JPS5947256B2 - Fluid sensing element - Google Patents

Description

[Detailed description of the invention] The present invention relates to a fluid sensing element.

Oil leaks from oil pipelines, oil tanks, etc. can cause pollution, fires, etc., so the law requires that pipelines and tanks be equipped with leakage detection devices. In addition, leakage from gas station tanks, solvents, flammable liquids, and toxic liquids used in chemical factories from liver equipment and pipes can also cause fires and local pollution, so leak detection devices are also installed in these equipment. It is desired that it be installed. Various methods have already been proposed for detecting the various types of leaks as described above, among which methods using light and electricity as detection means are listed below.

1) A method that utilizes the attenuation of the amount of light transmitted when liquid enters the detection element (optical fiber line).

2) A method of detecting changes in capacitance of a cable when a liquid to be detected enters a cable consisting of two conductors separated by a porous insulating material using a pulse reflection method (U.S. Patent Law No. 3981
18).

3) A method for detecting short circuits between two conductors caused by leaking liquid coming into contact with a cable that is separated by an insulating material that is dissolved by the liquid to be detected, causing the insulating material to flow out (Canadian Patent No. No. 775758).

4) A method in which a float with two electrodes is floated on the water surface and the electrical resistance between the electrodes increases when an insulating liquid flows on the water surface.

However, the above method has advantages and disadvantages.

For example, methods (1) and (2) are easily affected by moisture or water vapor, method (3) has low reliability and problems with short-circuit sparks, and method (4) has problems with water (liquid) levels. It has drawbacks such as being susceptible to the influence of waves, and the adjustment of the electrodes being delicate and unstable. As a result of various studies on electrical leak detection methods, the present inventor found that insulating liquids and vapors with low surface tension easily penetrate into continuous porous polytetrafluoroethylene resin (PTFE) materials containing conductive substances. The inventors discovered the phenomenon that the electrical resistance of the material increases significantly when it becomes wet or absorbed, and based on this fact, the present invention was completed.

That is, the present invention provides continuous porous PT containing a conductive substance.
This fluid sensing element is characterized in that at least two conductors are separated by an FE material.

The continuous porous PTFE material containing the conductive material used to make the fluid sensing element of the present invention includes PTFE powder and, for example, graphite powder and liquid lubricants (petroleum, petroleum, etc.).
A continuous porosity green PTFE molded product obtained by molding a mixture with a liquid hydrocarbon such as naphtha by compression, extrusion and/or rolling, and removing the liquid lubricant from the molded product by heating or extraction; Or the unsintered PTF
E The molded product is heated to a temperature of about 300°C to 360°C, that is, PT
An incompletely fired continuous porous PTFE molded product stabilized by heating to a temperature around 327°C, which is the melting point of FE, to such an extent that the PTFE is not completely fired, or the unfired PTFE molded product.
The FE molded product (it is better not to contain a liquid lubricant), for example, Japanese Patent Publication No. 48-44664, Japanese Patent Publication No. 51-189
Continuous porous unfired expanded PTFE material whose specific gravity is further reduced by spreading about 1 to 15 times in at least one direction by a method such as that described in No. 91;
and the above-mentioned expanded porous PTFE material at a temperature around the melting point (327°C) of the tetrafluoroethylene resin, for example, about 300°C to
Expanded porous PTF heated to a temperature of 360° C. to increase the dimensional stability of the material or to incompletely sinter it.
It is an E material. In the present invention, the conductive substance added to the tetrafluoroethylene resin powder is a powdery substance having conductivity, such as graphite powder, carbon black powder,
Carbon fiber, various metal powders (powders of platinum, gold, silver, titanium, tantalum, etc.), cermets (titanium carbide, chromium, etc.), metal nitrides (titanium nitride, aluminum nitride, etc.), metal borides (boron, etc.) titanium oxide, zirconium boride, etc.), metal silicides (molybdenum silicide, etc.), metal oxides (CU2O, TiO, .VO, .
MnO,. COONiO. ．． Examples include oxides such as ZnO, CaO, etc., which are given conductivity by making the oxygen ratio non-stoichiometric), metal semiconductors, and organic semiconductors.

In addition to adding conductive substances to PTFE,
It may also be filled with colored pigments or fluororubber for strength maintenance. The mixing ratio of the PTFE powder and the conductive substance should be within a range that imparts conductivity to the mixed molded product and that the elasticity of the PTFE remains to a level suitable for processes such as extrusion, rolling, and stretching. Usually 5-70% by weight (PTF
The proportion of conductive substance in E) is preferably 10.about.50% by weight.

The conductive substance and PTFE powder are mixed by uniformly mixing the PTFE powder containing the conductive substance in the above amount using a rotary mixer, and then adding about 20% by weight of a liquid lubricant to the mixture. or by adding a mixture of a conductive material and a liquid lubricant to the PTFE powder in a V-blender, or by extrusion, rolling, or stretching of PTFE (sheet tape PTFE powder or its compact is impregnated with a solvent in which a conductive substance is dispersed, or a precursor of a liquid conductive substance is contained in the PTFE powder or its compact, and then chemical and/or physical treatment such as heating and hydrolysis is applied. It may also be deposited as a conductive material by other means.

In any of the above cases, if there is a risk that the contained conductive substance may leak out, it may be fixed to the base material with a filler. The at least two conductors separated by the continuous porous PTFE material containing a conductive substance obtained as described above may be made of any electrically conductive material, such as copper, silver, gold, Copper, aluminum, or an alloy based on them, and is mainly used in the form of a wire or foil.

Next, the method of isolating at least two conductors using the continuous porous PTFE material basically involves separating the two conductors using the resin material, but there is a more specific method. To explain this with reference to the drawings, as shown in FIG. 1, one conductor is the center conductor 1, the other is the outer conductor 3 of the braided structure, and the PTFE material 4 is connected between these conductors.
A coaxial structure isolated by a large number of conductors 2 as shown in Figure 2.
a flat cable-like structure embedded in the resin material 4 with the conductor 1 kept in a parallel and spaced relationship, or as shown in FIG. A typical structure is that the conductors 1'' are arranged separately.

In addition to the above structure, two
A pair wire structure made of parallel or twisted conductors,
The fluid of the present invention may also include a structure in which the coated conductors of the paired wires are further provided with an outer coating and the outer coating is peeled off only at important points, an electrode type in which two conductors are used as electrodes and the space between them is separated by the resin, or an appropriate combination thereof. The object of interest of the sensing element.

Also, by keeping at least two conductors parallel and spaced apart,
The conductive PTFE material 4 or the conductive PTFE 4
After sandwiching the sandwiched body with stretched or unstretched unsintered PTFE sheets, or further supplying a narrow PTFE tape so as to cover the conductor part of the sandwiched body so as not to cover the part between the conductors, and crimping it, The PTF around the conductor can be heated by heating only the covering of the conductor from the outside, or by passing a desired current through the conductor and heating it to 327°C or higher using Joule heat.
Sintering only the E material strengthens the adhesion between the conductor and the insulator, which reduces the electrical contact resistance between the conductor and conductive PTFE of the cable-shaped fluid sensing element of the present invention, improves the detection sensitivity, and improves the mechanical strength of the element. It also improves.

Furthermore, as described above, at least two conductors in contact with or surrounded by the conductive PTFE material 4 may be selected to allow gas and the fluid to be detected to pass therethrough, but not to allow liquids with high surface tension (such as water) to pass therethrough. It may be further sandwiched, surrounded and crimped with an unfired or semi-fired PTFE sheet or a stretched porous PTFE sheet, which is a permeable sheet material, and then the required portions are fired by an appropriate means in the same manner as above. You can.

The fluid detected by the fluid detection element of the present invention (detected fluid) includes, for example, liquid hydrocarbons such as petroleum, gasoline, and heavy oil, various organic solvents such as carbon tetrachloride, and MEK, or various liquids with high vapor pressure. steam (e.g. MEK
N gas, such as ammonia).

When using the fluid sensing element of the present invention, the element is installed continuously or at predetermined intervals along the fluid container or transfer pipe as described above.

The content liquid (liquid to be detected) is then discharged from these containers or tubes.
or if the gas leaks, it contacts the fluid sensing element of the present invention and penetrates into the continuous porous PTFE material containing the conductive material separating the conductors of said element. Significantly reduces the electrical conductivity of the material (i.e.
(increase insulation resistance). This increase in resistance can be detected by a detector connected to the conductor of the element via a lead wire, and the presence or absence of fluid leakage can be determined. Furthermore, if the fluid detection element of the present invention is fixed to a suitable float and positioned so that the water surface is between the electrodes, if oil leaks onto the water surface, it will come into contact with and be absorbed by the element and Since the resistance value changes, it can also be used to detect oil films caused by oil leaks.

In addition, this detection method uses the change in electrical resistance as it is, compares it with standard resistance, connects a bridge and reads the change, and detects reflection from different points of characteristic impedance and its time delay. How to use (
TDR method). Furthermore, as a method for connecting the fluid sensing elements of the present invention, it is possible to connect a plurality of elements in series or in parallel (such as in the case of TDR method). Furthermore, when measuring the electrical resistance of an element, it is also possible to increase the voltage applied to the element to cause it to self-heat due to the resistance heating effect, thereby making it easier to detect highly viscous liquids (for example, heavy oil). In addition, in the case of a fluid detection element as shown in Fig. 3, the covered conductor 1 and the conductor plate are placed at a predetermined distance, and the place where the fluid to be detected (usually the ground) flows is used as part of the medium for detection. It is also possible to do this. The change in electrical resistance due to the intrusion of fluid in the fluid sensing element of the present invention is greater than the initial value V2, and since it utilizes volume changes based on continuous pores rather than changes in surface phenomena of substances, Since it is essentially stable against environmental conditions and is mainly composed of water-repellent PTFE, it has the advantage of being less susceptible to moisture and water vapor and having excellent heat resistance and weather resistance.

The present invention will be further explained below with reference to Examples, but the present invention is not limited thereto.

Example 1 A mixture of PTFE powder, 15% by weight of graphite powder, and a liquid lubricant (approximately 20 cA) was compressed and extruded, and the resulting rod was rolled to a thickness of 0.1
A PTFE tape with a width of 4 fmN was wound on a copper wire with a diameter of 1.5 mm to form a cylinder with an outer diameter of 3 m1, and a copper wire with a diameter of 0.1 fTrHL was braided onto the cylinder to form a coaxial cable.

This material was made into a length of 3 cm, and lead wires were attached to the inner and outer conductors to form a fluid sensing element as shown in Fig. 1. M
The table below shows the results of measuring changes in electrical resistance after immersion in EKl gasoline, kerosene, C heavy oil, and water. Example 3: The temperature of the immersion liquid and the fluid sensing element was 20°C.
The relative humidity in the room was 56%.

Furthermore, once the device was used in an experiment, it could be used again once the liquid that entered it volatilized. Since there is a correlation between volatility and electrical resistance, the element can also be provided with a function of identifying liquids based on their volatility. Example 2 The amount of graphite powder mixed into PTFE powder is 50% by weight
and Example 1, except that the expanded PTFE material was unsintered.
A fluid sensing element of the same shape was made in the same manner as above, and its characteristics were measured, and the results are shown in the table below.

This was placed in a constant temperature bath at 300°C for 30 seconds, then stretched 1.5 times in the longitudinal direction, and then slit to a width of 4 m. A conductive continuous porous PTFE tape was placed on top of the copper wire with a diameter of 1.5 mfL. It was wrapped to form a cylinder with an outer diameter of 3 (Frm), and a copper wire with an outer diameter of 0.1 fm was braided thereon to form a coaxial cable.

Is this thing 30 long? A fluid sensing element as shown in FIG. 1 was obtained by cutting the material into pieces and attaching lead wires. One of the devices left its PTFE layer unfired, and the other amateur device was placed in a constant temperature bath at 350° C. for 1 minute to incompletely bake and stabilize the resin bath. When these devices were tested by immersing them in MEKl gasoline, kerosene, C heavy oil, and water, their electrical resistance changed as shown in the table below.
The liquid and sample temperatures during the test were 20° C., and the relative humidity in the room was 56%. Furthermore, the fluid sensing element used in the experiment could be used again once the liquid evaporated. Example 4 12 copper wires with a diameter of 0.254 m, copper wire spacing 1.
12? , conductive PT mixed with 15% by weight of graphite
A flat cable-shaped fluid sensing element as shown in FIG. 2 was made using two FE sheets (thickness: 0.1 wft).

The length of this element was 30 wm. Every other lead wire was grouped together to form two poles. Changes in electrical resistance of this element due to vapor volatilized from the liquid as shown in the table were measured at liquid temperature, room temperature of 25° C., and room relative humidity of 71%.

The saturated vapor pressure was used as the pressure of the detected vapor generated from the liquid. Furthermore, this fluid sensing element regained its original function after the invading vapor was volatilized.

Example 5 Two copper wires with a diameter of 0.29 m were prepared for electrodes, held in parallel at an interval of 25fr1f1, and two unfired conductive PTFE sheets (thickness 0.1fWi) and then extruded and/or rolled two more sheets of unfired PTF.
A flat cable-shaped fluid sensing element was made by sandwiching and crimping the E sheet (thickness: 0.3 wn) and integrating it.

Next, a current of 10 A is applied to each copper wire of this element for 1 minute to generate heat, and the conductive PTFE sheet and unfired PTFE sheet surrounding the conductor are fired, and the conductor, conductive PTFE, and PTFE sheet are bonded together. Intensified contact. The fluid detection element thus obtained was cut into a length of 40 rfrm, one end of each copper wire was drawn out about 101E, and a plastic-covered electric wire was connected as a lead wire to the drawn out portion.

In addition, both cut surfaces of the fluid sensing element were sealed with epoxy resin to prevent water from entering the sensing element from both end faces. Using this fluid detection element, we conducted an experiment to detect an oil film when oil leaks onto the water surface. First, the device was held half submerged in water in a water tank with its cross section vertical, so that the water surface was approximately at the center between two electrodes of the device.

Claims (1)

[Scope of Claims] 1. A fluid sensing device characterized in that at least two conductors are isolated by disposing a continuous porous polytetrafluoroethylene resin (PTFE) material containing a conductive substance in at least a portion thereof. element. 2. The fluid sensing element according to claim 1, wherein the conductive substance is carbon powder. 3. The fluid sensing element according to claim 1, wherein the continuous porous PTFE material is a green PTFE molded product. 4. The continuous porous PTFE material is incompletely fired PTFE.
The fluid sensing element according to claim 1, which is a molded product. 5 The continuous porous PTFE material is an unfired stretched porous P
The fluid sensing element according to claim 1, which is a TFE molded product. 6. The fluid sensing element of claim 1, wherein the continuous porous PTFE material is an incompletely fired expanded porous PTFE material.