US2732849A

US2732849A - Theoretical bulb only

Info

Publication number: US2732849A
Authority: US
Publication date: 1956-01-31
Anticipated expiration: 1973-01-31

Description

Jan. 31, 1956 A. J. ROSENBERGER ETAL TEMPERATURE COMPENSATED TRANSMITTER Filed April 13, 1953 THEORETICAL BULB ONLY CONSIDERING BULB EXPANSIO BULB AND ELEMENT NOT N) BULB AND ELEMENT CON- SIDERING BULB EXPANSION RESULTANT CURVE WITH COM PENSATI NG BULB ATTORNEYS.

TEMPERATURE COMPENSATED TRANSMITTER Albert J. Rosenberger and Frederic R. Alex, Chicago, TEL, assignors to Republic Flow Meters Company, Chicago, llll., a corporation of Illinois Application April 13, 1953, erial No. 343,534 7 Claims. or. 137--85) This invention relates to temperature compensated transmitters and more particularly to instruments for producing a regulated force for indicating orcontrolling in response to pressure developed in a temperature responsive bulb.

Transmitters have been employed for many indicating or control operations which will produce either a regulated fluid pressure or electrical force in response to changes in a physical condition. When such transmitters are used with temperature responsive bulbs they have a drooping rather than a straight line characteristic due partially to the fact that the element connected to the bulb is outside of the heated space in which the bulb is mounted and partially to expansion of the bulb itself in response to temperature. Also because the element is outside of the heated space it is subject to changes in ambient temperature to produceanother error in the final output.

Attempts have been made to correct these difliculties by using a dummy element acting in opposition to the main element. Such systems have not been entirely successful because they are not affected equally by changes in ambient temperature and because the dummy element does not compensate for the drooping characteristic of the primary system. Linkages which attempt to compensate for the drooping characteristic are effective only over a narrow range.

It is one of the objects of the present invention to provide a temperature compensated transmitter in which the normal drooping characteristic is substantially fully compensated.

Another object is to provide a transmitter employing two bulb and element systems in which the secondary bulb is mounted inside of the primary bulb. This arrangement introduces a time delay into the response of the secondary bulb which results in minimizing the effect of time delay in response of the main bulb.

A further object is to provide a temperature compensated transmitter employing two bulb and element systems in which the two bulbs are formed of materials having difierent coefiicients of expansion.

The above and other objects and features of the invention will be more readily apparent from the following description when read in connection with the accompanying drawing, in which:

Figure 1 is a sectional partially diagrammatic view of a temperature compensated transmitter embodying the invention, and

Figure 2 is a graph indicating the operating characteristics of various parts of the transmitter.

The transmitter may be constructed as shown in Figure 1 with a supporting plate or frame carrying a bearing post 11 on which a balance beam 12 is pivoted intermediate its ends. The beam 12 is urged in one direction by means controlled by movement of the beam to rebalance it. As shown, the beam carries a valve member 13 which moves toward and away from a nozzle 14- fixed on the frame 10 and supplied with fluid such as air nited States Patent 0 under pressure through a restricted orifice 15. Thus a regulated pressure is produced back of the nozzle which may be transmitted to an indicator 16 and to any desired control mechanism through pipe 17. The regulated pressure also acts on a diaphragm 18 which engages the beam through a push rod 19 to urge the beam in a counter-clockwise direction about its pivot.

The beam is urged in a clockwise direction by a main pressure sensitive element shown as a flexible bellows 21 mounted on the frame 10 and engaging the beam to the left of its pivot. The element 21 is connected in closed fluid communication with a gas filled bulb 22 by a capillary tube 23. The bulb 22 may be placed in the space whose temperature is to be measured so that expansion of the gas therein in response to temperature increases will increase the pressure acting on the element 21 and will produce a corresponding increase in the regulated pressure. The initial filling pressure of the bulb and element system may be compensated by a calibrating weight 24 slidable on a lever 25 which is connected to the beam through a rigid strut 26.

Functioning of a bulb and element system as so far described is illustrated in the upper curves on Figure 2 wherein the abscissa represent absolute bulb temperature in degrees F. and the ordinates represent gas pressure. Theoretical bulb pressure, disregarding the remainder of the system, follows the straight line 27 whose slope depends on the filling pressure of the bulb. When the whole system is considered, however, the curve has a drooping characteristic as shown by the curve 28 due to the fact that the element and the connecting tubing are outside of the heated space and remain at the existing ambient temperature. It will be noted that the curves coincide at the existing ambient temperature, assumed to be 535 degrees absolute, and that the curve 28 will shift up or down with changes in ambient temperature.

Another error is introduced by expansion of the bulb which results in an increase in the volume of the system and causes a further droop in the curve. The actual overall curve when all factors are taken into account is shown by the dotted line 29 which is obviously not very satisfactory, particularly when it is remembered that this curve wiil shift in response to changes in ambient temperature.

To compensate the system and produce a more nearly linear output, a second bellows element 31 is mounted on the frame 10 and engages the beam to no right of its pivot. The element 31 is preferably identical to the element 21 and acts on the beam through the same lever arm although it could be of a difiierent size with its lever arm adjusted accordingly. The second element 31 is connected in closed fluid communication with a bulb 32 smaller than the bulb 22 by a capillary tube 33.

The two elements 21 and 31 will be affected equally by changes inatmospheric pressure and act oppositely on the beam so that the eilect of atmospheric pressure changes will be fully compensated.

The net force exerted on the beam will be the difference between the forces exerted by the main and second ele- ;ments so that if the curves of the two bulb and element systems could be made identical a straight line output would result. This cannot be fully accomplished, however, without making the two bulb and element systems identical in which case they would cancel each other and there would be zero output.

To elfect compensation for ambient temperature changes the bulbs must be of different sizes and filled with gas at different pressures. Filling at different pressures is necessary to produce differences in the pressures acting on the elements in response to bulb temperature changes so that the instrument can operate. if the bulbs were of the same size the pressure changes in the two systems would be difierent due to ambient temperature changes andp'top'ercompensation would not be produced. Therefore-,- the-compensatingbulb-is made-smaller thanthe main bulb by an amount such that the relative sizes and filling pressures of the two bulbs will be in the same ratio. Thus .a change in ambient temperature 16111 611 05661116 516516 7 willbe increased andmaybe made very similar '15 that ofthe main systemi Therefore; the diflfe'rences' between the pressures developed in'the-two systems maybe made to approach astraight line very- 0105611 Th'e actual'net cur-Ve isavery' fiat-S which follows th'etheQ'retic'aI straight line closely enoughfor' most'commercial applications;

Assuming a'volurn'e ratio of elem'ent 21*to'bulb 22'of 5652116 5 pressure spread for the-system of 120 pounds from degrees to 800 degrees Feand an-element to bulb ratio of with a pressure'sp'read of 40' pounds for the element and bulb system 31*, 32 the' following'values are 800 F. is less than two pounds in 126 pounds or about 1.6%. A further advantage not indicated by thesefigures is obtained by placing the second bulb inside of the main bulb which creates a greater time lag in the response of the second bulb than in the response of the first bulb. This means that during temperature changes the response of the second bulb will lag well behind that of the main bulb to compensate for the time delay inresponse of the main bulb. Thus a control device operated by'the-instrument will be actuated more quickly to bring the temperature back to the desired value. v 7

According to another feature of the present invention, more accurate compensation can be obtained by forming the compensating'bulb 32 of a material having a higher coefficient of thermal expansion than the bulb 22. This not only produces a greater drooping characteristic for the system 31, 32 but decreases the droop of the system 21, 22 by increasing the space within the bulb 22occup'ied by the bulb- 32 as the-temperature'increases. in a system with the same relative sizes as that described above .in which the elements are formed of a. material whose coefficient of thermal expansion is .000018, the main bulb 22 is formed of 49 nickel steel whose coefficient of thermal expansion isQOOOOlS and the inner bulb 32 is formed of 18-8 stainless steel whose coefficient of thermal expansion is .00003 the following values are obtained:

Ambient temperature 535 F. absolute 0 F F. 200F. 400 F. 600 F. 500 F.

Main Bulb 22 75. 55671 57. 33669 106. 65652 13676216 165. 90250 19413645 Inner-Bulb 32.. 29. 57612 33. 90952 40. 55505 50.55919 59.56022 65.23555 Base 1.516...." 46.62657 53. 52657 66.02657 56.02657 106. 02657 126. 02657 22-52-".-. 45. 95059 53.52687 66.09577 56.17297 10610225 125. 59760 Deviation; 04625 07190 14610 07541 12927 Ambient temperature 585 F; absolute 0 F. 75 F. 200 F. 400 F; 600 F. 500 F obtained when carbon-steel:

the bulbs are both formedof ordinary Ambient temperature 535 F. absolute With this system it will 'be seen' that the maximum deviationover the 800 degree range is about .16 pound 0 F. 75 F. 200 F. 400 F. 600 F. 500 F.

Main Bulb 22 75.61703 57. 33669 106. 51556 136. 29503 164. 99242 192. 67263 Inner Bulb 32.. 29. 55541 33. 50952 40. 59555 50. 72015 60. 04957 68. 62757 Base Line 46.02657 53. 52 59 66. 02657- 56. 02657 10602657 126. 02657 22-32..- 46. 05562 53. 52659 65. 91725 55. 57455 104. 94255 124. 04476 Deviation 03175 0 10959 45202 1. 05402 1. 95211 Ambient temperature 585 F. absolute 0 F. 75 '5. 200 F. 400 F. 600 F. 500 F.

Main Bulb 22 75.52954 57. 62339 106.94268 136. 99751 166. 02779 104. 09306 Inner Bulb 32... 29. 75591 34.11376 41. 03762 51. 41550 61. 04450 69. 93753 Base Line 46.02657 53. 52659 66. 02657 56.02657 106. 62657 126. 02657 2232..-...... 46.04363 53. 50963 65. 90506 35. 55171 104. 95329 12415573 Deviation 01676 01724 12151 44516 1. 04355 1. 57114 A typical operating curve for this system is shown at 34 in Figure 2'with the deviations .from the'baseline exaggerated. 7

It will thus be seen that when both bulbs are brought in'126 pounds'or less than'onetenth of onepercent. It will be apparent thatmanyofthe advantages. other than the anticipating effect could be obtainedif the'bulb 32 were mounted outside of the bulb 22 and were made of to the same temperature the maximum deviation over 75 a material having a higher coeificient of thermal "expansion but the arrangement shown produces the best results and is preferred.

While one embodiment of the invention has been shown and described in detail, it will be understood that this is illustrative only and is not to be taken as a definition of the scope of the invention, reference being had for this purpose to the appended claims.

What is claimed is:

1. A temperature compensated transmitter comprising a movable balance member, means controlled by movement of the balance member to urge the balance member in one direction, a first pressure responsive element acting on the balance member to urge it in the opposite direction, a second pressure responsive element acting on the balance member to urge it in said one direction, a first temperature responsive bulb in closed fluid communication with the first element, said bulb and first element being filled with a gas which will expand in response to temperature increases and being subject to the tempera ture to be measured, and a second temperature responsive bulb of smaller size than the first bulb mounted within the first'bulb and in fluid communication with the second element, the second bulb and element being filled with a gas which will expand in response to temperature increases.

2. A temperature compensated transmitter comprising a movable balance member, means controlled by movement of the balance member to urge the balance member in one direction, a first pressure responsive element acting on the balance member to urge it in the opposite direction, a second pressure responsive element acting on the balance member to urge it in said one direction, a first temperature responsive bulb in closed fluid communication with the first element, said bulb and first element being filled with a gas which will expand in response to temperature increases and being subject to the temperature to be measured, and a second temperature responsive bulb of smaller size than the first bulb mounted within the first bulb and in fluid communication with the second element, the second bulb and element being filled with I a gas which will expand in response to temperature increases, said second bulb being formed of a material having a higher coefficient of thermal expansion than the material of which the first bulb is formed.

3. A temperature compensated transmitter comprising a pivoted balance beam, means controlled by movement of the beam to produce a regulated force, means responsive to the regulated force to urge the beam in one direction, first and second pressure responsive bellows units acting on the beam to urge it in opposite direction a first gas filled bulb in closed fluid communication with the first bellows unit, and a second gas filled bulb of smaller size than the first bulb mounted within the first bulb and in closed fluid communication with the second bellows unit.

4. A temperature compensated transmitter comprising a pivoted balance beam, means controlled by movement of the beam to produce a regulated force, means responsive to the regulated force to urge the beam in one direction, first and second pressure responsive bellows units acting on the beam to urge it in opposite directions, a first gas filled bulb in closed fluid communication with the first bellows unit, and a second gas filled bulb of smaller size than the first bulb mounted within the first bulb and in closed fluid communication with the second bellows unit, the second bulb being formed of a material having a higher coetficient of thermal expansion than the first bulb.

5. A temperature compensated transmitter comprising a pivoted balance beam, means controlled by movement of the beam to produce a regulated force, means responsive to the regulated force to urge the beam in one direction, first and second pressure responsive elements acting on the beam to urge it in opposite directions, and first and second gas filled bulbs in closed fluid communication with the pressure responsive elements respectively, said bulbs being subjected to the temperature to be measured and being formed of materials having different coeflicients of thermal expansion. 7

6. In a temperature compensated measuring system, a pair of pressure responsive elements connected to oppose each other, a pair of gas filled bulbs in closed communication with the elements respectively, one of the bulbs being smaller than the other and being mounted inside of the other, and said one bulb being formed of a material having a higher coefficient of thermal expansion than the other.

7. In a temperature compensated measuring system, a pair of pressure responsive elements connected to oppose each other, a pair of gas filled bulbs in closed communication with the elements respectively, one of the bulbs being smaller than the other and being filled with fluid at a lower pressure than the other, said one of the bulbs being formed of a material having a higher thermal coefficient of expansion than the other.

References Cited in the file of this patent UNITED STATES PATENTS 1,289,435 Fulton Dec. 31, 1918 1,749,010 Weber Mar. 4, 1930 1,920,827 Wunsch Aug. 1, 1935 2,379,124 Wasson June 26, 1945 2,477,835 Smith Aug. 2, 1949 2,588,621 Eckman Mar. 11, 1952