Thermoelectric converters (thermocouples)
Principle of operation, circuits for connecting and using a thermocouple, calibration, measurement accuracy. Alloys for thermocouples, manufacturing.
The operating principle of a thermocouple is based on the thermoelectric effect, which consists in the fact that in a closed circuit consisting of two dissimilar conductors, a thermoEMF (voltage) arises if the junctions of the conductors have different temperatures. If we take a closed circuit consisting of dissimilar conductors (thermoelectrodes), then thermal emf E(t) and E(tо) will appear at their junctions, which depend on the temperatures of these junctions t and t 0 . Since the considered thermoEMFs turn out to be connected counter-currently, the resulting thermoEMF acting in the circuit will be determined as E(t) - E(t 0 ).
If the temperature of both junctions is equal, the resulting thermal EMF will be zero. In practice, one of the thermocouple junctions is immersed in a thermostat (usually melting ice) and the temperature difference and the temperature of the other junction are determined relative to it. The junction that is immersed in the controlled (tested) environment is called the working end of the thermocouple, and the second junction (in the thermostat) is called the free junction.
For any pairs of homogeneous conductors, the magnitude of the resulting thermoEMF does not depend on the temperature distribution along the conductors, but depends only on the nature of the conductors and on the temperature of the junctions. If the thermoelectric circuit is opened in any place and dissimilar conductors are included in it, then provided that all the connection points that appear are at the same temperature, the resulting thermoelectric emf in the circuit will not change. This phenomenon is used to measure the thermoEMF value of a thermocouple. The EMF arising in thermocouples is small: it is less than 8 mV for every 100° C and, as a rule, does not exceed 70 mV in absolute value.
Using thermocouples, you can measure temperatures in the range from -270 to 2200° C. To measure temperatures up to 1100 0C, thermocouples made of
base metals, for measuring temperatures in the range 1100 to 1600° C - thermocouples made of noble metals, as well as platinum group alloys. To measure even higher temperatures, thermocouples made of heat-resistant tungsten-based alloys are used.
Currently, platinum, platinum-rhodium, chromel, and alumel are most often used for the manufacture of thermocouples.
When measuring temperature over a wide range, it is necessary to take into account the nonlinearity of the thermocouple conversion function. For example, the conversion function of copper - constantan thermocouples for a temperature range from -200 to 300° With an error of approximately ± 2 μV is described by the formula
E = At^2 + Bt + C,
where A, B and C are constants, which are determined by measuring thermal emf at three temperatures, t is the temperature of the working junction at° C.
The time constant (inertia) of thermoelectric converters depends on the design of the thermocouple, the quality of the thermal contact between the working junction of the thermocouple and the object under study. For industrial thermocouples, the time constant is several minutes. However, there are also low-inertia thermocouples whose time constant lies in the range of 5 - 20 seconds and even lower.
The measuring device is connected to the thermocouple circuit into the free end of the thermocouple and into one of the thermoelectrodes.
As noted above, when measuring temperature, the free end of the thermocouple must be at a constant temperature. If the length of the thermocouple itself is not enough, then to lead this end to a zone with a constant temperature, wires are used that consist of two cores made of materials (metals) that have the same thermoelectric properties as the electrodes of the thermometer.
For non-precious metal thermocouples, extension wires are most often made from the same materials as the main thermoelectrodes. For thermocouples made of noble metals, extension wires are made of other (not expensive) materials that develop in pairs with each other in the temperature range 0 - 150° With the same thermo EMF as the electrodes of the thermocouple. For example, for a platinum-platinum-rhodium thermocouple, extension thermoelectrodes are made of copper and a special alloy, which form a thermocouple identical in thermo-EMF to a platinum-platinum-rhodium thermocouple in the range 0 - 150° C. For a chromel-alumel thermocouple, extension thermoelectrodes are made of copper and constantan, and for a chromel-copel thermocouple, extension thermocouples can be basic thermoelectrodes made in the form of flexible wires. If the extension thermoelectrodes are connected incorrectly, a significant error may occur.
In laboratory conditions, the temperature of the free end of the thermocouple is maintained at 0° By placing it in a Dewar flask filled with crushed ice and water. In industrial environments, the temperature of the free ends of a thermocouple usually differs from 0° C and is usually equal to room temperature (room temperature). Since calibration of thermocouples is carried out at a temperature of the free ends of 0° C and calibration tables are given relative to 0° C, then this difference may be a source of significant error; To reduce the indicated error, as a rule, a correction is introduced into the thermometer readings. When choosing a correction, both the temperature of the free ends of the thermocouple and the value of the measured temperature are taken into account (this is due to the fact that the thermocouple conversion function is nonlinear); this makes it difficult to accurately correct the error.
To eliminate the error, automatic correction for the temperature of the free ends of the thermocouple is widely used. To do this, a bridge is connected to the thermocouple and millivoltmeter circuit, one of the arms of which is a copper thermistor, and the remaining arms are formed by manganin thermistors. At the temperature of the free ends of the thermocouple equal to 0° C, the bridge is in equilibrium; when the temperature of the free ends of the thermocouple deviates from 0° C, the voltage at the output of the bridge is not zero and is added to the thermoEMF of the thermocouple, while introducing a correction to the readings of the device (the correction value can be adjusted with a special resistor). Due to the nonlinearity of the thermocouple conversion function, full compensation of the error cannot be achieved, but the indicated error is significantly reduced.
In practice, when using a thermocouple, the following connection diagrams are most often used (depending on the required accuracy). For example, a copper (M) - constantan (K) thermocouple is taken:
With excited Resonant Microcluster Structures (RM) and Supercoherent Radiation (SR) ( sb22.pdf , sb22.htm , ikar.pdf , sb43-1.pdf , sb43-1.htm , svg_avt.pdf , sb44-2.pdf , sb44-2.htm).
Fig.2. "DSI", mini set: 1,2 - auxiliary electrodes, 3 - EMF sensor, 4 - multimeter. Registration of EMF: K - initial tap water EMF=+197.5 mV; A - activated water after installation "Emerald-SI" (mod.01os-50) EMF=- 196.5 mV.
- Method for recording the properties of a nonequilibrium liquid (Shironosov V.G. - Method for determining the activity of a structured liquid. Application for invention of the Russian Federation No. 2007127132 dated July 16, 2007 pat_2007127132.pdf. International application for an invention under PCT A18058 dated July 14, 2008).
- Method for detecting cluster structure and microclusters of liquid (Shironosov V.G., Kuznetsov E.V. Application for invention of the Russian Federation No. 2007127133 dated July 16, 2007 pat_2007127133.pdf. International application for an invention under PCT A18056 dated July 14, 2008).
Features of measuring redox potential for nonequilibrium systems in the region of negative values. cm. faq.htm reply from 05/11/2009 -
...Easy to use portable commercial devices (pencils) are made on the basis of electrodes, the secrets of which are not disclosed. Calibration of such devices against standard solutions of red and yellow blood salt in the region of positive ORP does not provide any guarantee that the readings will be correct in the case of negative ORP. The use of platinum electrodes and standard reference electrodes (for example, silver chloride) seems to guarantee the correct result at first glance.
However, the purity of the platinum electrode is of great importance here. The measured quantity is the potential difference between the two electrodes. The input resistance of the measuring circuit is large, but not infinite; it is usually 10^10 - 10^12 Ohms.
see http://www.o8ode.ru/article/onew/water_ovp/
Part 1. ORP measurement
... It has been found that when making measurements using platinum electrodes, the size of the electrode area, the “smoothness” of the surface, the treatment of the electrode before measurements, as well as the structure of the metal are of great importance.
The larger the electrode area, the higher the purity of the processing, and the special methods used to remove oxide layers, the more sensitive the electrode is to changes in the oxygen content in water and has a more negative potential value.
So, for example, we took a batch of 100 pieces of platinum laboratory electrodes of the EPL-02 type, manufactured at the Gomel ZIP (Belarus), and carried out measurements in water with different oxygen contents. The platinum in these electrodes is a ball with a diameter of approximately 1 mm, fused into the glass. The potential spread of such electrodes at the water level, which has a potential of minus 200 mV, was 150 mV. When viewing the surface of platinum under a microscope, it is clear that the surface is uneven, pitted with pits that appeared when the platinum was processed in a gas burner.
Much better reproducibility is obtained if you take platinum in the form of a polished wire with a diameter of more than 1.5 mm and a length of 2-3 mm or in the form of a disk with a diameter of 1 cm, electrodes from YuMO (Germany).
In addition to the quality of the platinum surface and its area, the treatment of the electrode in certain reducing solutions is important.
We analyzed the “quality” of platinum electrodes in portable ORP meters, usually made in China. Unfortunately, due to the saving of platinum (the cost of platinum is now almost 2000 rubles/g), all electrodes do not meet the above requirements for obtaining sufficiently reliable and reproducible results.
To summarize, it can be largely stated that in the absence of redox systems of the Fe^2+/Fe^3+ type in water, the potential of the platinum electrode (ORP) is largely determined by the amount of dissolved oxygen.
 |
Fig.3. An example of recording with two ORP meters from one batch of ORP: buffer solution ORP_001=+281 mV , ORP_002=+ 289 mV.
sizing solution per unit. Hydrogen electrodes are not used in production measurements, as they are inconvenient to use.
8.1.1. Measuring cell pH meter
IN Due to the fact that the electrode potential cannot be directly measured, the potentiometric method uses a galvanic cell in which one electrode is a measuring electrode and the other is a reference (or auxiliary) electrode, the potential of which does not depend on the concentration of the solution ions being studied. The measuring electrode is placed in the analyzed
liquid medium, a potential jump EX is created on it, determined by the concentration of ions in this medium. The potential of the reference electrode must always remain constant regardless of changes in the composition of the medium.
IN Glass electrodes are used as measuring electrodes, the indicator part of which is made of special types of glass with a hydrogen function. Calomel or silver chloride electrodes are usually used as a reference or auxiliary electrode. They belong to the so-called second kind of electrodes, which consist of a metal, its sparingly soluble salt and a readily soluble salt with the same anion as the sparingly soluble salt.
A general view of a cell with a glass measuring electrode is shown in Fig. 1, where 1 is a glass indicator electrode, 2 is a calomel reference electrode.
The EMF of the electrode sensor of a pH meter consists of a number of potentials:
E cell= E k+ E in+ E x+ E av+ E d,
where E k is the potential difference between the contact auxiliary electrode and the solution filling the glass electrode; E ext – potential difference between the solution and the inner surface of the measuring membrane; E x – potential difference between the outer surface of the glass membrane and the controlled environment (function of pH); E cf – potential difference at the mercury (Hg) – calomel (Hg 2 Cl 2 ) boundary; E d – diffusion potential at the interface of contact between two media – KCl and the controlled environment. Chlo-
Potassium Ride KCl acts as an electrolytic key connecting the analyzed solution to the electrode.
Rice. 1. Electrical circuit of the pH meter measuring cell
In this case, the values of Ec, Ein, Ein are constant and do not depend on the composition of the analyzed medium. The diffusion potential E d is very small and can be neglected. Thus, the total emf is determined by the activity of hydrogen ions: E cell = E x + E.
Thus, E cell =f(pH), that is, E cell is a linear function of pH, which is used in the electrical measurement of pH.
The dependence of the EMF of the electrode cell E cell on pH is determined by the electrode properties of the glass and is characterized by the slope coefficient S of the characteristics of the electrode system S = E/ pH. Changing the temperature of the analyzed solution affects the EMF of the electrode system, changing the slope of the nominal static characteristic (NSC) of the measuring electrode. If we express this dependence graphically (Fig. 2), we will get a bunch of intersecting lines. The coordinates of the point of intersection of the lines are called the coordinates of the isopotential point (EH, pHH) and are the most important characteristics of the electrode system, which are used to guide the calculation of the temperature compensation circuit of the pH meter. Temperature compensation for changes in the EMF of the electrode system, as a rule, is carried out automatically (using a TS included in the circuit of an industrial pH meter converter).
>> R ST.
Rice. 2.NSH measuring electrode
A measuring cell with a glass electrode can be represented as an equivalent circuit (Fig. 3). The resistance R cell is very high due to the high resistance of the glass electrode membrane R st (R cell 500 MΩ). Therefore, the flow of small currents through the internal resistance of the cell will cause a large measurement error:
UВХ = E-CELL – I-CELL R-CELL; UВХ = EYCH .
From the last equality it is clear that the main requirement for measuring U VX = E YAC can be met if R VH >> R YAC, i.e.
R BX
Rice. 3. Equivalent circuit of the measuring cell
8.1.2. Industrial converters pH meters GSP
A set of automatic industrial pH meter consists of a submersible sensor (type DPg-4M) or a main sensor (type DM-5M), a high-resistance measuring transducer and a secondary GSP device for general industrial use. The task of the measuring device included in the pH meter kit is to measure the EMF of the electrode system, which, under constant temperature conditions, is a function of pH.
Accurate measurement of the EMF of the measuring cell of a pH meter, which is a low-power source, is associated with significant difficulties. Firstly, a current whose density exceeds 10–7 A/cm2 cannot be passed through the measuring cell, since the phenomenon of polarization of the electrodes may occur, as a result of which the electrodes fail. The second significant difficulty is that when directly measuring the EMF of a pH meter cell with current consumption, for example with a millivoltmeter, an electrical circuit is created through which a current flows, determined by the sum of the internal resistance of the measuring electrode (about 500...1000 MOhm) and the resistance of the measuring device. In this case, a number of conditions must be met: the measuring current must be less than the polarization current of the electrodes; the internal resistance of the device must be at least 100 times higher than the resistance of the glass electrode, which, however, conflicts with the requirement for high sensitivity of the device. In this regard, converters with direct EMF measurement are practically not used.
The only method that satisfies all the requirements for measuring the EMF of a pH meter cell is the compensation (potentiometric) or zero measurement method, the main advantage of which is the absence of current at the time of reading. However, one should not assume that with the compensation method the electrode is not loaded at all, and therefore the phenomenon of electrode polarization is excluded. Here, the flow of current (within 10-12 A) is explained by the fact that during the measurement process there is always an imbalance, and at the time of measurement, compensation is achieved only with the accuracy with which the sensitivity of the null indicator allows.
Currently, to measure the EMF of an electrode system with a glass electrode, only electronic null indicators (measuring transducers) with static compensation are used. A simplified block diagram explaining the principle of operation of such a converter is shown in Fig. 4. The converter is a direct current amplifier, covered by a deep negative feedback of the feedback on the output current, which ensures a high input resistance. The amplifier is built according to a circuit that converts direct voltage into alternating voltage with subsequent demodulation.
Rice. 4. Block diagram of the method for measuring the EMF of a pH meter cell
The measured EMF E IYA is compared with the voltage U OUT generated by the flow of the output current of the amplifier I OUT through the resistor R OS. The difference between these voltages U IN =E IA -U OUT is received at the input of the amplifier. If the gain k = U OUT /U IN, then E IYA = U OUT / (1+1/k). With a sufficiently large value of k (k 500) E IA U OUT I OUT R OS, i.e. The output current strength is practically proportional to the input signal from the pH-meter measuring cell.
The use of static compensation makes it possible to reduce the current consumed from the measuring cell during the measurement process by many times.
This principle is implemented in almost all industrial pH-meter converters: pH-201, P201, P202, P205 (semiconductor element base) and P215 (using standard microcircuits).
8.1.3. Description of the converter P – 201
Industrial converters type P201 are designed to measure the activity of hydrogen ions (pH value) of solutions and pulps in systems for automatic control and regulation of technological processes.
The converters are designed to work in conjunction with any commercially produced pH sensitive elements, such as DPg-4M; DM-5M and others.
The converter has voltage and current outputs for connecting secondary devices with corresponding inputs
signals. |
|||
Main technical characteristics: |
|||
measurement limits |
from –1 to 14 pH |
||
limit of permissible basic reduced |
|||
errors: |
|||
a) by DC output signals and |
|||
DC voltage |
|||
b) according to the indicating device |
|||
glass measuring resistance |
|||
electrode |
|||
auxiliary electrode resistance |
|||
settling time |
no more than 10 s |
||
output current |
|||
output voltage |
|||
from 0 to 10 100mV |
|||
The converter is designed for installation in close proximity to industrial units. The transducer may consist of a narrow-profile indicating device and the transducer itself, installed on one common panel or separately, or only one transducer. The appearance of the device is shown in Fig. 5.
Casing 1 is made of sheet steel, cover 2 is cast, made of aluminum alloy. On the front side of the cover there is an inscription with the device index, a cap 3 and a threaded plug 4.
Rice. 5. Appearance of the P201 converter
A frame is installed inside the casing, which serves as the basis for installing all the blocks and elements of the device. The front panel of the converter, located under the cover, contains the axes of variable resistors designed to change the measurement limits of the converters. The block with clamps for external electrical connections is located in a closed compartment; access to it is provided from the rear wall of the case. The wires are inserted into the compartment through four glands in the bottom wall of the device (Fig. 6).
Rice. 6. Diagram of external electrical connections of the P-201 converter: TRM – universal meter-regulator; TKR – block of temperature compensation resistors
8.1.4. Checking and calibrating an automatic pH meter
The current verification of an automatic pH meter consists of comparing its readings with the readings of a control device. If there is a significant discrepancy, the readings of the device being tested are corrected using a compensator or by changing the calibration of the transducer using the adjustment knobs. Except
In addition, it is necessary to periodically carry out more detailed checks of the sensor and converter.
Checking the sensor includes the following operations:
1) a thorough external inspection, especially of those parts that come into contact with the measured medium;
2) checking electrical circuits, especially the insulation resistance of the glass and reference electrode circuits from
relative to the housing, which must be at least 1012 Ohms and 2108 Ohms, respectively;
3) checking the characteristics of the electrode system using buffer solutions with a known pH value using a control laboratory pH meter.
Converter verification includes:
1) determining the main measurement error of the transducer and adjusting its calibration;
2) determination of additional measurement errors of the transducer from changes in the resistance of the glass electrode R CT, changes in the resistance of the reference electrode RCP
And change in the potential of the controlled solution E X .
To calibrate the scale of pH meters, you must have a simulator of the electrode system I-01 or I-02.
The electrode system simulator allows you to check the performance of the pH sensor - meters; the influence of changes in the resistance of the electrodes and the voltage between the solution and the body of the unit on the readings of the device; noise immunity of pH meters.
Using the simulator, you can reproduce the following parameters of the electrode system:
a) voltage equivalent to the EMF of the electrode system, ranging from 0 to 1000 mV;
b) resistance equivalent to the resistance of the glass electrode: 0; 500 and 1000 MOhm;
c) resistance equivalent to the resistance of the auxiliary electrode: 10 and 20 kOhm;
d) voltage equivalent to the EMF “ground - solution”: 0 and
The simulator is the electrical equivalent of the electrode system (Fig. 7) and is designed in the form of a portable device housed in a steel case with a removable cover.
E Z Rv
Rice. 7. Equivalent circuit of the electrode system simulator: R И – resistance of the measuring glass electrode; R B – resistance of the auxiliary electrode; E – total EMF of the electrode system: EZ – EMF “ground - solution”.
On the front panel of the simulator there are terminals for connecting it to the pH meter being verified using the cable that is included in the kit. There are also knobs for setting the required output voltage, electrode resistance, potential of the controlled solution, etc.
8.2. EQUIPMENT AND DEVICES
1. Industrial converter P-201.
2. Electrode system simulator I-02.
3. Meter-regulator universal multi-channel TRM 138.
8.3. SEQUENCE OF WORK COMPLETION
1. Assemble the installation for testing the converter P-201 using the I-02 simulator in accordance with the diagram in Fig. 8, connecting the output of the simulator to the “Meas” and “Acc” inputs of the converter via a coaxial cable.
2. Prepare the simulator for work. To do this, press the simulator switches: “R And ” – button 500; “EЗР”,”RВ” – buttons
“00” for EZR and “010” for RB; “POWER” – “EVN” and “On” button.
3. Apply supply voltage to the stand.
Rice. 8. Verification scheme: 1 – simulator of the I-02 electrode system; 2 – electrode system; 3 – high-resistance converter P-201; 4 – multi-channel meter-regulator TRM 138
4. Use arrows ^ v on TRM 138 to select channel No. 5, through which the EMF is measured.
5. Check the converter.
For this:
5.1. On the “E, mV” switch buttons of the simulator, dial the EMF value corresponding to the pH value of the digitized scale mark. The “EX, mV” switch is set to the “+” or “-“ position depending on the sign of the EMF in the calibration table.
5.2. Take readings using the I-02 simulator. Determine the main measurement error at RВ =10
kOhm; EЗ =0. The main error is checked at all digitized scale marks during forward and reverse stroke and is calculated using the formula = [(E –E 0 )/(E K –E H )]100%, where E 0 is the tabular value (the actual value of the emf of the electrode system corresponding to this digitized scale mark, mV; E – actual EMF value, mV; E K, E N – EMF values corresponding to the final and initial scale marks.
6. The verification results should be presented in the report.