US2984784A - Moisture testing instrument - Google Patents

Description

May 16, 1961 c. A. MEAD 2,984,784

MOISTURE TESTING INSTRUMENT Filed May 6, 1958 "A A 5a I/VVJ/VTOZ (dryer A Mad United States Patent 2,984,784 MOISTURE TESTING INSTRUMENT Carver A. Mead, Pasadena, Calif., assignor to Moisture Register Company, Alhambra, Calif., a corporation of California Filed May 6, 1958, Ser. No. 733,333

3 Claims. (Cl. 324-65) My invention relates generally to moisture measuring devices and more particularly to a new and useful electrical indicating device for measuring the moisture content of lumber and the like. a

Lumber which is saturated with moisture has a much lower electrical resistance than lumber which is entirely dry with zero percent moisture content. The resistance of wood decreases generally with increasing moisture content and this is true for most other materials which resemble wood thatcan absorb and retain moisture. Plaster is such an example. The difiiculty encountered with material such as lumber is that the moisture is mostly contained beneath a hard and dry surface, although a significant moisture content is usually evenly distributed internally throughout a piece of lumber. The moisture content of green lumber decreases as the lumber is cured and becomes ready for general use.

It is an object of my invention to provide means for accurately measuring the internal moisture content of lumber and the like. I i Another object of the inventionis to provide a moisture content measuring device in which an accurate indication is presented and there are no variations in correct reading over awide measuringrang'e. t

Anotherobject of the invention is to provide a multiple range moisture content measuring device wherein clear indications are available throughout the entire range of moisture content measurement. t t

A further object of my invention is to provide a compact moisture content measuring device of simple yet reliable construction andcircuitry. I

Briefly, and in'gen'eral terms,the foregoing and other ancilliary objects'of my invention 'are' preferably accomplished by providing a probe having four sharp needles arranged in a rectangular pattern and connected electrically to work with an 'electiical indicating meten The two end pairs of needles are electrically connected tog'ether togive two parallel" resistance paths through a piece of lumber whenthe needles are plungered into it, an average internal lumber resistance being obtained between the' two end pairs of needles. This average lumber resistance forms part of a variable resist ance voltage divider in the electrical meter and the output of the voltage divider'is provided as an input signal to an electrometer tube connected as a cathode follower having a loadresistance which is high compared to l/g where g is the mutual conductance of the tube. The

cathode follower output is applied to one input of a transistor differential amplifier'having the other input connected to an-adjusta-ble potentiometer tap. The output of 'the transistor diiferential'amplifier is connectedto a galvanometerwhich is suitably calibrated to read percent moisture content-of wood in three ranges of increasing moisture content. The input impedance of the differential amplifier is high compared to the cathode follower load resistance. I

My invention includes other objects and features, some iof which together with theforegoing; will be set forth in the following detailed description of a. preferred embodiment of my invention, and the invention will be more fully understood by reading the description with joint reference to the single attached figure in which a circuit diagram of a preferred electrical indicating meter and a perspective of a preferred probe structure used with the electrical meter, is shown to fully describe my invention.

A preferred embodiment of my invention is illustrated in the single figure of the attached drawing. The moisture content of a piece of lumber L is to be measured with my invention. The invention generally comprises a probe 1 and an electrical meter 2. The probe I preferably includes a sturdy handle 3 which is firmly fastened to the top surface of a cubical block 4 made of insulating material such as transparent plastic, for example. Four slender and sharp steel needles 5a, 5b, 5c and 5d extend from pedestal bases which are attached securely to the lower surface of the cubical block 4 in a rectangular pattern. The bases of the steel needles 5a, 5b, 5c and 5d are threaded and can be screwed into tapped holes in the bottom of block 4 located at respective corners of a rectangle. The points of the steel needles describe a rectangle 1 inch wide by 1% inches long, for example, and the needles are very similar to ordinary steel phonograph needles. Two jumper strips 6a and 611 each having a pair of end holes coinciding with the end pairs of tapped holes are placed against the lower surface of the cubical block 4 beforethe steel needles are screwed to the cubical block 4. The threaded bases of the steel needles are passed through the end holes of the jumper strips and screwed tightly to the cubical block 4 such that the jumper strips 6a and 6b each electrically connect two end needles together. Leads 7a and 7b are connected respectively to jumper strips 6a and 6b as indicated, and lead 7a is connected back to terminal 8a while lead 7b is connected back to terminal 8b of the electrical meter 2. Thus, steel needles 5a and 5c are electrically tied together by jumper strip 6a, and steel needles 5b and 5d are electrically tied together by jumper strip 6b. Two parallel electrical paths (shortest) through the piece of lumber L are provided between steel needles 5a and 5b, and between steel needles 5c and 5d, respectively. An average lumber resistance R is eifectively connected between terminals 8a and 8b.

The average lumber (wood) resistance R for the rectangular probe needle pattern described can vary anywhere between megohms to 20 kilohms, approximately, depending upon the (increasing) moisture content of the lumber. Terminal 8a is connected to a lead 9 which is connected to the positive terminal of a voltage source E1 through switch Sla, and terminal 8b is connected to the pole of a three position selector switch S2. The pole of selector switch S2 can be selectively connected to the free ends of three resistors R1, R2 or R3,

the other ends of which are connected back to the negative terminal of voltage source E1. Selector switch S2 is a range selector switch for the electrical meter 2, for measuring increasing ranges of lumber moisture content. Terminal 8]; and the pole of selector switch S2 are connected to the control grid of triode connected electron tube V1, the anode and screen grid of which are both connected to lead 9. The filament of tube V1 is connected to a filament voltage source E2 through switch Slb which can be ganged to switch Sla. One side of the filament of tubeVl is connected to the negative terminal of voltage source E1 through load resistor R4. The average lumber resistance R in series with one of the resistors R1, R2 or R3 form a voltage divider, the output of which is impressed on the control grid of tube V1. Tube V1 is connected as a cathode follower having a load resistor R4.

The measurement of lumber moisturecontentclearly involves high resistances, and since such a circuit supplies the grid signal voltage, grid current becomes significant even where the grid bias voltageisnegative. This istrue because the control gridto cathode resistancecf; the. tube V1 is not then large in comparison withthe resistance of the voltage divider circuit that supplies the grid signal voltage. At the same time it is desirable that variation of mutual conductance g between tubes does not affect resultant accuracy of indication for meter 2. Thus, tube V1 is preferably an electrometer tube having a low value of g and usedin a cathode follower circuit having a load resistancehigh comparedto l/g theappIOXimate output impedance of the tube V1 alone, such that-gain is nominally one andchange in .output voltage is made substantially equal to the corresponding .change in input.voltage applied to thecontrolgridof the, tube V1. Ifthe mutual conductance g of they tube V1. changes inthe cathode follower. circuit where the load impedance R4. is large compared to l/g suchthatgain is nearly unity, the gain correspondingly only changes (decreases) negligibly. With a large. load impedance R4,and gain almost independent of variation of g theoutput signal follows the input signal linearly since, the tube current only changes very little in the large load impedance. The output of thecathode follower V1 is connected directly to the base of trasistor T1 which is connected with transistor T2 in a conventional difierential amplifier circuit. The input impedance of the differential amplifier circuit is selected to be comparatively much higher than (approximately 10 times) the cathode follower load resistance R4 and can be neglected from consideration of .external impedance connected across the output terminals of the tube V1.

The collectors of transistors. T1- and.T2 are both connected directly to lead9. and the emitters are. respectively connected through resistors R5 andR6 to .the negative terminal of voltage source E1. A galvanometer. G in series with a high resistor R7 are connected between the emitters of the two transistors T1 and T2. The, base of transistor T2 is connected. to an. adjust-abletap. of resistor R8 which is connected inseries with resistors R9 and R10 to form a potentiometer. The .free end of resistor R9 is connected to lead.9 .and the freeendofresistor R10 is connected back to the negative terminal of .voltage source E1.

The impedance of the galvanometer circuit including resistor R7 is high compared to the.dynamic..output .impedance of the transistor differential amplifier stages. This arrangement provides a. galvanometer. indication whichis substantially independent of the transistor parameters (variation) since the. transistor common emitter current gain is large compared to, forcxample, 1.

Operation of the instrumentis'begun withprobelfree and clear of lumber L or any. other similar material to be measured for moisture content. Switches. Sin and Slb are first closed, and selector switch S2 can. be switched to the highest range resistor R3. The tapof resistor R8 is adjusted so that thecurrent flow. through transistors T1 and T2 are substantially equal and galvanometer G reads zero. The probeneedles are then pushed into lumber L. If the wood is very hard, the steel needles can be forced almost completely into the material by striking the top of handle 3 witha hammer or othersuitable implement. The output of the voltage divider thus formed is impressed on the control-grid of tube VLcausing tube current to increase if moisture content is sufficiently high. Base voltage of transistor T1 is increased.

suitably calibrated in percent moisture for wood, and;

other materials such as plaster, as desired. If a reading is not first obtained, the selector switch S2 "can be.

switched, progressively to lower moisture content ranges (R2, R1) until a reading is obtained. It is noted that the circuit is unique, such that only one adjustment of R8 is necessary for all ranges in the usual operation of the invention.

Values and types ofcomponents providing a highly satisfactory working model of my invention having three ranges ofmoisture content readings forv wood are listed below as an operating example. This listing is not intended to limit or restrict the breadthandscppe of the invention. Switching of S2 from the resistorsRl, R2 and R3, in sequence, provide moisture content reading ranges of 6 to 13%, 12 to 21% and 20 to 70%, respectively. Scale divisions are. nonlinear, the first (lowest) moisture content scale begins with wide divisions which generally decrease in angular width progressively through all three scales in a semi-exponential manner. The center portion of each scale, moreover, has wider divisions than at the ends. of thescale. This effect, how ever, is. due to characteristics of-the voltage, divider-in the circuit.

R1 1000 megohms.

R2 6.3 megohms.

R3 47 kilohms.

R4 62 kilohms.

R5 20kilohms.

R6 20 kilohms.

R7 39 kilohms.

R8 Q. 20 kilohms.

R9- 68 kilohms.

R10 39 kilohms.

E1 22.5 volts (mercury cells). E2 1.34 volts (mercury cells). V1 5886;

G -20 0-micro a mperes.

A novel and useful multiple range device which is compact and extremely accurate for measuring the internal moisture content of lumber is thus provided. The invention is not limited to use in measuring moisture content of lumber since the indicating meter can be easily calibrated for diiferentmaterials. No fluctuations or errors in readings occur with this circuit, as maybe found in other instruments of comparablecomplexity. An electrometer tube drawing an ex treme ly. low grid current in a cathode follower circuit; having ahigh load impedance compared to 1/g of thetube results in no variations in reading, and bias.stabilityfis'verygood.

In order to comply with thestatute, the invention has been described in language moreor less-specific .as to structural features. It is tofbe understood, however, that my invention is not limited to the specific features shown, but that. the means and construction herein ;disclosed comprise a preferred form of putting the invention into effect, and the invention is therefore claimedinany of its forms or modifications within the legitimate and valid scope of the appended claims.

I claim:

1. A. multiple range. instrument: o measuring the moisture content of lumber and the likehaving; a hard and dry surface, comprising: means for establishing a resistive path internally in the lumber, said.-patl 1 having first and second, ends; a plurality. ofrange resistors of progressively increasing resistances; switching means for selectively connecting a range resistorv in ,series to the second .end of said internal resistive -.path,to form a dividernetwork; a. source. of 'voltagepconnected across said divider network; a cathode.follower-including an electrometer tube having .ananode, control grid and cathode, and. aload resistor .having. .an impedance high compared to the reciprocal of the mutualnconductance of said tube, the cathode ..being....connected.;to;.one. .end of said load resistor, the anodelbeing.ccnnectediito the first end of said resistive path and corresponding end of the divider network, the other end of said load resistor being connected to the other end of said divider network, and the control grid being connected to the second end of said resistive path whereby an output signal obtained across said selected range resistor is applied to said cathode follower; a transistor emitter follower differential amplifier including first and second transistors each having a collector, base and emitter, a first resistor connected on one end to the emitter of said first transistor and a second resistor connected on one end to the emitter of said second transistor, the collectors being connected to the anode connected end of said source of voltage and the other ends of said first and second resistors being connected to the other end of said source of voltage, said difierential amplifier having a first input between the base of said first transistor and the other end of said first resistor, a second input between the base of said second transistor and the other end of said second resistor, and an output between the emitters of said transistors, the first input being connected to the load resistor of said cathode follower and having a high input impedance compared to the load impedance of said cathode follower; a potentiometer connected to said source of voltage and having an adjustable output signal applied to the second input of said difierential amplifier; a multiple range meter calibrated to indicate the moisture content of the lumber; and a resistor connected in series to said meter to form a series combination which is connected to the output of said difierential amplifier, the impedance of the meter and resistor series combination being high compared to dynamic output impedance of said differential amplifier transistor stages.

2. A multiple range instrument for measuring the moisture content of lumber and the like having a hard and dry surface, comprising: means for establishing a resistive path internally in the lumber including a probe having at least two electrically conductive needles that can be pushed into the lumber, said two needles being respectively first and second ends of the internal resistive path; switching means for selectively connecting a range resistor in series to the second end of said internal resistive path to form a divider network; a source of voltage connected across said divider network; a cathode follower including an electrometer tube having an anode, control grid and cathode, and a load resistor having an impedance high compared to the reciprocal of the mutual conductance of said tube, the cathode being connected to one end of said load resistor, the anode being connected to the first end of said resistive path and corresponding end of the divider network, the other end of said load resistor being connected to the other end of said divider network, and the control grid being connected to the second end of said resistive path whereby an output signal obtained across said selected range resistor is applied to said cathode follower; a transistor emitter follower differential amplifier having first and second inputs and an output, the first input of said differential amplifier being connected to the load resistor of said cathode follower and having a high input impedance compared to the load impedance of said cathode follower; a potentiometer connected to said source of voltage, an output signal being obtained from said potentiometer and applied to the second input of said differential amplifier; and a meter calibrated to indicate the moisture content of the lumber connected to the output of said differential amplifier.

3. A multiple range instrument for measuring the moisture content of lumber and the like having a hard and dry surface, comprising: means for establishing a resistive path internally in the lumber including a probe having a normally upright handle and at least four dependent, electrically conductive needles arranged in a rectangular pattern wherein end needles are electrically connected together to establish at least two parallel electrical paths between opposite end needles whereby an average internal lumber resistance is obtained between end pairs of needles when the needles of said probe are pushed into the lumber, said handle being centrally positioned with respect to the rectangular needle pattern and said resistive path having first and second ends; a plurality of range resistors of progressively increasing resistances; switching means for selectively connecting a range resistor in series to the second end of said internal resistive path to form a divider network; a source of voltage connected across said divider network; a cathode follower including an electrometer tube having an anode, control grid and cathode, and a load resistor having an impedance high compared to the reciprocal of the mutual conductance of said tube, the cathode being connected to one end of said load resistor, the anode being connected to the first end of said resistive path and corresponding end of the divider network, the other end of said load resistor being connected to the other end of said divider network, and the control grid being connected to the second end of said resistive path whereby an output signal obtained across said selected range resistor is applied to said cathode follower; a transistor emitter follower differential amplifier including first and second transistors each having a collector, base and emitter, a first resistor connected on one end to the emitter of said first transistor and a second resistor connected on one end to the emitter of said second transistor, the collectors being connected to the anode connected end of said source of voltage and the other ends of said rfirst and second resistors being connected to the other end of said source of voltage, said differential amplifier having a first input between the base of said first transistor and the other end of said first resistor, a second input between the base of said second transistor and the other end of said second resistor, and an output between the emitters of said transistors, the first input being connected to the load resistor of said cathode follower and having a high input impedance compared to the load impedance of said cathode follower; a potentiometer connected to said source of voltage and having an ad justable output signal applied to the second input of said differential amplifier; a multiple range meter calibrated to indicate the moisture content of the lumber; and a resistor connected in series to said meter to form a series combination which is connected to the output of said difierential amplifier, the impedance of the meter and resistor series combination being high compared to dynamic output impedance of said differential amplifier transistor stages.

References Cited in the file of this patent UNITED STATES PATENTS 2,547,650 McCool Apr. 3, 1951 2,558,945 Fritzinger July 3, 1951 2,587,771 Schoenbaum et a1. Mar. 4, 1952 2,659,048 Zabel Nov. 10, 1953 2,752,566 Quinton Jan. 26, 1956 2,771,583 Bloch Nov. 20, 1956 2,821,682 Bauer Jan. 28, 1958 2,841,765 Harrold July 1, 1958 2,870,404 Oxley Ian. 20, 1959

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