaporator. The sodium sulfide thus recovered may then be oxidized to produce free sulfer. It will be appreciated that the effluent from the second and third wash tanks is quite low in black liquor residues and direct recovery of sodium sulfide therefrom is not economically feasible. Therefore washing solution from the third and second stages is cascaded forward to stage 1 as shown.

In addition to the wash tanks the system of FIG. 9 incorporates holding tanks 88, 89 and 90, recirculation pumps 91, 92 and 93, controllers 94, 95 and 96, low level controls 101, 102 and 103, high level controls 104, 105 and 106, and valves 108, 109, 110 and 111. Controllers 94, 95 and 96 are constructed as previously described with their solid state membranes 39 made of a silver/silver sulfide composition as described in copending application Ser. No. 235,055. Valves 108 through 111 are all solenoid operated with valves 109, 108 and 111 respectively being actuated by controllers 94, 95 and 96. Each of the valves may also be actuated by the high and low level controls as illustrated.

The washing solution within tank 88 is maintained by controller 94 at a constant sodium sulfide concentration just below 10 grams per liter. This is accomplished as previuosly described by placing a sample of washing solution at the 10 gram concentration into reference tube 10. Whenever the concentration rises above 10 grams per liter, controller 94 actuates valve 109 admitting relatively more pure second stage washing solution into the first stage recirculation loop. Then as tank 88 becomes filled, high level control 104 actuates valve 110 sending first stage washing solution to the evaporator.

The operation of the second and third stages is similar to the operation of the first except for the control settings of controllers 95 and 96. Controller 95 maintains the second stage washing solution at a concentration of 3 grams of sodium sulfide per liter of solution and makes control corrections by opening valve 108 to admit third stage washing solution into the second stage recirculation system. Controller 96 actuates valve 111 to admit fresh water into the third stage wash as necessary for maintenance of a concentration of 1 gram sodium sulfide per liter of washing solution. High level controls 105 and 106 and low level controls 101, 102 and 103 performs a safety function only. Thus either of controls 102 and 106 can open valve 108 to admit third stage washing solution into the second stage recirculation system, but ordinarily opening commands will come only from controller 95. Accordingly fresh water enters to cascade through the system and exits with a constant sodium sulfide concentration of 10 grams per liter.

Alternative embodiments for the controller of this invention are disclosed in FIG. 11 and 12. These embodiments are similar to the preferred embodiment of FIG. 1 except for the capillary junction between the process solution and the reference solution. In FIG. 11 the junction is provided by a capillary passage 81 whereas in FIG. 12 the junction is provided by a capillary passage 82. In both cases the reference tube 80 is bonded in place. This contrasts with the tapered junction and removable tube 10 of FIG. 1.

In still another embodiment the capillary junction may be made so large as to permit a slow interchange of fluid between the reference chamber and the sample chamber. This alters the composition of the reference solution and makes it in effect a sample of process solution averaged over some past period of time as determined by the fluid interchange rate. Such an arrangement is desirable in cases wherein the reference solution has a tendency to deteriorate over a period of time, and it is not convenient to shut down and change the reference solution. Accordingly the controller continues to replenish the process stream at an average rate equal to the average rate during some period in the past. (Instantaneous replenishment follows an ON/OFF cycle in accordance with the conduction cycle of transistor 56) Thus there is compensation for reference solution degradation but with an inherent slight long period drift.

Obviously other junctions such as a porous plug, a salt bridge, or an electrically conductive reference tube wall may be employed. In any event it is important that the junction resistance be low relative to the resistance of probes 4 and 5 so as to maximize the sensitivity of the controller. For the preferred embodiment with the reference tube seated by a seating force ranging from 1 to 10 pounds, the junction resistance ranges from about 10 to 200 ohms. This compares with an individual probe resistance of about 7,500 ohms for a probe and made as described in application Ser. No. 235,055.