EVM-2 Moisture Meter for Sawn Timber

I. Sh. Solomonik

(Presented by the scientific seminar of the Radio Engineering Faculty)

In 1959, at the Department of Design and Production Technology of Radio Equipment, third-year students of the Radio Engineering Faculty, V. P. Rogalev and Yu. N. Poetov, under the supervision of the author of this article, modernized the EVM-1 instrument [1]. In doing so, they took into account the experience gained from four years of operation of the EVM-1 sawn-timber moisture meter at the Tomsk Weighing Equipment Plant. It was considered advisable to abandon the division of the moisture-measurement limits into a number of ranges, since the practical need for measurements of the relative moisture content of sawn timber does not go beyond the limits of 10–25%.

The absence of a range switch noticeably increases the operating reliability of the electric moisture meter, reduces the labor required for calibration work, and simplifies the design of the instrument. In connection with the transition to a single measurement range, the new version of the moisture meter uses an M-24 microammeter, providing reliable moisture readings with an accuracy of up to 1%. In the EVM-2 instrument, as in the EVM-1 instrument, an indirect method of measuring the moisture content of sawn timber is implemented—the input resistance of an electronic tube is compared with the resistance of a control section of the sawn timber.

Basic Technical Characteristics

1. The EVM-2 instrument is intended for rapid measurement of the relative moisture content of sawn timber without preliminary weighing and drying of control samples.

2. The limits of measurement of the relative moisture content of sawn timber are 10–25%.

3. The error in the moisture reading is not more than 1% when the line voltage varies by no more than ±15%.

4. Moisture is read from a microammeter by means of previously calibrated tables.

5. The instrument is powered from an alternating-current mains supply with a voltage of 220 volts and a frequency of 50 hertz.

6. The instrument is ready for measurements 2 minutes after being connected to the power supply.

7. The weight of the instrument is about 3 kg.

Fig. 1. General view of the EVM-2 instrument.

Schematic diagram of the EVM-2 moisture meter

Sensor

Fig. 2. Schematic diagram of the EVM-2 instrument.

Design of the Instrument

The EVM-2 electric moisture meter is made as a portable instrument measuring \(200 \times 215 \times 150\) mm. All the main units of the electrical circuit of the moisture meter are mounted on the front panel and on the horizontal chassis of the instrument. The instrument is closed by a metal casing, on the upper part of which a handle for carrying the instrument is installed; on the right side surface there is a hinged cover, and on the lower part four shock absorbers are fastened. The remote sensor, the sensor cable, and the power cord with plug are placed in a special compartment of the instrument.

The calibration table is fastened to the reverse side of the hinged cover. The main structural elements of the instrument are shown in Fig. 1, where 1 is the microammeter; 2 is the hinged cover with the calibration table; 3 is the remote sensor of the instrument; 4 is adjustment of the initial operating mode of the circuit.

Table 1

As can be seen from Fig. 2, the circuit of the instrument contains a measuring section, a resistance sensor, and a stabilized source of anode voltage. The equivalent circuit of the measuring section of the instrument is shown in Fig. 3, where \(R_x\) is the resistance of the monitored section of the sawn timber; \(R'_{v x}\) is the equivalent DC resistance of the grid--cathode section of tube \(L_2\), equal to

\[ R'_{v x} = \frac{R_{v x} \cdot R_3}{R_{v x} + R_3} \simeq R_3, \]

where \(R_3\) is the leakage resistance of tube \(L_2\), since \(R_{v x} \gg R_3\) at a large initial grid bias;

\(i_g\) is the current flowing in the test specimen;

\(U_g\) is the voltage controlling the anode current of tube \(L_2\);

\(E_g\) is the voltage drop across the part of the cathode load \(R_5\).

Schematic Diagram of the Instrument

The schematic diagram of the EVM-2 moisture meter and its specification are given in Fig. 2 and Table 1.

When the moisture content of the wood changes, the current \(i_g\) flowing in the test specimen changes and, consequently, so does the control voltage \(U_g\). Since there is no grid current in the grid circuit, we find the control voltage of tube \(L_2\) from the expression

\[ U_g = i_g R'_{\mathrm{in}} = \frac{E}{R'_{\mathrm{in}} + R_x} R'_{\mathrm{in}} . \]

To determine the optimum operating mode of the instrument, we find the ratio

\[ \frac{U_g}{E_g} = \frac{1}{\dfrac{R_x}{R'_{\mathrm{in}}} + 1}. \]

Having plotted the dependence

\[ \frac{U_g}{E_g} = \varphi\!\left(\frac{R_x}{R'_{\mathrm{in}}}\right), \]

(Fig. 4), we establish the following features of the circuit operation:

a) At low wood moisture content, when \(R_x > R'_{\mathrm{in}}\), it is necessary to have a high input resistance of the grid circuit of tube \(L_2\), since this increases the accuracy of the instrument reading.

b) At high wood moisture content, when \(R_x < R'_{\mathrm{in}}\), it is necessary to reduce the input resistance of the grid circuit of tube \(L_2\), since otherwise slight fluctuations of the supply voltages may lead to large errors in the readings.

c) The optimum operating mode of the circuit when measuring the relative moisture content of sawn timber within the range of \(10 \div 25\%\) should be considered the mode corresponding to the ratio

\[ \frac{R_x}{R'_{\mathrm{in}}} = 0.1 \div 1. \]

At such values of the ratio \(\dfrac{R_x}{R'_{\mathrm{in}}}\), it is possible to make an instrument with a sufficiently linear moisture-reading scale.

d) The maximum moisture content of the wood corresponds to the minimum deflection of the microammeter pointer, since in this case

Fig. 3. Equivalent circuit of the measuring part of the EVM-2 instrument.

Fig. 4. Dependence

\[ \frac{U_g}{E_g} = \varphi\!\left(\frac{R_x}{R'_{\mathrm{in}}}\right). \]

the maximum negative bias is applied to the grid of tube $L_2$.

As a result of experimental studies and on the basis of an analysis of the circuit operation, it was found that, when measuring relative moisture content below 25%, it is necessary to use a resistance $R_3 \simeq R_{\mathrm{in}}' \simeq 4$ megohms.

The stabilized anode-power supply consists of a two-half-period rectifier on two semiconductor diodes $D_1$ and $D_2$, a stabilovolt $L_1$, and a $\Pi$-shaped filter consisting of two electrolytic capacitors $C_1$ and $C_2$ and a resistance $R_1$.

Capacitor $C_4$ protects the measuring circuit from self-excitation and from the influence of high-frequency pickup.

Capacitor $C_3$ also protects the microammeter winding from external pickup. Resistance $R_2$, shunting the microammeter, is selected so that tube $L_2$ operates in a mode ensuring the production of approximately linear calibration curves and, at the same time, eliminates the danger of damaging the microammeter frame.

A standard TS-1 transformer from the “Taiga” record player is used as the power transformer.

The mains voltage (220 volts) is supplied to the step-up winding $A_1A_2$; this provides a reduced filament voltage for tube $L_2$ (about 6 volts), which increases the input resistance of the tube and, consequently, increases the sensitivity of the circuit for measuring small relative moisture contents.

In addition, by connecting two mains windings $C_1C_2$ and $C_3C_4$ in series, we obtain a lead from the midpoint of the transformer’s anode winding.

The purpose of the remaining elements of the instrument’s schematic circuit requires no additional explanation.

The resistance sensor is made in the form of two steel needles fastened to a wooden handle. The needles are connected to the measuring circuit of the instrument by a two-conductor shielded cable 750 mm long. During measurements the needles are driven into the tested lumber specimens to a depth of 0.23 of the specimen thickness.

Calibration of the Instrument

Calibration of the instrument is carried out by the laboratory (gravimetric) method of measuring the relative moisture content of wood and is the most responsible and labor-intensive operation in adjusting the electromoisture meter.

On the basis of the measurement results, a calibration table is compiled, and from it a working graph is constructed showing the dependence of the microammeter-pointer deflections on the value of the relative moisture content. An example calibration curve is given in Fig. 5.

The technological process of compiling calibration tables is reduced to the following operations:

1. From the drying chamber, lumber specimens most representative in assortment and wood species are selected at various stages of drying.

2. These specimens are kept for 2 hours under normal conditions until they cool.

3. From the tested specimens, control sections at least 100 mm long are cut.

1. Prepare the electric moisture meter for measurements; to do this, connect the instrument to the mains and, using the knob of potentiometer \(R_5\) (setting “200”), set the pointer at the division \(200\ \mu\text{A}\).

Fig. 5. Sample calibration characteristic.

1. Drive the needles of the remote sensor into the control section to their full depth along the wood fibers; in doing so, the pointer of the microammeter will stop at some division between “0” and \(200\ \mu\text{A}\).

2. After recording the established reading of the microammeter, the assortment of sawn timber, and the species of wood, weigh the control section on analytical balances with an accuracy of not less than 0.2%.

3. Obtain the initial weight \(P_1\) of the control section (it should be kept in mind that delay in weighing distorts the correctness of the calibration, since thin control specimens dry very rapidly in air, especially in summer).

4. After the initial weighing, the control sections are placed in a drying oven and dried to an absolutely dry state at a temperature of \(100^\circ\text{C} \pm 5^\circ\). (The absolutely dry state of the wood is established by periodic weighing every 1–2 hours. The first control weighing during drying of soft species is carried out no earlier than after 6 hours, and of hard species after 10 hours. Drying is considered complete when the last two weighings give one and the same weight.) Record the final weight \(P_2\).

The moisture content of the control section is determined from the formula

\[ B = \frac{P_1 - P_2}{P_2}\,100, \]

where \(B\) is the relative moisture content in %,
\(P_1\) is the initial weight of the control section,
\(P_2\) is the final weight of the dried control section.

1. Based on the results of measurements of the various control sections, fill in calibration tables for each type of sawn timber.

2. Using the tabular data, compile working calibration graphs.

REFERENCES

1. Solomonik I. Sh. Moisture meter for sawn timber EVM-1, NTB of the Tomsk Council of the National Economy, Nos. 1–2, 1958.