US2515785A

US2515785A - Electrical piezometer apparatus

Info

Publication number: US2515785A
Application number: US789683A
Authority: US
Inventors: Clarke C Minter
Original assignee: MINTER INSTR CORP
Current assignee: MINTER INSTR CORP; MINTER INSTRUMENT Corp
Priority date: 1947-12-04
Filing date: 1947-12-04
Publication date: 1950-07-18
Anticipated expiration: 1967-07-18

Description

5 July 18, 1950 c. c. MINTER 2,515,785

ELECTRICAL PIEYZOMETER APPARATUS Filed D80. 4, 1947 ,.4 @v9-jif Z4 -L I15 ze EPE' EE- a I Heftige/EE ATTORNEY Patented .uiy 18, 1950 ELECTRICAL PIEZOMETER APPARATUS Clarke C. Minter, Washington, D. C., assignor to Minter Instrument Corporation, New York, N. Y., a corporation f New York Application December 4, 1947, Serial N o. '789,683

4 Claims. (Cl. 201-48) The present invention relates to pressure measuring devices and describes novel apparatus for quantitatively converting mechanical displacements due to pressure changes into equivalent changes in the condition of an electrical circuit and constitutes an improvement over the electrical piezometer shown and claimed in my prior Patent No. 2,358,467, issued September 19, 1944.

The most common means for converting mechanical displacements into changes in an electrical circuit is that in which the mechanical displacement is made to actuate a movable contact sliding over a suitable resistance which is part of an electrical circuit. This type of `pickup is not very satisfactory owing to variable friction at the sliding contact.

In my above noted prior patent the use of Sliding contacts is eliminated and I utilize the variation in thermal conductivity of a confined body of vgas (helium at 1 mm. pressure) as the volume of a spring-loaded bellows which is varied by external pressure to change the resistance of a current-heated lament forming part of a Wheatstone bridge circuit. The present invention employs the same basic principle, namely, the effect of pressure on the rate at which a current-heated lament loses heat in a confined body of gas. |The improvement of the ,present invention consists in arranging the currentheated filaments in such a manner that when the external pressure decreases, for example, one of the nlaments increases in resistance While the other decreases. l

The present invention together with the objects thereof may be best understood as the specification proceeds when taken in conjunction with the accompanying drawing wherein:

Fig. l is a cross-sectional View showing one form which the presentinvention may take;

Fig. 2 is a cross-sectional View taken on the line III- II of Fig. 1;

Fig. 3 is a view similar to that of Fig. 1 but showing another modication which the present invention may take;

Fig. 4 is a sectional view taken `on the line IV-IV of Fig. 3, and

Fig. 5 lis a graph showing the variation in the thermal conductivity of helium with chan-gesV 'in pressure.

Referring now to the drawings .in detail, the device as shown in Fig. 1 comprises .a cylinder 5, hermetically sealed at one end by a threadedly engaging end plate 6 and having an inlet 1 at its opposite end, to which a conduit or the like from a source of variable pressurais adapted to threadedly engage a projecting bib 8. The endplate 6 has an expansible and contractible bellows 5 hermetically sealed to its inner surface leaving the opposite at end I-l of this bellows free to move. Also welded or otherwise afxed to the end fplate 6 is a U-shaped strap or bracket I2 which, at its inner end or base of the U-shape, has secured thereto a second expansible and contractible bellows I3 which is thus disposed within the outer bellows 9. It will also be noted that a further U-shaped strap or bracket I4 is welded or otherwise secured to the movable flat end lil of the outer bellows 9 and at its other end the strap Ill is secured to the far end I5 of the bellows I3 opposite to the end to which the bracket I2 is connected. Also hermetically sealed through the end plate 6 are a plurality of leading-in conductors I6, II and I8, with the conductors I1 and I8 being connected to and supporting a lament 20 Within the interior of the outer bellows 9 and the conductors I6 and I8 extending hermetically through the end of the inner bellows I3 and connected to a lilament 22 disposed interiorly of the inner bellows I3.

The entire unit as described, being carried by the end plate 6, is accordingly assembled and secured to the end plate prior to insertion in the cylinder 5. After assembly, the bellows 9 and I3, which are designed to have substantially equal free internal volume, so as to compensate for external temperature changes, are then filled with a suitable gas at a pressure of about 1 mm. of mercury and having high thermal conductivity, such as hydrogen or helium, and sealed ofi at the tips 23. I prefer to employ helium as the filling gas because it is less likely to diiuse through the walls of the bellows. Upon completion of the assembly of the various elements, the end plates 6 carrying such elements, is then screwed into the cylinder 5 in an air-tight manner, by utilization of a suitable gasket 24. Although the complete electrical circuit has not been vshown herein, the filaments 2li and 22 may be connected by the leading-in conductors I6, II and IB to the identical Wheatstone bridge arrangement in the same manner as the filaments in my above noted Patent No. 2,358,457, to quantitatively measure pressure variations as therein described.

In the modication of Figs. 3 and 4, the

lilaments

30 and 32 are mounted in cells or recesses 33 and 34 formed in the end plate 35. Also in this modification the end plate 35 is of slightly larger diameter and threadedly engages an annular collar 36 formed on the cylinder 31 with a ,3 sealing gasket 38 positioned therebetween to make the connection air-tight. A sealed closure 39 constitutes the outer wall of the cells 33 and 34 as well as the outer wall of the end plate 35. In lieu of the inner bellows do being supported by a strip or bracket as in Fig. 1, its end 42 is supported from the end plate 35 by a U-shaped hollow tubular member 43 which forms a passageway from the cell 34 to the interior of the inner bellows 49. Also the end plate 35 is provided with a passageway @9 which establishes communication between the cell 33 and the interior of the outer bellows 45. In all other respects the modcation of Figs. 3 and 4 is the same as that of Figs. 1 and 2, with the strap 46 tying the mov able end 41 of the outer bellows 45 to the end 48 of the inner bellows 49, as previously -described relative to Figs. l and 2. Also in this latter modification the end plate 35 is provided with four hermetically sealed leading-in conductors 49, 5l), 52 and 53, with the leading-in conductors 5o and 52 being connected together externally of the unit instead of internally, as is the common lead I8 of Fig. l.

Fig. 5 shows how the thermal conductivity of helium varies with pressure with the latter constituting the abscissaand absolute thermal conductivity constitutingthe ordinate. The line A shows the approximate thermal conductivity of air for comparison purposes while Vthe curve B shows the variation of helium. It will thus be noted that if the pressure of the gas is low (1 mm'. of mercury or less), the thermal conductivity thereof varies very rapidly with pressure (density).

Referring again to Figs. 1 and 2 when external gas pressure enters through the inlet 1, it acts on the flat end I l! of bellows 9. The response of the 4dat end I3 is controlled by the stiffness of the outer bellows 9 and the inner bellows I3 which are so chosen that when the external pressure exceeds 1 mm. l-Igabsolute, the flat end II) moves toward the end plate 5. Due' to the connection of the straps Il between the flat bellows end II'I and the end I5 of'bellows I3, the flat end I5 is also moved toward the end plate 5, thus expand-V ing the inner bellowsv I3 to the same degree' that the outer Abellows 9 is compressed. At the samev time the pressure (density) of the gas within the bellows 9 and I3 is compressed and expandedA respectively. The compression of the gas within the free space vof bellows 9 increase its thermal conductivity, as can be seen by reference toV Fig."

5, with the result that the current heated iilament 23, which may be connected to a Wheat'- stone bridge arrangement, will lose heat at a faster rate thus causing its temperature and electricalresistance to decrease in proportion to the increase in pressure 'exerted on the ilat end Il! of the bollows 9.

vThe simultaneous expansion of bellows I3', due to movement of its end plate I5 by theyU -shaped strap I4 causes a decrease in the gas pressure within the bellows I3 thereby decreasing the thermal conductivity of the gas. This in turn results in the heated filament 22, which may likewise be connected to the Wheatstone bridge arrangement, losing Vheat at a slower rate and causing an increase in the temperature land Are,- sistance of the filament 22. Thus the

iilaments

20 and 22 are inversely responsive to pressure changes exerted on the flat bellows end II) and by connecting these filaments in a Wheatstone bridge arrangement, as shown in my above noted prior patent, such pressure changes caribe readily indicated on a suitable meter, or the changes in the resistance of the filaments utilized to automatically operate machinery or the like.

When the external pressure is about 1 mm. Hg

the two bellows 9 and I3 are completely relaxed,

due to the internal gas pressure of the helium being so chosen that the total inward force exerted by the pressures within bellows 9 and I3 on the ilat bellows end I0, is equal to the external force exerted on this bellows end I9. This means that -in which P is the external pressure applied to lbellows end I3, P9 is the pressure in bellows 9 Pis is the pressure in bellows I3 and A10 and A15 are the area of the respective bellows end plates IIt and It.

lunit is responsive is controlled by the stiffness of the bellows 9 and I3 together with the spacing between the angular projection of the U-shaped bracket Il-i relative to the fixed end of the bellows I3, when the vbellows 9 and I3 are in relaxed condition, with-V the maximum displacement of the bellows ends I and I5 depending on the elastic limit of the bellows 9 and I3.

The modification of Figs. 3 and 4 operates in the identical mannerY as that above described, the only difference being that the laments 39 'a and 32 oi' vthismodication are not within the confines of the'respective bellows 139 and 45. Instead, the pressure changes within the bellows li5 are transmitted tothe filament 3U through the passageway l and in a similar manner, the

`pressure changes within the inner bellows 4I! A'provided whichvis exceedingly sensitive in its operation. v-Bythe utilization of electrically heated filaments subjected to changes in pressure of a fluid having high thermal conductivity, the temperatureA and hence Vthe electrical resistance of these laments is simultaneously varied inverselyproportional to changes in pressure of an external force. Thus by suitably connecting these ilaments ina measuring or a control circuit, the circuit is thus responsive to external pressure changes which can be accurately measured or employed to accurately control machinery andthe like. p

Although one specific embodiment of the present invention has been shown and described, it is to be understood that still further modications thereof may be made without departing from the spirit and scope of the appended claims.

I claim:

l. Apparatus for converting mechanical dis placements of a flexible member into equivalent changes in the condition of elements of an electrical circuit comprising two bodies of helium two sensitive bellows having their fixed ends suitably attached to a base plate and their free pressure-responsive ends connected together, a conducting filament mounted in each of said two bodies of helium gas, and means for connecting said conducting filaments into an electrical circuit.

2. Apparatus for converting mechanical displacements of a ilexible member into equivalent changes in the condition of elements of an electrical circuit, comprising two bodies of helium gas, each at a preselected density and independently confined in the free internal volumes of two unequally sensitive bellows, the smaller being suitably mounted inside the larger with the fixed ends of both of said bellows suitably attached to the same base and their free pressure-responsive ends connected together, a conducting lament mounted in each of the said two bodies of gas, and means for connecting said conducting filaments into an electrical circuit.

3. Apparatus for converting mechanical displacements of a flexible member into equivalent changes in the condition of elements of an electrical circuit comprising two bodies of helium gas, each at a preselected density and independently conned in the free internal volumes of two unequally sensitive bellows, each of said bellows communicating through suitable conduits with two similar closed cells, a conducting filament mounted in each of said cells, and means for connecting said filaments into an electrical circuit.

4. Apparatus for converting mechanical displacements of a flexible member into equivalent changes in the condition of elements of an electrical circuit comprising two bodies of helium gas, each at a preselected density and independently conned in the free internal volumes of two unequally sensitive bellows, the smaller being mounted inside the larger with the Xed ends of both of said bellows attached to the same base plate and their free pressure-responsive ends connected together, two similar closed cells, two passageways each of which connects the interior of one of said bellows with one of said closed cells, a conducting filament mounted in each of said cells, and means for connecting said conducting filament into an electrical circuit.

CLARKE C. MINTER.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,358,467 Minter Sept. 19, 1944 2,400,467 Ruge May 14, 1946