TDS-meter - a prefix to a multimeter. Scheme, description

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TDS-meter - a prefix to a multimeter. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Measuring technology

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The prefix, offered to the attention of readers, expands the capabilities of the M-830B (DT-830B) multimeter, allowing you to measure the concentration of salts dissolved in water. With its help, you can evaluate the suitability of drinking, the "quality" of distilled water, distinguish real mineral water from fake. The set-top box is powered by a multimeter and does not require additional connections inside it.

Water, as you know, is necessary for the existence of living organisms. Suffice it to say that in the human body it makes up about 65% of its mass, is contained in all cells and tissues, and all life processes proceed with its participation. A vivid example confirming the importance of this substance: in the presence of water, a person can live without food for about a month, without water - only a few days.

However, not all water, even if it is transparent and clean in appearance, is suitable for drinking, therefore, before it enters the water supply system, it undergoes appropriate purification. The quality of tap water largely depends on the amount of salts it contains. According to the sanitary standards of the State Committee for Sanitary and Epidemiological Supervision of Russia, the total concentration of salts dissolved in water (the so-called total mineralization) should not exceed 1000 mg/l [1]. Water with a higher salt content is considered mineral water. Special equipment is used to measure the degree of mineralization.

Due to the deteriorating environmental situation around the world, many companies have begun to produce devices for express analysis of water parameters. One of them is the Zepter TDS meter [2], which measures the amount of dissolved particles (Total Dissolved Solids - hence the name - TDS meter) per million water molecules. Numerically, its readings are equal to the total mineralization, measured in milligrams per liter. The price of a TDS meter is quite high - $112.

However, such a device can be assembled independently, and the cost of its manufacture will be small. The principle of measuring such devices is based on the dependence of the electrical conductivity of water on the amount of dissolved salts. It is known from physics that the conductivity of a solution is determined by the formula [3]

δ = F z+ n0+(u++u-)/NA,

where F = 96,5 103 C/mol is the Faraday number; NA = 6,02 1023 mol-1 - Avo-gadro number; z+ - valency of positively charged ions in solution; n0+ is the number of positively charged ions per unit volume of the electrolyte; u+, u- are the mobility of positively and negatively charged ions, respectively. The formula clearly shows that the conductivity is proportional to the concentration of dissolved compounds. Of course, it depends on the solute and the temperature of the solution [4], but it is believed that an average concentration of 1000 mg/l approximately corresponds to an electrical conductivity of 0,2 S/m [5].

Thus, to determine the degree of mineralization of water, it is enough to measure its electrical conductivity or resistance. To exclude the influence of solution electrolysis on the result, measurements must be carried out on alternating current.

The proposed device is made in the form of an attachment to the widely used multimeter M-830V [6] or its analogue DT-830B, which converts the measurement results of conductivity into voltage. It is powered by a voltage of 3 V from the internal stabilizer of the ICL7106 microcircuit of the multimeter. The current consumption when the sensor electrodes are not immersed in water does not exceed 0,25 mA.

The measurement error of the device was estimated by comparing its readings with the readings of the aforementioned Zepter TDS meter. In the concentration range from 0 to 1200 mg/l, it does not exceed ±10%. If the salinity is more than 1200 mg / l, the error increases sharply due to an increase in the current consumed by the attachment and the low load capacity of the stabilizer. It should also be noted that when using the set-top box with the DT-830B, the measurement error may be somewhat higher, since the load capacity of the stabilizer of the frameless analog of the ICL7106 microcircuit, usually installed in these multimeters, is extremely small.

The schematic diagram of the attachment is shown in fig. 1. As you can see, it is assembled on only two microcircuits and two transistors. On the ICL7660A (DA1) chip, a voltage polarity converter is made. A different polarity voltage is necessary in order for an alternating current to flow through the sensor electrodes.

(click to enlarge)

On the op-amp DA2.1, a generator of differently polar symmetrical rectangular pulses with a repetition rate of about 170 Hz is assembled. This signal is amplified by a current amplifier based on transistors VT1, VT2, the emitter circuit of which includes a conductivity sensor, a current-measuring resistor R6 and a thermistor RK1, which partially compensates for the dependence of water conductivity on temperature.

The AC voltage from the current-measuring resistor is fed to the non-inverting input of the DA2.2 op-amp, which acts as a half-wave rectifier and a non-inverting amplifier with a gain of about 12. To compensate for the zero bias voltage of this op-amp, voltage is applied to the inverting input through the resistor R9 from the resistive divider R5R7R8.

So that the minus sign is not displayed on the multimeter display, the output voltage of the set-top box must be positive. Since the supply voltage of the positive polarity is stabilized by the internal stabilizer of the ICL7106 microcircuit of the multimeter, and the stability of the negative polarity voltage is low, the DA2.2 op amp is turned on by a non-inverting amplifier. The voltage filtered by the R12C7 circuit is fed to the input of the multimeter, which is connected to measure DC voltage. The voltage measured by the multimeter in millivolts corresponds to the total mineralization in milligrams per liter.

All elements of the device, with the exception of the sensor and thermistor, are placed on a board made of foil fiberglass (Fig. 2).

TDS-meter - attachment to the multimeter

The board is designed for the use of fixed resistors MLT, trimmer SP5-2, oxide capacitors K50-16 (C1, C2, C4), the rest of the capacitors are almost any low-voltage ceramic ones. The pins of the X1-XXNUMX connectors, with which the set-top box is connected to the corresponding sockets of the multimeter, are soldered from the side of the printed conductors.

Instead of the ICL7660A chip, it is permissible to use the ICL7660 or the domestic analogue KR1168EP1. We will replace the KR1446UD2A op-amp with any of this group, as well as KR1446UD4A-KR1446UD4V, however, the current consumed by the prefix in the latter case will increase. It is possible to use the OU KR1446UDZA-KR1446UDZV, but they have a different "pinout", so the printed circuit board will need to be adjusted.

Care must be taken when installing op amps: like other CMOS microcircuits, they often fail from the effects of static electricity.

The transistors of the series indicated in the diagram can be replaced by any low-power ones of the corresponding structure. Diodes - any low-power pulse, for example, the KD521 or KD522 series. The prefix uses the MMT-9 thermistor, however, almost any one with a negative TKS and a resistance of approximately 620 to 750 Ohms will do.

The drawing of the sensor is shown in fig. 3.

TDS-meter - attachment to the multimeter

It consists of a base 1 - a plate of foil-coated fiberglass with a thickness of 2,5 ... 3 mm and the sensor itself - two metal pins 4 with an anti-corrosion coating (it is convenient to use silver-plated or gold-plated pins of a suitable diameter from a detachable connector). The holes in the base must be drilled on a drilling machine and in such a way that the pins are inserted into them tightly (this will ensure their parallelism). Fix the pins by soldering to the foil. Then, approximately in the middle of the base, parallel to the short side, a piece of tinned wire 5 with a diameter of 0,6 ... 0,8 mm and a length slightly less than the diameter of the thermistor 3 is soldered to a larger foil. after that, flexible stranded insulated wires 2 are soldered to its second terminal and both foil pads.

When using a different type of thermistor, the dimensions and number of pads of the base foil may need to be changed, as long as the thermistor is securely soldered to the foil. It should be remembered that the accuracy of measurements depends on the quality of manufacture of the sensor, therefore the diameter of the pins, the length of their protruding parts from the base and the distance between them must be maintained within the values indicated in Fig. 3 limits.

The device does not need to be adjusted. The only thing to do is to connect it to a multimeter, turned on at the voltage measurement limit of 2000 mV, and set zero readings with a trimming resistor R7. To check, a 1,5 kΩ resistor is connected to the sensor contacts: the multimeter should show a voltage of about 1000 mV.

When working with the device, it should be remembered that the thermistor has thermal inertia, so readings can only be taken 1 ... 1,5 minutes after the sensor is immersed in water (when they stop changing).

Literature

.
.
Yavorsky B., Detlaf A. Handbook of Physics. - M.: Nauka, 1968.
<tos.urc.ac.ru/~tex/sensor/ion2.htrn>
Afonsky A., Kudrevatykh E., Pleshkova T. Compact multimeter M-830V. - Radio, 2001, No. 9, p. 25-27.

Author: V. Chudnov, Ramenskoye, Moscow Region

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