US2240955A - High frequency wattmeter - Google Patents

Description

May 6, 1941.

E. MITTELna/Julu4 HIGH FREQUENCY WATTMETER Filed April 22, 1938 fag! URI Il U 25 Patented May 6, 1941 HIGH FREQUENCY WATTMETER Eugen Mittelmann, Vienna, Germany Application April 22, 1938, Serial No. 203,661 In Austria April 26, 1937 4 Claims.

This invention relates to a method of and apparatus for the measurement of power which is absorbedin the output circuit of a high frequency generator, and while my invention is applicable to the measurement or determination of the power which is absorbed or utilized in the output circuit of whatever purpose for which said output is utilized, the invention has special application to .the determination of the power absorbed by thel patient in connection with the treatment thereof by short wave or high frequency therapy.

The object of my invention is to provide a method of and apparatus for the determination of a component of the total power absorbed in an oscillatory circuit, which component can be referred to as that portion of the total power which is useful for a specic purpose as distinguished from that which constitutes losses in the apparatus itself or due to other factors.

In the application of my invention to short wave therapy where only a fraction of the total high frequency power is absorbed by the patient under treatment, the remamder constituting radiation losses, leakage losses, etc., my object is to measure the power absorbed by the patient independently of total power in the patient's circuit, whereby it can be accurately known what component or factor of the total power is utilized in the treatment of the patient.

My invention will be readily understood by reference to the accompanying drawing wherein is illustrated diagrammaticaliy the output circuit of a short wave or high frequency transmitter or generator embodying my invention.

Referring to the accompanying drawing,

Fig. 1 is a diagrammatic view of the secondary or output circuit of a high frequency generator, showing the instrumentalities which I utilize in carrying out my invention; and

Fig. 2 is a similar view to Fig. 1 in which a modified form of instrumentality is utilized.

In both of the figures of Vthe drawing, A represents a source of high frequency oscillations coupled to a secondary or output circuit B, by means of a transformer C, all in a manner well known in the art. The source of high frequency A may be of any type well known and sui-table for the purpose since my invention is not concerned with said apparatus as such.

The oscillatory output circuit B is shown as including the two conductors I and 2 extending from the secondary of the transformer C and terminating in sui-table electrodes 5 which are spaced apart, and between which the object 6 is positioned, which object 6 represents the load which is to be measured as to its power absorption.

If the high frequency is to be applied as treatment to a patient, then it is to be understood that the object 6 is intended to represent the patients body or some portion thereof positioned between the electrodes, as is well known and generally understood in this art. The output circuit in apparatus of this character generally `has a variable condenser 4 bridged across the deflection of the pointer is proportional to the square of the voltage or current. Any standard or well known device of this type may be utilized. That which is shown in Fig. l is of the hot wire thermocoupie and galvanometer type, while that shown in Fig. 2 is of the dynamometer type having a moving coil and an exciting coil and utilized in connection with an electronic tube operating in the square portion of its characteristic.

In Fig. 1 the thermocoupie 9 is bridged across the output circuit conductors I and 2 by means of conductors 1-1, which include the condensers 8 8.

The thermocoupie 9 is connected to the terminals of the galvanometer I2 by means of conductors I0-III in the usual manner in instruments of this character. The galvanometer I2 has a pointer I3 which moves over a calibrated dial I2a, the deflection of the pointer being proportional to the square of the voltage in the circuit including the thermocoupie. The dial is calibrated in equal divisions and in the present instance I have shown the dial as calibrated in divisions from 0 to 100. v

In accordance with my invention, I provide a shunt circuit around the galvanometer, which shunt includes a variable resistance I 4 for the purpose of varying the sensitivity of the galvanometer. This shunt includes the conductors I5 which are connected to lthe terminals of the galvanometer and one of which is connected to a terminal of the resistance I4. The other conductor is connected to a contact member I6 movable over the resistance I4. Alongside the path of the contact member I6 is a scale or dial I1 which is calibrated preferably in divisions from 0-100, corresponding to the dial of the galvanometer, and the contact member carries a pointer I8 which cooperates with the dial to indicate the resistance value by its position relative to the dial.

Since it is desired that the shunt resistance I4 affect the sensitivity of the galvanometer in certain mathematical ratios, as will hereinafter be explained, it is necessary in the first instance to calibrate the dial I'I of `the resistance in accordance with that ratio rather than have the dial II read true values of the resistance Il. It is adjusted so that regardless of what the resistance may happen to be, that resistance is correct for adjusting the galvanometer to read a value for power absorbed by the body in the cirsuit when the reading of the galvanometer pointer I3 on its dial I2a and the reading of the dial I1 are the same under certain preliminary conditions defining a typical ratio for a given electrode condition. It will be seen, from later discusson, that the ohmage gradient of the resistance is conveniently constructed so as to make the division of the dial I1 linear.

The principle upon which this measurement of the power absorption of the object is accomplished by the above described apparatus is explained by the following equations.

The secondary circuit B of Fig. 1 may be denoted byl two equivalent circuits: (1) a simple coil in parallel with a variable condenser and a single resistance R0 and (2) the same circuit with a second resistance R1 in parallel with Ro.

The iirst equivalent circuit represents the secondary circuit B with no load,or the object E not in position between the electrodes .fi- 5. Under these conditions, certain reactive and resistive elements are introduced into the circuit B and they make up circuit losses, i. e., leakage losses, radiation losses, all absorptive of power from the circuit at no load for any given electrodes. Since the circuit is at resonance and all reactive factors are balanced, lthe total power absorbed at no load is a function of the constant Ro (for any given electrodes), Ro representing the equivalent parallel circuit loss resistance. The resonant voltage developed in the secondary circuit is eo (no load), and, of course, the relation between eo and Ro is:

eo=k Rn (1) where 7c is a comparative factor. In other words, at resonance, the voltage is proportional tothe parallel circuit resistance or under no loa-d conditions to the parallel equivalent circuit loss resistance.

With the introduction of a body into the circuit and between the electrodes, more reactive and resistive elements are coupled cinto the circuit. The circuit is once more tuned to resonance, resulting in a new voltage e1 and compensating for reactive effect of the newly added elements. Hence in circuit 2 no new reactive elements are present. This leaves the'resistive element introduced by Vthe body 6. In effect, we have added the so-called equivalent parallel patient resistance to the circuit denoted by R1.

Again, we see that the voltage e1 is proportional to the combined parallel resistance, or

RRi Ro +R1 where, of course, K is a comparative factor.

From the above two equations, it will be seen that R1=f(e1) or R1 is purely a function of e1 (4) Keeping .this in mind, and recalling that the power absorbed by R1 is that which We desire, and that it depends upon the voltage of the circuit, we see that if we can measure the voltage at resonance with a load in position, and if we know the resistance of the load (that is, the equivalent parallel Vresistance effect which the load causes to be added to the circuit), we can determine the power absorbed by the load by the well known power equation:

where, if E is in volts, R in ohms, P is given in watts.

Since R1 is a pure function of e1 for any condition of no load, i. e., same electrode leads, etc., we can determine a proportion corresponding to a solution of Equation 3 above involving initial no load voltage eo and any load voltage e1 (for the same power input, of course). According to Equation 3, R1 is going to be proportional to the damping of voltage of said circuit caused by the insertion of a load, i. e., the ratio of load voltage e1 to the difference between no load voltage eo and load voltage e1. Clearly, since Ithe equation holds true for any ratio, R1 will remain a pure function of e1 (any voltage) as explained above, and hence, constant for any given load, the no load conditions being set.

Every change of power will vcau-se a change of both no load and load voltage, so that conditions of vEquation 3 are always met. Hence, if we set our value corresponding to factor introduced in the power Equation 5 by R1 to satisfy the law of Equation 3 upon the resistance I4 (for the purpose of operating upon the galvanometer reading in accordance with Equation 5 above, as will be hereinafter explained) we can forget about it for any power delivered to the output circuit. The meter, if calibrated in power units, will read any power absorbed by the same load for the entire range of power which the transmitter is capable of producing or within the range which the meter can read.

It is understood that the value R1 can never be actually measured or known, and that the values e1 or eo need not be metered. All that is necessary is to find the ratios in a manner to be described hereinafter and to set this up as a factor inuencing the value of resistance I4. The import of this is that the measurement of power absorbed by the body 8 becomes a simple expedient. By merely determining the ratio represented by the constant value Ri from any damping caused in the voltage by the addition of/ where P1 is the power labsorbed by the body 6/ at a power input such as to give any circuit voltage E, R1 representing the equivalent loss resistance which will be constant for that particular load regardless of power absorbed.

Practically, in my invention all measurements are made by means of a hot wire thermocouple circuit or any similar square law measuring device.

In the galvanometer l2, thedeiiection represented by the pointer I3 thereof is proportional to the voltage measured and the sensitivity of the galvanometer or --KgEgo' (7) where equals deflection and Kg equals a constant factor depending upon physical constance of the galvanometer; Eg equals/voltage of the galvanometer circuit; r equals sensitivity of the galvanometer. Since we know that R1 is a pure function of e1, and is a constant quantity representing a percentage voltage damping of circuit caused by the insertion of a known load, if we adjust the sensitivity a for that given body, or load, to be proportional to the reciprocal of the equivalent parallel resistance of that body, or

where K' is of course a comparative factor. Then, substituting our R1 values back into Equation 7, we get The galvanometer, we know, reads a deflection proportional to the circuit voltage squared; hence, Eg is proportional to E2 or E2 K Rl Comparing this with Equation 6 above, we immediately see that the galvanometer now reads a quantity proportional to P1, the power absorbed by the load. l

All that remains, therefore, is the calibration of the galvanomefter to read exactly in watts, and the adjustment of the scale I1 of the resistance I4 to cause the sensitivity of the galvanometer to be v for any given E.

The mechanical procedure of determining the range of ohmage for the resistance I4, is no part of this inventionjgffbut suffice it to say that the equation for shunting galvanometers to alter the sensitivity thereof may be used for calculation; that is, for any desired value of a, or

theresistance of the shunt could be determined from the following equation:

R +R. r--RI (ll) where Rg is the resistance of the galvanometer, Re is the resistance of the shunt, a is the sensitivity factor. Y

Since R1 is purely dependent on e1, therefore, the factor introduced by the sensivity adjustment of the galvanometer can be pure operation upon the voltage read. The setting of the resistance will be controlled by the voltage damping in the circuit for any ratio fio-81 (see Equation 3). This becomes understandableK when comparison is made with Equation 1|1 in which, since the shunt resistance Rs is being decreased (see Fig. 1 of drawing), the value R1 is exactly proportional to a quantity RR, Hence the required sensitivity may be controlled by variation of the shunt resistance I4 in 'accordance with the damping of voltage for any two values eo, e1.

Advantage is taken of this fact in calibration of the scale I1 as will be described.

The procedure in Calibrating the resistance I4 is as follows:

The output circuit loss without a load or object between the electrodes is first determined in terms of resonance voltage eo, corresponding to the no-load loss resistance Ru. This is done by tuning the output circuit to resonance by means of the variable condenser 4. The galvanometer will indicate a maximum Voltage, which by suitable means of output control, for instance by varying the anode voltage of the oscillating tubes of the generator, is adjusted so that the pointer I3 indicates the full scale reading, which for illustration We will assume to be on the scale I2a. This gives a reading on the meter corresponding to en. The value of Rs is now maximum and the scale I1 is caused to read zero. Next a load is placed between the electrodes, which for illustration we assume to be one which will absorb say 60 watts of power, and the circuit is again tuned to resonance by the condenser 4. The galvanome'ter pointer will then indicate a lower reading on the dial, say 25. The change in reading now corresponds to the damping of the circuit voltage signifying eo-ei and gives the crteron upon which a calibration of the resistance I4 may be based. We now proceed to adjust resistance I4 until the pointer I3 of the galvanometer indicates 60 on the scale I2a, thus making the galvanometer reading correspond to that which we know is to be the power absorption constant. Hence, any power can now be supplied to the load by varyingthe voltage E and the cor' rect value thereof will be'read on the dial I2a..

Hav-ing now made this adjustment, the scale I'I is marked at the position at which the pointer I8 stands. However, the numeral marked on the scale l1 rls 25 so that it will correspond to the galvanometer reading of 25 which was indicated as corresponding to the value of e1 before the resistance was adjusted. This setting corresponds to the damping of the circuit B caused by the additional introduction of the equivalent loss resistance R1. The instrument will read the correct value of power absorption for any arbitrary value of the high frequency voltage across the circuit B which mightrbe possibly adjusted by any known means of output control, for instance by varying the anode voltage of the oscillating tubes oi the generator A. For any load causing the same percentage reduction of the resonance voltage eo of the unloaded circuitl B, there is an infinite number of possible values of power absorption correctly indicated by the instrument, corresponding to the amount of high frequency energy whichmight be administered by means of an arbitrary output control. Having now determined one value of the parallel resistance I4 which changes the galvanometer in a correct reading wattmeter and located it on the dial II of resistance I4, the dial can now be calibrated into proper divisions throughout its range by computation and without the necessity of utilizing other known loads.

It is to be understood that the procedure just referred to r4is that which is used in the factory to calibrate the apparatus, and that having once calibrated it will read the correct power absorption by the load 6 for any arbitrary values of the voltage across the circuit B and the equivalent loss resistance R, respectively.

Having now calibrated the instrument, the

procedure for the measurement of the absorbed power by an object placed in the circuit is very simple. The circuit is rst tuned to resonance by condenser 4 and without the object, and the output control of the generator A adjusted so that the pointer I3 of the galvanometer will indicate full scale reading.` The o bject is then placed between the electrodes and the circuit again tuned to resonance. 'I'he pointer of the galvanometer is thereby deflected to indicate a certain value. The pointer I8 of the resistance is then moved until it indicates the corresponding value mark on dial I'I that was indicated by the galvanometer. T he adjustment of the resistance changes the sensitivity of the galvanometer proportional to the loss conductance 1 R1 introduced by the object, and causes the galvanometer pointer to move to a reading on its dial I2a, which directly indicates the power absorbed by the object. Any desired value of powe absorption might be administered to the object by l varying the output control of the generator, thus varying the value of the high frequency voltage across the circuit B. Correct values of the power absorption by the object will be indicated regardless of the amount of output developed in the output circuit. The resistance I4 is thus set once by means of the scale I1 for any run; that- In Fig. 2 of the drawing there is shown another form of instrumentality. An electronic tube E is bridged across the conductors I and 2 of the output circuit and is connected with the moving coil 20 of a galvanometer F of the dynamometer ty'pe. The excitation coil 2| of the galvanometer is in the circuit 22 which includes the variable resistance 23 so that the sensitivity of said galvanometer can be varied by adjusting the resistance and thereby the exciting current.

'Ihe inyention has been described in` connection with an indicating device which utilizes the resonant voltages of the circuit, but it is to be understood that indicating devices may be used which utilize resonant current in the circuit as the motivating factor.

I claim:

1. In an apparatus for the determination of the power absorbed by the load in an oscillatory circuit, the combination of an electrical device with a square law characteristic, including a meter and a dial associated therewith connected across the circuit thereby adapted to indicate a value proportional to the square of the voltage of said circuit, a variable resistance in parallel with the meter of said device for varying the sensitivity thereof and thereby operating upon said value with a ratio factor to produce a final value indicated upon said meter proportional to the power absorbed by said load, a scale associated with said resistance for indicating the portion thereof shunted, said meter dial being calibrated to indicate the second value in power units, said resistance scale having been calibrated to correspond with the percentage no load voltage damping caused by said load, the insertion of the amount of said resistance thereby scaled being that which will cause the sensitivity of said meter to be inversely proportional to the equivalent loss resistance which would be introduced into Said circuit by said load regardless of power absorbed thereby.

2. In an apparatus for the determination of the power absorption in a load in an oscillatory circuit, the combination of a hot wire thermocouple with a galvanometer for measuring the voltage across the circuit, and indicating a value corresponding to the load component, the power absorption of which is to be measured, a calibrated resistance in parallel with said galvanometer for changing the sensitivity thereof in denite relation to said load component and the corresponding value being indicated by said galvanometer, said relation being dened by the formula l Rl corresponds to the sensitivity of said galvanometer, Pi being the power absorbed by the load. E corresponding to the voltage across the circuit and R1 corresponding to the equivalent parallel resistance inserted into said circuit by said load.

3. In an apparatus for the determination of power absorption in a load in an oscillatory circuit, the combination of an electrical device with a square law characteristic, a variable resistance in parallel with said device, and a calibrated scale in connection with said variable resistance for adjusting the sensitivity of said electrical device to a value proportional to the equivalent loss conductance introduced into the circuit by the load, the power absorption of which is to be measured.

4. The method of indicating upon an electrical measuring instrument the power absorbed by a load in a high frequency output circuit, including the steps of impressing upon the instrument a voltage proportional to the square of the resonant no-ioad voltage of the circuit, inserting the load into the circuit and thereby adding an equivalent loss conductance into the circuit to cause a damping of the voltage therein to make a corresponding change in the voltage impressed on the instrument, and then adjusting the sensitivity of said instrument so as to render the same proportional to the equivalent loss conductance inserted into said circuit by said load; said instrument thereby reading a quantity proportionai to the value of the power absorbed.

EUGEN NUTTELIVIANN.

CERTIFICATE OF C ORRECTI ON Patent, No. 2,240,955. my 6, 19141.

EUGEN MITTELMANN.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correotion' as follows: `Page 2, first column, line Z-Zlg., for "crsuit" read -circuit; page 5, second column, line 56, for "criteron" read -criterion; page b, first column, line 52, for "l R1" read --R-- and that the said Letters Patent should .ve read with this correction therein that the seme may conform to the record of the case in the Patent Office.

signed and sealed this 15th dew of July, A. D. 1914i.

Henry Van Arsdale,

(Seal) Acting Connnis sioner of Patents.

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