A viscometer having a fluid volume-displacer or driver, such as a speaker membrane, and a pressure sensor or detector, such as a microphone membrane, forming the inside surfaces of a cavity that is sealed from the ambient environment of the viscometer except for a controlled leak such as a capillary tube. An electrical signal from the sensor or detector is processed to indicate viscosity of the fluid in the cavity. Additionally determined from the viscosity are heating value, oxygen demand and other thermophysical properties of the fluid. Also, absolute pressure is derived after viscosity is determined.

BACKGROUND
 The present invention pertains to viscosity detectors and particularly to
 delta-pressure-based sensors. More particularly, the invention pertains to
 viscosity sensors for determining the oxygen demand (for complete
 combustion) of a gaseous or liquid fuel for combustion purposes.
 Existing and recently proposed quasi-static viscometers are either fluid
 (i.e., gas or liquid) density and pressure-dependent and costly (such as
 vibrating wire or quartz crystal-based viscometers). Other viscometers
 suffer from additional fluid property dependencies (e.g., those involving
 thermally-driven capillary flow), are prone to drift due to deteriorating
 and leaky valves (as in viscometers dependent on capillary flow driven by
 periodic refill from a source of pressurized gas, valve closure and decay
 observation), or depend on their orientation (as with the falling ball
 viscometer).
 The proposed sensor measures a known property of fluids, viscosity. When
 applied to a combustion control system, it enables feed-forward operation
 and sensing in the mild pre-combustion environment; it is low-cost because
 the property can be simply proportional to the measured signal (in one
 preferred measurement approach) and it relates also simply to Wobbe
 number, oxygen demand or heating value of the fuel, so that the sensing
 error makes a relatively small contribution to the total combustion
 control error.
 This invention involves the use or application of a known property,
 viscosity, to combustion control. It is also about using a preferred
 approach to viscosity measurement to that application.
 Viscosity, .eta., may be known best for its linear relation to laminar
 volumetric flow (dV/dt) and pressure drop, .DELTA.p, in a capillary (of
 radius, r.sub.c, and length, L.sub.c), as shown in equation (1).
EQU dV/dt=.pi..DELTA.pr.sub.c.sup.4 /(8L.sub.c.eta.) (1)
 One first notes the potential of viscosity as an individual property for
 combustion control when searching for low-cost means to compensate for
 variabilities in natural gas composition, and for a way to determine
 heating value without combustion, which includes analytical determination
 via correlations involving k(T.sub.1), k(T.sub.2) and .eta., i.e., in
 conjunction with other properties.
 Here, the viscosity of the fuel, and, previously, the stack O.sub.2
 concentration were for indicating predicted or actual changes in the
 fuel's oxygen demand, D.sub.O2. FIG. 1 shows graphically a comparison
 between .eta., curve 23, and other fuel gas thermophysical (Q.sub.i)
 properties, i.e., .rho., density, curve 26; k, thermal conductivity, curve
 24; c.sub.p, specific heat curve 25; and C.sub.v, thermal anemometer
 correction factor, curve 27; and how well they correlate individually with
 oxygen demand of fuel, D.sub.O2. .eta. exhibits a most advantageous,
 monotonic decrease as D.sub.O2 increases, although c.sub.p appears
 promising as well. The c.sub.p value of noncombustible CO.sub.2 (8.83
 cal/(mol.multidot.K); 8.60 for H.sub.2 O) lies between that of CH.sub.4
 and C.sub.2 H.sub.6 (8.50 and 12.42), but all .eta.-values of
 noncombustible gases O.sub.2, N.sub.2, CO.sub.2 (except H.sub.2 O) lie
 above that of CH.sub.4.
 By including two or more fuel properties into a correlation with heating
 value or D.sub.O2, the achievable accuracy increases significantly (note
 cited art below), but at the penalty of significant cost increases as
 well, because of the need for digital processing for determination of
 c.sub.p. The above is based on the assumption that control of emissions
 and efficiency are prime goals of any combustion control; this is most
 closely achieved by operating under constant stack-O.sub.2 or excess air,
 which in turn is met by maintaining a constant air flow and adjusting fuel
 flow in response to its composition variations, which change D.sub.O2 and
 m*. Half of its density variation is taken care of by the factor m*, as it
 affects all orifice- or venturi-controlled flow control situations. The
 aim of adjusting fuel flow to counter variabilities in Wb, Wobb number, is
 similar but less correct (if one aims at conserving the A/F (air-to-fuel
 ratio) and emissions) and goes back to the definition of the Wobbe number,
 Wb=.DELTA.H/m*, with .DELTA.H=heating value rather than O.sub.2 demand and
 m*=(M.sub.gas /M.sub.air).sup.0.5. M is moles, Wb is closely aligned with
 Bn (Bonne number=D.sub.O2 /m*), as long as non-hydrocarbon fuel
 constituents such as H.sub.2 and CO are absent. A correlation of D.sub.O2
 or Bn with viscosity may be determined with a formula D.sub.O2
 =A+B.eta..sup.C or Bn=A'+B'.eta..sup.0.5, respectively. A and B are
 correlation coefficients and C is a correlation exponent. A' and B' are
 similarly correlation coefficients. The correlation is like that of
 natural law. Related information is in FIG. 6, page 21, of "Microsensors
 for Fluid Properties", by U. Bonne and D. Kubisiak, Scientific
 Honeyweller, Sensors Issue (1996). Additional information is in U.S. Pat.
 No. 5,486,107 by U. Bonne, issued Jan. 23, 1996 and entitled
 "Determination of Fuel Characteristics", which is herein incorporated by
 reference.
 To illustrate the significance of the proposed, viscometer-based combustion
 control system, Table 1 compares some parameters relevant to the quality
 of a combustion control system based on thermal conductivity versus
 viscosity sensors. As shown, on all counts, the viscosity-based system
 lists more advantageous values such as smaller sensor output dependence on
 pressure and temperature but larger dependence fuel-gas composition or
 fuel concentration in a fuel+air mixture. The latter parameter was
 included to quantify the merits of direct measurement of thermo-physical
 properties of the fuel+air mixtures; as shown, measurement of viscosity or
 thermal conductivity in a premixed fuel+air mixture makes the pressure,
 temperature and humidity effects much larger than the sought fuel property
 effects. A similar case can be made for the measurement of .eta. or k in
 the stack gases.
 Table 1 indicates advantages of viscosity versus thermal conductivity as
 D.sub.O2 or .lambda., wavelength, sensors. .lambda.=(actual fuel/air
 ratio).div.(stoichiometric air/fuel ratio). This table indicates that
 viscosity is approximately two times more sensitive to changes in .lambda.
 and D.sub.O2 than thermal conductivity, but thirty percent less sensitive
 to variations in pressure and temperature. That means viscosity detection
 results in a several times more accurate sensor than thermal conductivity,
 for D.sub.O2 or .lambda. measurement. The gas G20 is methane and G271 is a
 gas mixture of 74 percent methane and 26 percent nitrogen. p is pressure
 in bars, and T, temperature, is in degrees Celsius. W is the dependent
 variable, measuring the desired property (.lambda. or D.sub.O2).
 Sensitivities of k and .eta. are relative to variability in nitrogen
 content of fuel mixed with air, .lambda., T, p and nitrogen content of
 pure fuel gas.
 TABLE 1
 .differential.W/.differential.x
 W = k W = .eta.
 Dependence Conditions % %
 1. .differential.W(.lambda.)/.differential. (fuel + air) G20 + air vs G271
 + air 0.2687 -0.3532
 .lambda.= 1.05; 15.degree. C.
 2. .differential.W(.lambda.)/.differential..lambda. .lambda.= 1.05 vs. 1.10
 0.0930 -0.1229
 G20 + air
 3. .differential.W(D.sub.o2)/.differential.Gas G20 vs G271 (26% N.sub.2)
 6.7490 -13.782
 15.degree. C.; 1 bar
 4. .differential.W(D.sub.o2)/.differential.T T = 20 vs T = 15.degree. C.;
 G20 1.6320 1.3860
 5. .differential.W(D.sub.o2)/.differential.P P = 2 vs 1 bar; G20; 0.1938
 0.1043
 For the most desirable property (k or .eta.), the values of W for rows 1-3
 should be the highest and 4-5 the lowest. Viscosity is obviously the
 preferred choice.
 SUMMARY OF THE INVENTION
 The viscometer disclosed here does not rely on the availability of
 pressurized gas, a microsensor or on thermal drivers, and its output is
 independent of absolute pressure (to the extent that viscosity is). It is
 not sensitive to orientation and can be fabricated at low cost. The
 present viscosity sensor is used for determining the oxygen demand of
 various gaseous mixtures. It has a fluid volume-displacer, actuator or
 driver, such as a speaker membrane, and a pressure sensor or detector,
 such as a microphone, with a chamber or cavity and a controlled leak
 between the cavity and the ambient environment of the viscosity sensor.
 The driver, the leak and sensor electronics can be assembled from
 commercially available and inexpensive components. In sum, the sensor has
 low manufacturing costs, and good accuracy, reliability, intrinsic safety
 and long service life.

DESCRIPTION OF THE EMBODIMENT
 A viscometer 10 of FIG. 2 makes use of a linear relationship between
 laminar volumetric flow (dV/dt) through a controlled leak, for which we
 shall choose at first a capillary 11, (of radius, r.sub.c, and length,
 L.sub.c, of capillary tube 11) and viscosity, .eta., for a pressure
 difference, .DELTA.p. In FIG. 2, a diameter of capillary 11, 2r.sub.c, is
 shown for clarity of the drawing. The mathematical relationship of these
 parameters is shown in equation (2).
EQU dV/dt=.pi..DELTA.pr.sub.c.sup.4 /(8L.sub.c.eta.) (2)
 Instead of using a thermal source, a valve or a mechanical flow is used to
 induce a repeatable but time-dependent flow and enable the observation of
 a pressure drop (or rise), .DELTA.p, time constant, .tau., or phase lag,
 .delta.. Viscometer 10 disclosed here is designed to induce a repeatable
 but time-independent flow to enable the observation of a steady .DELTA.p
 when the rate of volumetric displacement by an actuator 12 and the actual
 leakage flow become equal for a few milliseconds (ms). This is sketched
 out and represented in FIGS. 3 and 4. The .DELTA.p signals for three
 gases, Ar, N2 and C.sub.3 H.sub.8, are marked as curves 13, 14 and 15,
 respectively, and the volume change .DELTA.V.sub.c, in percent, of cavity
 16, shown by saw-tooth curve 17, represents the volumetric change induced
 by actuator or driver 12.
 If it can be assumed that the displacement of a sufficiently strong
 actuator is independent of the type of gas, actuator 12 can be driven in a
 "saw tooth" mode and at constant frequency, f, and the above equation (1)
 indeed establishes itself; then all types of gases will eventually reach
 their own .DELTA.p in the chamber, but at the same capillary 11 flow rate,
 thus enabling the determination of each fluid's viscosity via measurement
 of .DELTA.p and use of equation (2). If one designs viscometer 10 to meet
 the first two above-noted assumptions, the remaining question is whether
 there will be enough time in one half-cycle to establish the above
 equality. If the capillary radius r.sub.c is too small, the .DELTA.p(t)
 will also be a saw-tooth-like function (one may neglect adiabatic heating
 effects for small .DELTA.V.sub.c); but if capillary 11 is large enough,
 its flow and .DELTA.p will only increase until dV/dt=.DELTA.V.sub.c f/2,
 regardless of the viscosity of the gas in the viscometer 10, and then
 remain at that value until the end of the saw-tooth 17 period. The
 viscosity then results from equation (3).
EQU .eta.=.pi..DELTA.pr.sub.c.sup.4 /(4.DELTA.V.sub.c fL.sub.c) (3)
 This relationship is illustrated by the results of calculations with a
 numerical model in FIGS. 3 and 4, whereby for a given r.sub.c, L.sub.c and
 f, the cavity volume would be incremented by a small amount corresponding
 to a time step, .DELTA.z=0.1 ms, which would change the cavity pressure by
 an amount corresponding to PV=nRT, which in turn would start the flow
 through the capillary and remove some of the pressure change during
 .DELTA.z, until a balance between dV/dt=dV.sub.c /dt=.DELTA.V.sub.c f/2
 and .DELTA.p is reached. These figures show that: (1) steady .DELTA.p
 values can be achieved towards the end of the saw-tooth periods; (2) the
 values of such steady .DELTA.p periods are proportional to the viscosities
 of the indicated gases C.sub.3 H.sub.8, N.sub.2 and Ar (83, 178 and 224
 .mu.P at 20.degree. C. and 1 atm, respectively); (3) the R.sub.V
 =.DELTA.V.sub.c /V.sub.c, r.sub.c and L.sub.c values need to be and can be
 chosen to both achieve a steady .DELTA.p period and laminar flow (Re&lt;2300)
 in the capillary, as indicated by the Reynolds Number, Re=2rv.rho./.eta.,
 for the frequencies of 124 and. 324 Hz, and the lowest .eta./.rho.-gas,
 which was propane in FIGS. 3 and 4; the 324 Hz frequency was chosen to be
 away about equally from higher harmonics of 50 and 60 Hz; and (4) the time
 constants to reach the steady .DELTA.p-period are longer for higher
 viscosity fluids and lower pressure gases. The time constant results are
 illustrated in FIG. 5.
 FIG. 3 is a graph of .DELTA.p, pressure amplitude in cm water column (WC)
 versus z or time in ms. This graph shows an oscillatory volume 17 and
 .DELTA.p of a leaky cavity 16 for three gases Ar, N.sub.2 and C.sub.3
 H.sub.8, as represented by curves 13, 14 and 15, respectively. The
 oscillatory volume 17 of cavity 16 is the result of electrical input to
 actuator 12. The pressure sensor changes, as indicated by curves 13, 14
 and 15, are detected by sensor 20. These plots were taken at 22.degree. C.
 and a pressure of 0.7 bar. The frequency, f, is 132 Hz. R.sub.v is one
 percent. The pressure equilibration times to 63% of the final .DELTA.p,
 .tau., are 0.40, 0.23 and 0.153 ms for the three gases, respectively. The
 cavity volume at rest, V.sub.co, is 0.15 cm.sup.3, and capillary length
 L.sub.c and radius r.sub.c are at 0.3 cm and 0.17 cm, respectively. The
 maximum Reynolds number, Re.sub.max, is 752.
 FIG. 4 is a graph of .DELTA.p, pressure amplitude in cm WC versus time in
 ms, z. This graph shows an oscillatory volume 17 and .DELTA.p of leaky
 cavity 16 for three gases Ar, N.sub.2 and C.sub.3 H.sub.8, as represented
 by curves 13, 14 and 15. These plots were taken at 22.degree. C. and a
 pressure of 0.7 bar. Frequency f is 323 Hz, which is about 2.447 times
 faster than f for FIG. 2. R.sub.v is 0.5%, .DELTA.z is 0.05 ms and the
 linear excitation, .tau., is 0.18 ms. V.sub.co and L.sub.c are the same as
 for FIG. 3. The capillary radius, r.sub.c, is larger at 0.2 mm. The
 maximum Reynolds number is 785. Note that the amplitudes of corresponding
 curves 13, 14 and 15 are about half of those for the curves in FIG. 3.
 However, the amplitude differences between curves 13, 14 and 15 appear to
 be more distinguishable in FIG. 4 than FIG. 3.
 FIG. 5 is a graph of .DELTA.p, pressure amplitude in cm WC versus z, time
 in ms. This graph shows an oscillatory volume 17 and .DELTA.p of leaky
 cavity 16 for N.sub.2 at three different absolute pressures, p.sub.a.
 Curve 14 is for N.sub.2 at 0.7 bar, curve 18 is for N.sub.2 at 1.0 bar and
 curve 19 is for N.sub.2 at 1.5 bar. This data was taken from device 10 at
 22.degree. C. and a frequency of 122 Hz, and R.sub.v is equal to 1%.
 V.sub.co is 0.15 cm.sup.3, L.sub.c is 0.3 cm, r.sub.c is 0.14 mm and
 Re.sub.max is at 846. These curves in FIG. 5 show that the
 .DELTA.p-equilibria reached are independent of absolute pressure. But they
 also show that the time constants to reach equilibrium, .tau., are
 pressure dependent: time constants of about 0.29, 0.23 and 0.194 ms were
 derived for the pressures of 0.7, 1.0 and 1.5 bar, respectively, which may
 also serve to determine absolute pressure after viscosity has been
 determined: p.sub.a.tau..sup.2.apprxeq.constant.
 To implement viscometer 10, one needs to consider volumetric drivers 12,
 pressure sensors 20 and possible system limitations. FIGS. 6 and 7 show
 additional embodiments 21 and 22, respectively, featuring interfaces or
 coupling devices 28 and 29 that may be baffles or at least partial
 barriers between drivers 12 and sensors 20. Barrier 28 or 29 is for
 preventing transport from driver 12 to sensor or detector 20 any physical
 energy (i.e., mechanical, electrical, and/or thermal) which may hinder,
 delay or inhibit the transfer of .DELTA.p information from driver 12 to
 detector 20. Baffle 28 or 29 may prevent such possible distortion or
 dilapidation by effecting diffusion, attenuation or other appropriately
 affecting function.
 Viscometer 21 has a baffle 28 situated in cavity 16 between actuator 12 and
 sensor 20. Baffle or diffuser plate 28 has apertures or holes 32 so that
 driver 12 can affect sensor 20 via the tested gas and apertures 32. The
 gas enters cavity 16 via capillary 11. A front view of diffuser plate 28
 is shown in FIG. 6a, taken along reference line A--A in FIG. 6. Referring,
 now, to FIGS. 7, 7a, and 7b, viscometer 22 has a baffle, bar or damping
 channel 29 situated in cavity 16 between actuator 12 and sensor 20. A
 passage or hole 30 provides for passage of the tested gas so that driver
 12 can affect sensor 20, for viscosity determination of the gas. The
 controlled cavity leak is though capillary opening 11.
 Actuators may be of several kinds. To generate constant rate of volumetric
 expansion or contraction, one can consider a bulk (piezoelectric) PZT
 expansion, PZT bimorph actuators as used in tweeters and electromagnetic
 speakers as drivers 12. Volumetric expansion of piezoelectric transducers
 is attractive because of the large forces involved, which would not be
 affected by changes in gas density, although the displacements would be
 very small. Both PZT expansion and PZT bimorph actuators would need to be
 temperature-compensated, which would not be required in the instance of
 electromagnetic speakers.
 Measured center deflections were made of a bimorph actuator (PZT/brass
 lamination by Mallory Sonalert of Indianapolis, Ind., at $0.55/each at a
 25-99 quantity) consisting of a 15 mm OD/0.11 mm-thick brass support+ a 10
 mm OD/0.11 mm-thick PZT+9 mm OD/0.02 mm-thick Ag electrode film. Under the
 rated voltage of .ltoreq.25 V, the center deflection was about 0.8
 .mu.m/VRMS. Center deflection measurements were also made of an
 electromagnetic speaker (BRT1209P-01, from Int'l. Components Co. of
 Melville, N.Y., at a price for a quantity of 25-99 at $0.60/each) and
 rated at a maximum input of 1-2 VDC and 10 mA maximum. The center
 displacement of this 50 .mu.m-thick speaker disc was found to be fairly
 linear with input voltage (see curve 31 of FIG. 8). The displacement
 amounted to about 7.5 .mu.m/volt from -6 to +6 VDC. This displacement per
 volt (V) is thus about 10 times greater than the one from a PZT bimorph.
 Support of the speaker membrane 33 merits some careful consideration (note
 membrane 33 of designs 10 and 21 of FIGS. 2 and 6, respectively).
 Referring to FIG. 6, if chamber or cavity 16 is sealed to eliminate leaks
 across speaker membrane 33, then the side opposite to its V.sub.c side
 needs to be opened to an ambient/external fluid to avoid anomalous effects
 when the ambient pressure or temperature changes. If the volume on the
 membrane 33 side opposite of the cavity 16 side is not sealed leak-tight
 from cavity 16 (as in BRT1206P-01), then it competes with capillary 11
 and/or the leak functions as an orifice.
 Sensors may be of several kinds. Another function needed for viscometer 10
 is the .DELTA.p sensor 20 between ambient and V.sub.c. Microphones may be
 the lowest-cost choice to meet that need. The ideal sensor 20 would be a
 rigid microphone, i.e., lack of influence on V.sub.c via deformation, with
 a frequency-independent output. An electret microphone (by Panasonic of
 Secaucus, N.J., or Gentex of Carbondale, Pa., with a deformable membrane)
 was used in phase lag measurements. It provided an unamplified output of
 200 mV for about 50-60 Hz pressure variations of .ltoreq.1.2 cm of WC. The
 noise level was at 2 mV, which is equivalent to (1.2/1000)
 (2/200).multidot.10.sup.6 =12 ppm of pressure change in cavity 16, or
 0.012 cm of H.sub.2 O.
 An electromagnetic speaker (BRT1209P-01 by Int'l. Components Co.), was
 tested as a sensor 20, back-to-back to a similar unit serving as a driver
 12. However, their magnetic fields interfered, and thus shielding would be
 needed for good results.
 A MICRO SWITCH 24PC pressure sensor chip, from Honeywell Inc. of Freeport,
 Ill., mounted and sealed on a TO5 header (over a center hole to avoid any
 back-pressure build-up) served well as a sensor 20, in operation with one
 of the BRT1206P-01 speakers serving as an actuator 12. It was verified
 that the sensor 20 output followed the shape of the driver 12 excitation
 (sine, square or saw-tooth shape) with capillary 11 plugged. After
 unplugging the capillary, a balance between rate of cavity 16 volume
 change and capillary 11 flow was achieved, demonstrating the invention.
 During this dynamic balance, the established .DELTA.p was representative
 of the fluid viscosity. The length to diameter ratio of the capillary 11
 tube should be greater than four. The capillary 11 inlet should be smooth
 internally and at the edges to minimize turbulence.
 Experiments with the same driver 12, but a piezoelectric speaker serving as
 sensor 20, yielded a pressure-independent relationship that was as
 expected theoretically, .eta..apprxeq..DELTA.Gp.sup.0 (where
 .DELTA.G=pressure sensor output and p=absolute pressure) within a .+-.5%
 scope measurement error. This viscosity sensor consisted of simply
 epoxying an available $0.60 driver 12 and $0.55 sensor 20 back-to-back,
 and being operated at 60 Hz. In summary, very low-cost, off-the-shelf
 components show that fabrication of the present viscometer 10, 21 or 22
 can be very cost-effective.
 Viscometer 10 may have several limitations. First, one possible limitation
 is head pressure due to flow reversal. As the frequency of a flow driver
 increases, the inertial pressure generated at each reciprocating flow
 reversal increases. In order to stay away from such effects, one may
 calculate the frequency at which the capillary 11 pressure drop,
 .DELTA.p.sub.c, would equal the inertial pressure drop, .DELTA.p.sub.i, as
 shown by equation (4).
EQU .DELTA.p.sub.c =(.DELTA.V.sub.c f/2).multidot.8.eta.L.sub.c
 /(.pi.r.sub.c.sup.4) and .DELTA.p.sub.i =2.rho.v.sup.2
 /2=.pi.(.DELTA.V.sub.c f/(2.rho.r.sub.c.sup.2)).sup.2 (4)
 With the assumption that the steady volume flow in each direction,
 .DELTA.V.multidot.f/2, generates an average velocity, v=.DELTA.V.sub.c
 f/(2.pi.r.sub.c.sup.2), and an inertial pressure pulse upon reversal of
 2.rho.v.sup.2 /2, then for .DELTA.p.sub.c =.DELTA.p.sub.i, one gets
EQU 8.eta.L.sub.c =.rho..DELTA.V.sub.c f/(2.pi.) and f=16.pi..eta.L.sub.c
 /(.rho..DELTA.V.sub.c). (5)
 For .eta.=0.000178 g/(cm.multidot.s) or .nu.=.eta./.rho.0.153 cm.sup.2 /s
 for N.sub.2 at 20.degree. C. and 1 bar, L.sub.c =1 cm and V.sub.c =0.0001
 cm.sup.3, one gets f=77 kHz. For most applications, one will therefore be
 able to neglect this effect because it is generally small at low
 frequencies and it only occurs at each flow reversal.
 Second, the appearance of turbulence in a capillary 11 tube needs to be
 avoided. For the above example, with r=0.008 cm, one gets Re=2r.sub.c
 v/.nu.=1909 for N.sub.2 (but 6935 for propane). However, appropriately
 combined changes in r.sub.c, .DELTA.V.sub.c and f, provide a laminar range
 that is wide indeed. To minimize the onset of turbulence, the edges at the
 ends of capillary 11 should be made smooth.
 Third, The stability of the actuator, .DELTA.p, sensor and controlled leak
 (no plugging with time) are clearly critical to long-term, reliable
 service. The way to maintain stable displacement via circuitry, stable
 leak via a multiplicity of leaks in porous plate, and self-checking the
 sensor for proper operation and accuracy are recommended to overcome
 stability limitations.
 The components of the quasi-static viscometer 10 and their costs, include a
 saw-tooth generator at $0.4, a speaker (10-15 mm diameter) at $0.22-0.25
 (from DAI Ltd.), a microphone (6-10 mm diameter) at $0.22-0.25, a
 microphone amplifier and an analog-to-digital converter (A/D) for a
 digital output, at about $2.00, one or more 0.2-0.4 mm holes/capillaries
 of 3-6 mm in length, or equivalent controlled leaks made of porous
 materials, at $0.10, plus assembly, calibration and miscellaneous
 materials at about $3.00. Thus, the total cost of the sensor may be less
 than $6.00, so that its use and business potential is great.
 Referring to FIG. 9, features of the invention involve the combination of
 capillary 11 flow (known to be proportional to viscosity) to or from a
 cavity 16, an electromechanical fluid displacer/actuator 12 and a
 saw-tooth electronic drive 34 to enable the .DELTA.p in cavity 16 to
 stabilize during at least one of the two periods of each AC actuation
 cycle. The measurement of the established .DELTA.p at the end of (at least
 one or) each saw-tooth period is an indication of the desired viscosity.
 It is the .DELTA.p (but not .DELTA.p/p.sub.a) which is largely independent
 of absolute pressure, p.sub.a. .DELTA.p is sensed by sensor 20, which
 provides an output to an analog-to-digital converter 35, as shown in FIG.
 9. The digital output goes to processor 36 for processing. Processor 36
 has a digital or analog output that may be provided to some apparatus such
 as a combustion control or regulator. Also, an output is provided to
 indicator 37 that may provide readable information about the directly
 measured viscosity, as well as correlated properties such as oxygen demand
 or other parameters of the tested gas. The output of sensor 20, instead,
 may go directly to an anatlog indicator, processor or interface.
 An embodiment may have an actuator 12 that is a low-voltage,
 electromagnetic earphone speaker, rather than a piezo-electric tweeter or
 electro-static speaker, and in which the .DELTA.p sensor 20 is a
 microphone based on either piezo-electric (preferred), piezo-resistive,
 electro-magnetic, electret, carbon-contact or capacitance effects. The
 present invention may sense not only viscosity but also absolute pressure
 by further processing the determined viscosity and the initial time
 constant or phase lag between actuator 12 input and sensor 20 output, as
 shown in FIGS. 3-5.
 Viscometers 10, 21, and 22, of FIGS. 2, 6, and 7, respectively, have
 advantages over related art viscometers based on vibrating wires and
 quartz crystal oscillators, having pressure decay&lt;or phase lags, because
 they have no dependence on absolute pressure. Viscometers 10, 21 and 22
 also have advantages over viscometers based on thermal excitation, because
 they has no dependence on thermal conductivity or specific heat of the
 fluid being measured. They also has advantages over traditional
 viscometers based on capillary flow driven by a constant source of
 pressurized fluid, or based on timing of the fall of an object, in that
 there is no need for a costly source of constant pressure fluid, constant
 gravity magnitude and direction, or equipment for providing and measuring
 the falling object. The present viscometer are not sensitive to their
 mounting orientations. As noted above, besides viscosity, the present
 devices can also be used to sense absolute pressure by using the initial
 time constant or phase lag between actuator input and sensor output, which
 is a feature not available from related art viscometers.
 Other embodiments and variants of the present invention, not disclosed
 here, are covered by the claims and only limited in scope by the claims,
 which includes all equivalents thereof.