The second profession of an electrocardiographer. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Electronics in medicine

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In the process of creating amateur designs, tools and equipment may be required, which are often difficult to obtain. In such cases, ingenuity comes to the rescue, and craftsmen do everything necessary with their own hands. We will talk about how you can make a real thermal recorder from an old EK1T-03M type electrocardiograph (Fig. 1).

This device may be required to measure and record the temperature of a particular area (part) for some time. Let's explain with an example. When assembling a powerful computer with your own hands, you may encounter the problem of overheating of the processor. To cool it, you will need a cooling system that will certainly eliminate the risk of failure of the "computer brain". Well, if you own the appropriate calculation methodology. And if not? You can, of course, immediately turn on the computer and see if the processor "burns out" or not?

It is unlikely that you will be satisfied with the sad outcome of such an experiment... It is much better to check the performance of the cooling system under model conditions by measuring the rise in temperature of the heatsink with which the processor will subsequently come into contact. Using a model heat source and a thermal recorder, you can easily select the design parameters for which the processor will be guaranteed from overheating.

Rice. 1. Appearance of the device: 1 - power button; 2 - network on indicator; 3 - pen incandescence regulator; 4 - plug for connecting the lead cable; 5 - lead switch; 6 - pen shift regulator; 7 - calibration button; 8 - speed switch button; 9 - calm button; 10 - record button; 11 - sensitivity switch

It is useful to mention the principle of operation of a homemade thermal recorder. Methods for measuring temperatures using thermoelectric thermometers (thermocouples) are well known in the art. A thermocouple is two conductors of different materials that make up a common electrical circuit (see Fig. 3). If the temperatures of the junctions (junctions) of the thermocouple are not the same, then a thermo-EMF arises - and a current flows through the circuit. Thermo-EMF is the greater, the greater the temperature difference between the junctions. By connecting a thermocouple to a recorder, we get a device that allows you to record temperature changes over time.

The single-channel electrocardiograph EK1T-03M is well suited for recording thermo-EMF thermocouples. Indeed, the electrocardiograph is designed to measure voltages in the range of 0,03 to 4 millivolts (mV). The sensitivity of the device is set by the switch and can be 5, 10 and 20 mm/mV. At a recorder tape speed of 25 mm/s, a 50-meter-long thermal paper roll (Fig. 2) is enough for about half an hour of continuous operation, which is more than enough for amateur purposes.

Rice. 2. A roll of thermal paper 50 meters long provides 30 minutes of continuous recording of instrument readings

The use of an electrocardiograph as a temperature recorder, in fact, does not require any modifications. It is only necessary to have one or more additional thermocouples and know how to make all the necessary connections. Despite the fact that an instruction is usually attached to an electrocardiograph, we will dwell on some of the main points that are important from the point of view of measuring temperatures.

First of all, you need to connect a network cable to the device and the so-called "lead cable", usually consisting of five wires of different colors. According to the instructions, when measuring cardiograms, the wires of the lead cable are connected to the electrodes: R red - on the right hand; L yellow - on the left hand; F green - on the left leg; N black - on the right leg; With white - on the chest. To measure temperatures, we will use pairs of wires corresponding to three standard leads: 1. R red - L yellow (hand - hand); 2. R red - F green (right arm - left leg); 3. L yellow - F green (left arm - left leg).

On the top panel of the device there is a switch that sets the measurement mode for a specific lead number.

We set the switch to the position of the 1st lead and connect the thermocouple leads to the red and yellow wires. We lower the connection points of wires and thermocouples into a thermos with ice (below we will explain why this is done). Thermocouples for the other two leads are connected in the same way - they have corresponding switch positions. The temperature recorder is almost ready for operation.

Where can I get a thermocouple? You can also make it yourself using the technology that has been repeatedly described in the technical literature, and it hardly makes sense to dwell on this in detail. But if you have a multimeter, then look inside its original packaging. It is not known to many that the double thin wire in a white braid with a tiny ball at the end attached to the device is just a thermocouple.

Before recording thermocouple readings with a temperature recorder, the range of possible temperature measurements should be assessed. Recall that the EK1T-03M electrocardiograph is designed to measure voltages in the range from 0,03 to 4 millivolts (mV). In this case, the thermo-EMF of chromel-copel thermocouples in the temperature range from 0 to 100°C varies from 0 to 6,9 mV. That is, with such thermocouples, in this case, it is logical to measure temperatures of several tens of degrees. Thermo-EMF of chromel-alumel thermocouples in the temperature range from 0 to 100°C varies from 0 to 4,09 mV. Therefore, it becomes possible to register temperatures up to 100°C using an electrocardiograph. If, however, special platinum-rhodium thermocouples are used, then the upper limit of the determined temperature will approach 500°C.

When measuring with thermocouples, the cold junction correction must be kept in mind (Fig. 3).

Rice. 3. Types of thermocouples: 1 - hot junction; 2 - cold junction; 3 - cold junctions (chromel; alumel; copper)

The fact is that in the process of measuring temperature, one junction of the thermocouple circuit, the so-called cold junction, must be at 0 ° C (in a thermos with ice), and the other, the hot junction, in the medium whose temperature is to be measured. The thermo-EMF tables of various thermocouples are compiled specifically for the case when the cold junction is at 0°C. If for some reason it is not possible to place the cold junction in a medium with a temperature of 0°C and it is at room temperature (for example, at 20°C), then in this case the resulting thermo-EMF corresponds to the temperature difference between the hot and cold junctions and at To determine the temperature, you need to enter the so-called correction for the cold junction. To do this, the measured thermo-EMF should be added to the thermo-EMF corresponding to the temperature of the cold junction (20 ° C), and the temperature obtained should be determined using the tabular data.

Before turning on the device, it is necessary to install a roll of recording thermal paper into the compartment of the tape drive mechanism. The signal will be recorded with a special thermal pen, which heats up to approximately 300°C during operation. The electrocardiograph has a special calibration button "1mV", pressing which will allow you to record special calibration signals on the tape, indicating the sensitivity of the device (Fig. 4). The "M" button starts the tape drive mechanism of the registrar and the thermal pen records the change in the thermo-EMF of the thermocouple on the tape.

Rice. 4. Recording a 10mm/mV calibration signal

On fig. Figure 5 shows a record of the readings of a chromel-copel thermocouple when it is lowered every second into a container with heated water. The rise and fall of the hot junction temperature are clearly visible at the set sensitivity and a calibration signal of 10 mm/mV. The temperature recorder based on the old cardiograph works.

Rice. 5. Record temperature fluctuations in 10-15 degrees

Literature

1. Electrocardiograph EK1T-03M. Technical description and operating instructions, 2nd edition, 1985
2. Theory and technique of thermophysical experiment, Moscow, Energoatomizdat, 1985
3. Zubarev V., Alexandrov A., Okhotin V. Workshop on technical thermodynamics, Moscow, Energoatomizdat, 1986

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