GB2053479A

GB2053479A - Sensing Apparatus

Info

Publication number: GB2053479A
Application number: GB7923281A
Authority: GB
Original assignee: Johnson Controls Inc
Current assignee: Johnson Controls Inc
Priority date: 1979-07-04
Filing date: 1979-07-04
Publication date: 1981-02-04

Abstract

An air pollutant and/or combustion sensing apparatus includes a probe 1 having a metal plate 2 with a mounting stud 11. One or both surfaces of the plate has reactive surfaces 3, 4 secured thereto by adhesive 9. The surfaces 3 and 4 are formed from dielectric material containing an electric charge which develops a charge enhancing electric field. The dielectric material reacts with contaminating products to produce a detectable electric output. <IMAGE>

Description

SPECIFICATION Sensing Apparatus This invention relates to a sensing apparatus for environmental gaseous product sensing and has for its object to provide such an apparatus in a simple and convenient form.

According to the invention a sensing apparatus for environmental gaseous product sensing includes a sensing probe means mounted with an exposed sensing surface exposed to contact with a surrounding free space environment in which said products may arise and having an electrical signal lead means connected to said sensing surface for connection of the electrical potential of the surface to an output means for creating an electrical signal in the presence of environmental borne contaminating products, the improvement wherein said surface is formed of a dielectric material containing an electric charge developing a charge enhancing electric field, said dielectric material having a volume resistivity greater than 1 x 1012 ohm-cm at 50% RH and a water absorption capability of less than 1% by weight at 95% environmental RH, said dielectric material surface having a surface energy component due primarily to dispersion bonding forces with a minimum contribution from dipole-hydrogen bonding forces, said dielectric material reacting with said contaminating products and producing a detectable electrical output related to the environmental borne contaminating products.

According to a further feature of the invention a capacitance sensing apparatus for detecting the presence of environmental contaminating products comprises a capacitance sensing unit including a pair of spaced capacitance electrode means having a non-conductive free space therebetween; at least a first of said electrode means having a charged dielectric material forming a free surface spaced from the opposite electrode with an electric field at the surface, a dielectric material containing an electric charge developing a charge enhancing electric field, said dielectric material has a volume resistivity greater than 1 x 1012 ohm-cm at 50% and a water absorption capability of less than 1% by weight at 95% environmental RH, said dielectric material surface having a surface energy component due primarily to dispersion bonding forces with a minimum contribution from dipole-hydrogen bonding forces, said dielectric material defining an active material which interacts with the contaminating products to change the electrical output of the capacitance means, and an amplifying means having a high impedance input means connected to the capacitance sensing means to produce an amplified output of the capacitance sensing means.

The present apparatus includes a non-conductive sensor probe of an electric material specially charged and which contains either electric dipoles and/or electric monopoles to develop a charge enhancing field. The field can result from aligned dipoles or a charge deposited on the surface or preferably into the material. Useful materials particlarly include polytetrafluoroethylene (Teflon TFE), perfluoroalkoxy resin (Teflon PFA), fluorinated ethylene propylene copolymer, (FEP), polystyrene and polyethylene although other materials can also be used. The high volume and surface resistivity of such materials are important to hold the charge for an extended period of time. Such materials have a low water adsorption and adsorption characteristic. The non-conductive materials can be charged in any suitable manner.

The electric response to combustion products includes an initial pulse followed by a unique increasing ramp response function which is not noted in a non-charged, non-conductive adsorption sensor material.

In one preferred structure a conductive base plate has a support and connecting stud on the backside, with an electret intimately attached to the base plate to form a probe mounted within a suitable outer conductor which acts as ground shield. In one structure, the outer conductor is significantly spaced from the probe such that the probe acts as an adsorption sensing device. In another structure the outer conductor and ground shield is located in close spacement to the probe, or an additional plate electrode is so located, and a capacitor sensor is formed, with the high intensity electric field to the ground shield providing a capacitance response.

The magnitude of the signal is generally such that a high input impedance device which also has good electrometer characteristics should be employed. A suitable output signal detector is described in the specifications of United States patents 3754219 and 3989463. However, significantly less amplification is required to develop a useful output signal than with a non-charged probe.

The present invention has been found to provide a significantly greater sensitivity to particulate and gaseous mediums resulting from combustion and like environmentally borne products.

In the accompanying drawings: Figure 1 is a view of a novel sensing probe; Figure 2 is a block circuit diagram of a fire detector with the probe of Figure 1; Figure 3 is a graphical illustration of the sensitivity of charged electric to gaseous molecules; Figure 4 is a view similar to Figure 3 for a non-charged dielectric; Figure 5 shows a novel probe in a capacitor sensor; and Figure 6 is a view similar to Figures 3 or 4 for Figure 5.

In Figures 1-2, one embodiment of a sensing probe 1 is shown including a supporting metal plate 2 and sensitive reactive surfaces 3 and 4, on opposite faces of the plate. The reactive surface may however be on one face only of the plate. An outer support housing or case 5, having a pair of grounded cup-shaped perforated shield plates or electrodes 6 and 7, forms a free space between the probe 1 and the shield plate. Perforations 8 permit essentially free access of the surrounding environment into the space and into contact with surface 3.

The probe 1 includes a base metal plate 2 of a conducting material and the intimately attached reactive surfaces 3, 4, which may be a coating or a film-like material are secured by a suitable adhesive 9 such as FEP or PFA Teflon, a silicone contact adhesive or the like. Other means may of course be "employed. A supporting post or stud 11 may act as a support and circuit connector for probe 1. A high impedance detector and processing circuit 12 (shown in Figure 2) is preferably used to operate a suitable output such as an alarm circuit 1 3.

Each surface 3 and 4 is an electret which is defined as a charged insulating dielectric material which produces a permanent electric field. This field can result either from aligned electric dipoles, or from an extra charge deposited in or on the electret. An electret is the electrostatic analogy of a permanent magnet except that the dielectric material may contain bth~electric drpolesand electric monopoles, which explains why the dielectric may be polarized in many more ways than a magnetic material.

Holding the electric charge is of prime importance, since the electret life is a function of its charge holding ability. A principal property of the electret material which is responsible for this charge holding ability, is its electrical resistivity. Generally, the material must have a high volume and surface resistivity to hold the charge for an extended period of time. The volume resistivity should be greater than 1 x 1 o12 ohm-cm at 50% RH (relative humidity) and the surface resistivity should be greater than 1 x 1010 ohms/square at 50% RH. The preferred volume and surface resistivities are greater than 1x10'5 ohm-cm and 1 x 1015 ohms/square respectively.

In practical applications, a low water absorption at environmental relative humidities up to 95% RH (preferably less than 1% at 95% RH) determined on a weight basis after 24 hours of exposure is desired. Preferably the electret material has a surface energy component due primarily to dispersion bonding forces with a minimum contribution from dipole-hydrogen bonding forces; i.e. the surface has a minimum of polar functional groups. Preferably values of yh are less than 5.0 ergs/cm2. Typical preferred dielectric materials include polytetrafluoroethylene (Teflon TFE), polystyrene and polyethylene.

Other electret materials would include fluorinated ethylene propylene copolymer (FEP), perfluoroalkoxy resin (Teflon PFA), lonomer resin, and polypropylene. Any other insulating or dielectric which can be charged and can retain such charge may be used.

The sensing mechanism is two-fold in nature. The dielectric material functions (1) as an adsorber as in non-charged dielectric sensors, and (2) functions with an electrostatic type sensing reaction. The electret differs from a simple adsorption reaction in that its detection capability is further dependant on coulombic electrostatic forces.

The electric field associated with the electret material amplifies the adsorption and charge detection phenomena associated with non-charged dielectric materials. The electret's amplification of its non-charged adsorption capability would appear to be explained by the action of the electric field associated with the electret tending to align the adsorbed polar gas molecules, this increasing the effect of their induced field. The dielectric field also has the ability to attract charged aerosols, ion radicals and polar gas molecules. The degree of attraction associated with any such polar gas molecules which are present in the free space is of course dependant upon the degree the coulomb forces are able to overcome the thermal energy associated with the polar gases.

The second function results in a noticeably different product sensing probe signature, such as shown in Figs. 3 and 4.

Figs. 3 and 4 show two graphical illustrations, in which the curves A show similar traces 1 5 and 16 of the obscuration response with time of an U.S. Underwriters Laboratories (UL) smoke chamber employed in testing each sample and resulting from the insertion of a smoldering cotton wick into the chamber. Curves B are traces 1 7 and 1 8 of the response or signal characteristic of the probe. The trace for the charged dielectric ar electret probe shown in Figure 3 includes an initial pulse 21 followed by a distinct ramp function signal. The ramp signal 22 is shown reaching a consistent level 22a. This resulted from the saturation of the amplifier and would have otherwise continued to increase.In contrast the trace for the non-charged probe shown in Figure 4 shows an initial pulse 23 followed by an essentially constant and substantially reduced output signal portion 24. The charged dielectric with the adsorption enhancement field thus produces a distinctly different sensitivity characteristic, with the field maintaining a continuous response. The electret or charged dielectric provides detection of a wide spectrum of products generated by combustion and air pollution, including the various toxic and noxious polar gases and charged particles. The electret sensing probe with the continuous sensitivity thus is capable of sensing the fire in both the incipient smoldering and the flaming stages of combustion.

The amplified adsorption of the polar gases on the surface of the sensing element increases the magnitude of the charge induced in the electret's sensing surface. In order to sense this charge, a high input impedance detection circuit with good electrometer characteristics is generally required.

However, the gain of this amplifier can be lower than that required by a non-electret adsorption sensor.

As a result the complexity and therefore the cost of the sensing circuitry for the electret probe is considerably reduced. Generally, the output signal detection units disclosed in the previously mentioned patents can be satisfactorily employed with the probe described.

A wide variety of known methods are available for forming the electrets. The classical thermoelectric method employs a strong electric field applied to an insulator while it is heated and subsequently cooled to create aligned dipoles. Dipoles may also be formed by stressing or stretching appropriate ferroelectric materials. Charge injection may be produced by a corona discharge charging or by an electron beam.

The shielding of the electret from the ions in their surroundings is preferably provided by the illustrated ground shield members. The charge injected electrets are readily formed with lifetimes of several hundred years with proper shielding and thus may be employed where long unattended life is required.

The present invention thus teaches that a highly significant result is obtained by the use of a charged dielectric material wherein the signal changes are enhanced as a result of the interaction between the pollutant products and the field created by the charged material.

As previously noted, shield plate 6 may be located in close spacement to the probe unit 1 to form a capacitor sensor for sensing of fire generated products and like environmental borne products. A capacitor sensor may be uniquely formed as shown in Fig. 5.

In Fig. 5 a separate capacitor plate 25 is located in close spacement to the probe unit 1, and particularly the sensing surface 4. Plate 25 includes a connecting post which may also be a support and which is connected by lead 26a to probe lead 26, and thereby also to amplifier 12.

Plate 25 may also be connected to ground or any other reference potential. Fig. 5 shows a parallel plate capacitor although any other capacitive geometry could be employed which defines a free space between the electret sensing surface 4 and capacitor plate 25. Figure 5 produces an additional detection mechanism to that of a non-compacitive type probe sensing system. Both the adsorption response and the response based on which develops a multiple or built-up layer of charge of polar gas molecules on the surface 4 occur also in Figure 5. In addition the capacitor geometry creates a high intensity field which interacts with the particular matter in the smoke as well as with noncharged gas molecules with the induction of an induced dipole moment in the particulate matter and the gas molecules.The high intensity electric field tends to separate the negative and positive charge within the molecule and within the particulate. By locating the sensor plate elements close to each other and thereby forming a capacitor sensor the electric field intensity is increased to a level to provide effective and operable induced polarization of the particulate matter. For example, the electret can be readily charged to a typical operating level of 3,000 volts per square inch and the second plate 25 located from the probe in the order of one-eighth of an inch, thereby significantly increasing the field and creating the induced movement in the particulate matter and gas molecules.Although charging of a sensor as described above to any significant level can be expected to produce some improvement in response, we have generally found that the electric field strength of the sensing capacitor should be of the order of 3.1 50x 105 volts/m to obtain practically significant results.

Smoke as it enters the electric field becomes polarized with the positive induced charges equal in magnitude to the negative induced charges. In this process, electrons in the smoke are displaced from equilibrium, forming induced dipoles of polarization, P-Ngd. The induced smoke charge will always appear in such a way that the field of the electret set up by it (Es) opposes the electric field (Eo) of the capacitor. The resultant capacitor field E is the sum of E0 and E5 and has the same polarity as E0 except for the fact that it is smaller due to the induced smoke polarization which tends to weaken the capacitor's original external field. The weakening of the capacitor's electric field reveals itself as a reduction in the potential difference between the capacitor's plates.Here, V=Ed, where V=voltage.

E=electric field strength and d=distance. As shown in Fig. 6, the response of the capacitor sensor is a continuously increasing ramp. The induced dipole movements in the gas molecules and particles, thus tend to smooth the characteristics of the curve 27 and eliminate the drop in the output after the initial smoke charge 28 (Fig. 6) is detected.

The induced polarization of a molecule in the smoke cloud can also be described with reference to its structure. If no external field is present, the molecule is in its normal electrical configuration. In the capacitor's electric field, the electron cloud shifts from its normal electrical configuration to a more deformed polar configuration. The shift is such that the force on the electron cloud by the electric field of the charged plates and the force on the electron cloud due to the coulombic attraction between the charges are balanced, and stable equilibrium exists, where and

Therefore,

4nE,R3E=Nqd (5) The dipole moment of the molecule can therefore be written: P=Nqd=47rEOR3E=E (6) where a=47GE,R3 (7) is the molecule's electronic polarizability.The polarizability a: can be seen to depend not on the number of charges N of the molecule but rather on its radius R. One is therefore able to control the size of the particles being detected by controlling the magnitude of the electric field. The dipole moment can be seen to be proportional to the strength of the capacitor's electric field.

The phenomena may also be described, including the induced polarization mechanism, in terms of changes in a capacitor's relative permittivity. The passage of smoke forms a dielectric between the plates of a charged capacitor and will produce a change in the capacitor's relative permittivity. This change in permittivity will result in a change in the electric field intensity between the plates of the capacitor and thus, a change in the voltage across it. If no dielectric smoke is present, Gauss Law states:

If the dielectric smoke is present;

Qo D=- is the electric flux density A E=V/d is the electric field intensity Q, P1=- is the induced polarization A However, since D=EOErE (16) we can write

Since smoke detection is accomplished using induced polarization in a capacitor's electric field, the detector's particulate and liquid droplet sensitivity is proportional to the charge on the capacitor.

Where S=smoke sensitivity D=the capacitor's electric flux density A=capacitor's area E0=permittivity of dielectric Errelative permittivity of dielectric E=the capacitor's electric field intensity V=the capacitor's voltage d=the spacing distance of the capacitor plates.

The important aspect and feature of this invention is the exposure of the gaseous products to a significant area of the uniquely charged dielectric material in combination with a means to detect a change in its charge. The structure may vary from that of Figs. 1-6.

In relatively large areas, a plurality of sensing units may be distributed throughout the area and connected to a signal processing circuit or to individual processing circuits. The present invention thus further enhances the use of a non-conductive sensor whilst maintaining a relatively low cost unit having a long, reliable life. The invention can be employed in any smoke or pollution environment in which the products generated interact with the special charged surface means to produce a change in the surface charge.

Claims

1. A sensing apparatus for environmental gaseous product sensing including a sensing probe means mounted with an exposed sensing surface exposed to contact with a surrounding free space environment in which said products may arise and having an electrical signal lead means connected to said sensing surface for connection of the electrical potential of the surface to an output means for creating an electrical signal in the presence of environmental borne contaminating products, the improvement wherein said surface is formed of a dielectric material containing an electric charge developing a charge enhancing electric field, said dielectric material having a volume resistivity greater than 1 x 1012 ohm-cm at 50% RH and a water absorption capability of less than 1% by weight at 95% environmental RH, said dielectric material surface having a surface energy components due primarily to dispersion bonding forces with a minimum contribution from a dipole-hydrogen bonding forces, said dielectric material reacting with said contaminating products and producing a detectable electrical output related to the environmental borne contaminating products.

2. An apparatus of claim 1 wherein said dielectric material includes electric monopoles to form said electric charge.

3. An apparatus of claim 1 or 2 wherein said dielectric material includes aligned dipoles to form said electric charge.

4. An apparatus as claimed in any one of claims 1, 2 or 3 wherein said material includes a predominate portion of or is totally selected from polytetrafluoroethylene (TFE), perfluoroalkoxy (PFA), fluorinated ethylene propylene (FEP), polystyrene, polyethylene, polypropylene and ionomer resins.

5. An apparatus as claimed in any one of claims 1, 2, 3, or 4 wherein said dielectric material includes surface charges establishing an electric field at the sensing surface.

6. An apparatus as claimed in any one of claims 1, 2, 3, 4, or 5 wherein said dielectric material includes entrapped electric charges establishing an electric field at the sensing surface.

7. A capacitance sensing apparatus for detecting the presence of environmental contaminating products, comprising a capacitance sensing unit including a pair of spaced capacitance electrode means having a non-conductive free space therebetween; at least a first of said electrode means having a charged dielectric material forming a free surface spaced from the opposite electrode with an electric field at the surface, a dielectric material containing an electric charge developing a charge enhancing electric field, said dielectric material has a volume resistivity greater than 1 x 1012 ohm-cm at 50% RH and a water absorption capability of less than 1% by weight at 95% environmental RH, said dielectric material surface having a surface energy component due primarily to dispersion bonding forces with a minimum contribution from dipole-hydrogen bonding forces, said dielectric material defining an active material which interacts with the contaminating products to change the electrical output of the capacitance means, and an amplifying means having a high impedance input means connected to the capacitance sensing means to produce an amplified output of the capacitance sensing means.

8. The apparatus of claim 7 wherein said dielectric material includes electric monopoles to form said electric charge.

9. The apparatus of claims 7 and 8 wherein said dielectric material includes aligned dipoles to form said electric charge.

10. The apparatus of claims 7, 8 or 9, wherein said material includes a predominate portion or is totally selected from polytetrafl uoroethylene (TFE), perfluoroalkoxy (P FA), fluorinated ethylene propylene (FEP), polystyrene, polyethylene and ionomer resins.

1 The apparatus of claims 7, 8, 9 or 10 wherein said dielectric material includes surface charges establishing an electric field at the sensing surface.

12. The apparatus of claims 7, 8, 9, 10 or 11 wherein said dielectric material includes entrapped electric charges establishing an electric field at the sensing surface.

13. The apparatus of any one of the claims 7, 8, 9, 10, 11 or 12 wherein the second electrode means is a grounded conductive member surrounding the first electrode means and being apertured to permit relatively free movement of the surrounding environment between the electrode means and into engagement with said active material, the first electrode means being an element mounted within the second electrode means and having said material on the opposite surfaces.

14. The capacitance sensing apparatus of any one of the claims 7, 8, 9, 10, 11, 12 or 1 3 wherein said capacitor has a field strength of at least 3.1 50x 105 volts/m.

1 5. The apparatus of any one of the claims 7 through 14 wherein the electrode means are plates, and an outer ground shield means encloses the plates.

1 6. The apparatus of claim 1 5 wherein the capacitor plates are connected to each other and to the input of the amplifying means.

1 7. The apparatus of claim 1 5 wherein one capacitor plate is connected to ground.

1 8. A sensing apparatus for environmental gaseous product sensing comprising the combination and arrangement of parts substantially as hereinbefore described with reference to Figures 1-4 of the accompanying drawings.

1 9. A sensing apparatus for environmental gaseous product sensing comprising the combination and arrangement of parts substantially as hereinbefore described with reference to Figures 5-6 of the accompanying drawings.