JPS5832652B2 - How to calibrate an electronic thermometer - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to electronic thermometers, and in particular to thermometers that have a narrow temperature range but need to accurately measure absolute values and changes thereof, and in particular, a method for calibrating the same. Regarding.

Conventionally, electronic thermometers have used a thermometer, such as a thermistor, whose resistance value changes depending on the temperature to measure the change in resistance value to determine temperature.

The temperature measurement range is very narrow (e.g. 35°C to 42°
C), and the accuracy is better than o,i °C.

Therefore, since the resistance change of the resistance temperature detector is minute, precise calibration of the resistance temperature detector has not been easy.

Figure 1 shows the temperature change in the resistance value RT of the thermistor. As the temperature T rises, the resistance value RT decreases, and although it is generally not a straight line, it can be considered to have a characteristic close to a straight line in a very narrow temperature range JT. .

These characteristics vary widely depending on the thermistor on the other side, so it is almost impossible to make a thermistor alone with exactly the same temperature characteristics.

Therefore, by connecting a series resistor, a parallel resistor, or both to the thermistor, linearity is improved and a composite resistor with the same temperature characteristics is created.

For example, if you connect an ordinary resistor R whose resistance value does not change with temperature in parallel to the thermistor RT as shown in Figure 2, the combined resistance RX
is as shown in the graph in Figure 2a.

In other words, the resistance value decreases and the temperature changes (temperature coefficient α)
is smaller, but linearity is improved.

Fig. 3 shows a case where a resistor R is connected in series to the thermistor RT, and the resistance values are added and the combined resistance RX becomes as follows, and moves in parallel as shown in the graph of Fig. 3a.

Therefore, by connecting a resistor in parallel to the thermistor to obtain a value for determining the temperature coefficient, and then connecting the resistor in series and moving the whole resistor in parallel, it is possible to synthesize a resistance temperature detector having desired temperature characteristics.

However, it is not easy to obtain a composite resistance with desired temperature characteristics using such conventional methods.

This is because it is difficult to match the temperature coefficients.

Now, considering Fig. 2, the temperature coefficient decreases as the resistance value of the parallel resistor R decreases, so the resistor R with an appropriate resistance value is By connecting them in parallel, the desired temperature coefficient can be obtained without fail.

To determine this temperature coefficient, it is necessary to measure resistance values at at least two temperatures.

Therefore, to actually determine the value of the combined resistance, a certain temperature T is required.

First measure the resistance value at T 1 (T
1>To), and if the temperature coefficient determined by this measurement is larger than the target value, the resistance value of the parallel resistor R is further reduced.

If you change the resistance value of the parallel resistor R, not only the temperature coefficient but also the absolute value of the combined resistance will change, so T again.

The value of the parallel resistance R must be set by repeating operations such as measuring the resistance values at two points, T1, and determining the temperature coefficient.

This is very complicated and impractical.

Therefore, there is also a method of calculating it.

In a narrow temperature range as shown in Figure 2a, the temperature change can be approximated by a straight line, and if the temperature coefficient is α, then the following equation is obtained.

Therefore, the desired temperature coefficient is α. The temperature coefficient of the combined resistance is α.

To adjust the temperature, first set the temperature T.

, T1, measure the resistance of the thermistor or a thermistor connected to a parallel resistor, and determine the resistance value RTo and temperature coefficient α at T-To, and perform the above (3), (
By calculating the resistance value R from the equation obtained from equation 5) and connecting resistors of that value in parallel, a composite resistance having a desired temperature coefficient can be obtained.

Further, by connecting a series resistor to this, a composite resistor having desired characteristics as a whole can be obtained.

However, with this method, the operation of calculating and determining the appropriate resistance value is complicated, and it is difficult to make adjustments when it is assembled into a device as a thermometer, or when the thermistor changes over time and requires adjustment. However, this method is extremely inconvenient in practice.

This invention was conceived with these points in mind.
The purpose of this is to create an electronic thermometer that can be easily and accurately calibrated and that can measure temperature accurately even if there are variations in the characteristics of the resistance temperature detector or changes over time. A calibration method is provided.

That is, in the present invention, the reference temperature T is determined by using a resistance temperature detector, a reference resistance, and a common resistance common to them.

The resistance value and apparent temperature coefficient of the resistance temperature detector can be changed independently, making adjustment for temperature calibration extremely easy.

An embodiment of the present invention will be described below with reference to the drawings.

First, a thermometer that measures temperature with high precision will be described.

Now, the resistance value Rt is expressed by the formula Rt=Rs, which is similar to the formula (3) above.
(1-αo(T-'ro)) -・-・The resistance temperature detector Rt that changes as shown in (7) and the reference resistance Rs whose resistance value does not change depending on temperature are shown in Figure 4. The resistance value is measured using the same measuring circuit M.

Then, from equation (7) above, becomes.

Therefore, an accurate thermometer can be obtained by providing an arithmetic circuit that calculates equation (8) from the measured value of (Rt/Rs) after the measuring circuit and displaying the calculated value on a display device.

In this case, naturally the resistance temperature detector Rt to be used is as described in (7) above.
must have the properties of expression.

However, as mentioned above, since there are variations in the temperature characteristics of individual RTDs, the above (7)
The values of Rs and α0 in the equation are different, and if used as is, an error will occur in the displayed temperature.

Therefore, in this embodiment, the following steps are taken.

Now, the temperature characteristics of the resistance temperature detector Rt are the same as the above equation (3). Suppose it was.

This resistance temperature detector (it may be one that has already been synthesized)
A resistor r1 whose resistance value does not change depending on temperature is connected in series with Rt.
Furthermore, as shown in FIG. 5, a common resistor r2 whose resistance value does not change depending on temperature is connected in series with the reference resistor Rs to form a changeover switch S1 measurement circuit M1 arithmetic circuit exactly the same as that shown in FIG. The temperature is measured using the L1 display circuit.

Then, the measurement circuit M uses the series composite resistance R1-(of the reference resistance circuit and the common resistance circuit) instead of Rt and Rs in FIG.
R80r2) and the series combined resistance R2 (Rt+r1+r2) of the temperature-measuring resistance circuit and the common resistance circuit, and therefore the arithmetic circuit performs the calculation in place of equation (8) to obtain the result of equation (10) above. temperature T
Display.

However, this generally does not display the correct temperature.

Since the characteristic of the temperature sensing resistor Rt at this time is the above equation (9), the correct temperature is from the above equation (9), and the calculation result of the above equation (10) is the above equation (ll) at all measured temperatures. Only when they are equal will the thermometer in Figure 5 give a correct reading.

Therefore, first, the resistance temperature detector Rt is set to the reference temperature of T-To, and the thermometer displays the correct reference temperature T at that time.

The resistance value r1 of the resistor r1 is adjusted so as to show the following.

In this case, as can be seen from equation (9) l (to) above, it should be as follows.

Next, when formula (10) is modified and further calculations are made using formulas (9) and (12), we get:
becomes.

Therefore, by adjusting the resistance value of the common resistor r2 (l + θ
)α0−α, the above (15) EE is completely the above (1)
1) is equal to the formula, and thus each resistance rl r r2
By setting the resistance value of , the device shown in FIG. 5 can always display the correct temperature.

At this time, the resistance value r2 of the common resistor r1 is (l+θ)α
0−α and the above formula (12).

However, in reality, the temperature of the resistance temperature detector Rt is set to a predetermined temperature T,
(for example, T, >To), the resistance value of the common resistor r2 may be adjusted so that the thermometer display at that time becomes the predetermined temperature T1.

As is clear from the above equation (15), even if the resistance value of the common resistor r2 is changed, when the reference temperature T=To, the thermometer always displays the correct reference temperature T.

is displayed. Therefore, even if the resistance temperature detector has characteristics that are slightly different from the desired characteristics, the temperature of the resistance temperature detector Rt can be adjusted to the reference temperature T by using the configuration shown in FIG.
.

The display at that time is the reference temperature T. The resistance value of the resistor r1 is adjusted so that the temperature of the resistance temperature detector Rt is set to a reference temperature T, for example.

By adjusting the resistance value of the common resistor r2 so that the display becomes the predetermined temperature T1, which is higher than the predetermined temperature T1, the device is completely calibrated and the device always displays correctly for any temperature within the measurement range. become.

For example, in FIG. 5, the arithmetic circuit that performs the calculation of equation (8) is α.

=5.00X l o-3/'C, R5=IO, and Ω is used for 00.

If the temperature characteristic of the resistance temperature detector Rt is, each resistance r1. The resistance value of r2 may be 0, but it generally has different characteristics, for example, R1
=9.90 (1-5,15X 10-3(T-To)
) ・(18).

At this time, if the resistance values of each resistor r, , r2 are adjusted as described above, then each resistor r1 >
The resistance value rl > r2 of r2 is defined by the above (12),
From equation (16), it should be that r = lOOΩ and r2 = 197Ω.

Of course, RT if you want to make adjustments. There is no need to calculate the values of and α, and there is no need to calculate the above equations (19) and (20).

However, if the characteristics of the individual temperature measuring resistors Rt used are uniform to some extent, the range of the resistance value of each resistor r1+r2 will be limited and adjustment will be easier.

In this way, a series resistor r1 is connected to the resistance temperature detector Rt, a common resistor r2 is connected to the resistance temperature detector Rt and the reference resistor Rs,
Temperature calibration is extremely simple because the calibration of the absolute value of temperature is completed by simply adjusting at the reference temperature T-To using the resistor r1 and adjusting at the predetermined temperature T=T using the common resistor r2.

This is because this method has the excellent advantage that the absolute value of the resistance value and the apparent temperature coefficient at the reference temperature T-To can be adjusted completely independently (7).

Furthermore, since the entire device is calibrated rather than a partial portion, the influence of leakage resistance, etc., for example, is all compensated for, making it possible to perform highly accurate calibration.

Furthermore, since the display is usually done by digital display, precise adjustment is easy, and from this point of view it can be seen that the calibration method is simple and highly accurate.

Furthermore, no measuring equipment is required for calibration.

In addition, when a thermistor or the like is used as a resistance temperature detector, there is a strong possibility that the temperature characteristics will change over many years due to aging, but it is extremely easy to recalibrate such a device even in that case. be.

And from these things, a highly accurate thermometer can be obtained at a low price.

Note that the configuration of the present invention is not limited to that of the above embodiment.

For example, in the above embodiment, a resistor r1 is connected in series with the resistance temperature detector Rt, and the reference temperature T=To is set by the resistor r1.
However, as shown in FIG. 6, it is also possible to connect a resistor r1 in series with the reference resistor Rs, and use the resistor r1 to perform the adjustment at the reference temperature T=To. The same effects as in the example can be obtained.

Further, in this case, the resistance value of the reference resistor Rs may be directly changed.

Further, in the above embodiment, a resistor r1 is connected in series with the temperature sensing resistor Rt.
However, the resistor r1 may be connected not in series but in parallel to the temperature measuring resistor Rt or the reference resistor Rs, or may be connected in series and in parallel.

Furthermore, each resistor r1+r2 may be a fixed resistor after adjustment, or may be a volume or a combination of a fixed resistor and a volume.

Further, for example, the measurement circuit and the calculation circuit shown in FIGS. 4 to 6 do not necessarily have to be separated.

As described above, according to the present invention, even if there are variations in the characteristics of the resistance temperature detector or changes over time, precise adjustment is extremely easy, and calibration can be performed easily and accurately, resulting in accurate temperature measurement. It becomes possible to do this.

[Brief explanation of the drawing]

Figure 1 shows the temperature characteristics of the resistance value of a thermistor, Figure 2 shows the temperature characteristics of the thermistor resistance value.
Figures a and b show the effect when a resistor is connected in parallel to the thermistor, Figure 3 a and b show the effect when a resistor is connected in series to the thermistor, and Figure 4 shows the effect of connecting a resistor in series with the thermistor. FIG. 5 is a diagram showing the construction of an embodiment of the present invention, and FIG. 6 is a diagram showing the construction of another embodiment of the invention. Rt...Resistance temperature sensor, Rs...Reference resistance, S...Selector switch, M...Measurement circuit, L...Calculation Circuit, D...Display circuit, rl...Resistor whose resistance value does not change depending on temperature, r2...Common resistance.

Claims (1)

[Scope of Claims] 1. A reference resistance circuit whose resistance value hardly changes with temperature, a resistance temperature measurement circuit including a resistance temperature sensor whose resistance value changes with temperature, and common to these reference resistance circuits and resistance temperature measurement circuits. The connected common resistance circuit, the reference resistance circuit and the temperature-measuring resistance circuit are switched, and from the series combination resistance R1 of the reference resistance circuit and the common resistance circuit and the series combination resistance R2 of the temperature-measuring resistance circuit and the common resistance circuit. , (where To is the reference temperature and α0 is the temperature coefficient of the resistance bulb circuit) When calibrating an electronic thermometer, the temperature of the resistance bulb is is the reference temperature T. The calculated and displayed temperature is T. The resistance value of the resistance temperature detector circuit or the reference resistance circuit is adjusted so that the temperature of the resistance temperature detector is T. A method for calibrating an electronic thermometer, characterized in that the resistance value of the common resistance circuit is adjusted so that the calculated and displayed temperature becomes T1 when a predetermined temperature T1 that is different from the temperature T1 is set.