Resistance thermometer
RTDs are electrical devices that utilize the temperature dependence of the electrical resistance of conductors to the measurement of temperature.
Pure metals exhibit greater resistance changes as alloys and have a relatively constant temperature coefficient of electrical resistance. For precise measurements using corrosion-resistant metals, usually platinum, as these show very little aging and because the thermometer is to manufacture it with a low error limits. The temperature-sensitive sensor, the measured resistance can also ceramic (sintered metal oxides) or semiconductors exist, which can be much higher temperature coefficient than with metals and thus achieve much higher sensitivities, but with lower precision and considerable temperature dependence of the temperature coefficient itself they are referred to as resistors thermistors, with thermistor ( NTC thermistors ) are used in metrology rather than thermistors ( PTCs ).
Platinum resistance thermometers for industrial use consist of a measuring insert in a corrosion-protective fitting. The wiring of the measuring insert is often done in a connection head, from which the thermometer can be connected via cable to an external electrical measurement device. The measuring insert is an easily replaceable unit, usually with ceramic or stainless steel casing and terminal block; This use includes, at its end one or more platinum measuring resistors. The measuring insert shown in the image contains the terminal block except the terminal screws for wiring two spring-loaded mounting screws that provide the necessary contact pressure for good thermal contact with the thermowell.
Conventional thermometers measure the temperature on the basis of length or volume change of a substance and are only suitable as a reading devices. The advantage of the resistance thermometer is that they provide an electrical signal and are suitable for use in industrial metrology.
Parameters and limiting errors
Within small temperature ranges can often be the formula
Be applied. Are belonging to the Celsius temperature resistance belonging to 20 ° C and the resistance -related temperature coefficient at 20 ° C is known, the temperature can be calculated as follows:
The temperature coefficient is regarded as a material constant that indicates the relative resistance change per temperature change
Prerequisite for such a simple calculation is a limited measuring range or a constant temperature coefficient. The latter is the case of metals and silicon only approximately the case; Thermistors of barium titanate in this approximation is not provided.
Platinum
For those in industrial metrology widespread platinum resistance thermometer and the measuring resistors used in it there is a standardization in the other summands are specified for the function behind the linear term
- For the range = 0 ... 850 ° C:
- For the range = -200 ... 0 ° C:
The values shown in the table below are calculated with these equations. This domain do not tell up to which temperature sensing resistor or a measuring insert can be set aside; the permissible operating range depends on the materials used in total and is specified by the manufacturer separately. Platinum resistance thermometer can also be used to -250 ° C to 1000 ° C or an appropriate design in extrapolation of the curve.
As nominal value is specified, ie the resistance at 0 ° C. Preferably, the nominal value of 100 Ω is used; these sensors are called Pt100. Other common denominations are 500 Ω and 1000 Ω. Details are listed under the heading of platinum resistance.
A platinum thermometer is to be assigned by the manufacturer to an accuracy class. In the standard indicated that the absolute value maximum permissible measurement errors (limit deviations) for each class are defined:
Example for preferably used Class B: At 500 ° C deviations of the measured value will be accepted up to ± 2.8 ° C.
The temperature coefficient of the resistor is determined by the standard somewhat different than common ( and above)
Thus, the reference temperature of 0 ° C instead of 20 ° C. The average temperature coefficient over the range 0 ... 100 ° C is given by and is used as a characterizing value. With the linear approximation
Are in the range -20 ... 120 ° C, the differences in absolute value less than 0.4 ° C. You are not greater than the above limit error (MPE due to production variation ) in class B.
Usually the temperature is measured, and sought. The resolution ( " reversal " ) or after the linearization ( generating a non- linearly linked with the resistance, but with the temperature output signal ) is partially performed by an integrated in the measuring insert transmitter, see below for measurement circuits.
Nickel
Nickel, in comparison with platinum has a higher sensitivity, it provides at the same temperature change, a greater relative change in resistance. However, this material has been removed from the standardization. For the transition temperature was in the range of -60 ° C to 250 ° C, the equation:
With = temperature in ° C; = Nominal resistance at 0 ° C; = 5.485 ∙ 10-3 ° C -1; = 6.65 ∙ 10-6 ° C -2; = 2.805 ∙ 10-11 ° C -4; = -2 ∙ 10-17 ° C- sixth
In addition to the Ni100 with = 100 Ω the remarks were Ni500 with 500 Ω Ni 1000 and 1000 Ω in use.
According to the last only applied to the nickel measuring resistors and since April 1994 withdrawn DIN 43760 were considered as limiting errors:
- In -60 ° C to 0 ° C → = 0.4 ° C 0.028 ∙ | |
- 0 ° C to 250 ° C → = 0.4 ° C 0.007 ∙
Disadvantage compared to platinum resistance are the smaller temperature range (-60 ... 250 ° C) and the larger deviation limit, especially in the range below 0 ° C.
Silicon
Silicon measuring resistors are used in the range -50 ... 150 ° C. For its temperature response is valid in the range -30 ... 130 ° C according to the data the equation:
With = Celsius temperature; = 25 ° C; = Nominal resistance at 25 ° C; = 7.88 ∙ 10-3 ° C -1; = 1.937 ∙ 10-5 ° C-2.
In the datasheet referred to nominal values for 1000 Ω and 2000 Ω are at a measuring current of 1 mA indicated with tolerance 1 ... 3%.
Further, there is the same temperature range for integrated circuits with linearized output signal, for example, nominally 1 uA / K or 10 mV / K at a supply voltage of 4 ... 30 V; for example.
Thermistor
NTC thermistors exhibit a strong non-linear relationship between resistance and temperature. For the mathematical description of the behavior is a function of the absolute temperature in the mold is
This is an arbitrary reference temperature such as 293 ° K (20 ° C). The size is a material constant; Approximate value = 2000 ... 6000 K.
The relative tolerances on values are typical at 20 %, from 5%.
The temperature coefficient is defined here something different and is found to be in the limit of differentially small temperature changes
It illustrates a temperature increases steeply sloping but high at room temperature measurement effect.
Example: = 3600 K; = 300 K; = - 40 ∙ 10-3 K -1. This is the absolute value approximately ten times compared to the case of platinum.
By interconnecting with ohmic resistors, the problems of sample variances and the non-linearity can be reduced, but this also reduces the sensitivity of the measuring arrangement.
Thermistor can be used depending on the version, for example, in the range 0 ... 150 ° C. They are produced in rod, disc or bead form, partially covered with glass.
For resistance measurement of the resistance of a constant current must flow through. The voltage applied is an easily measurable, the resistance proportional signal. Frequently, however, one does not measure this voltage, but only its change from an initial value by means of a difference -forming circuit ( Wheatstone bridge). To keep the error due to self-heating low, the measuring current must be very low, typical for Pt100 not higher than a milliampere.
In industrial plants, large distances between sensor and transmitter are often to bridge with correspondingly long supply lines. To avoid influences of the resistances of the lines to the measured value, platinum resistance sensors are manufactured with three- or four-wire connection. Thus, a separate supply of the measuring current and the possible supply error can be compensated. Outside, the installation is strongly encouraged with three or four conductors. Alternatively, a first transmitter is already accommodated in the connection head.
Sources of error
As with all contact thermometers static and retarding heat conduction influences must be considered. When resistance thermometers also come in addition to the already treated sample variances and the influence of the resistance of the test leads as error sources:
A defective insulation resistance can be electrically viewed as a parasitic parallel resistance to the measuring resistor. He therefore means that evaluating components show a too low temperature. It usually occurs during production of the sensors due to the penetration of moisture into the insert, especially where mineral insulated cable with hygroscopic insulating material such as magnesium or aluminum powder. For platinum measuring resistors according to an insulation resistance of ≥ 100 M at a DC voltage of at least 100 V at room temperature is prescribed, but only ≥ 0.5 milliohms at 500 ° C and 10 V.
They are caused by the thermoelectric effect and caused by the use of different materials for the leads and the platinum sensor itself this way, for in a measurement application, which is about supply lines of nickel and a platinum-chip sensor with palladium connecting wires used several parasitic thermal voltage sources. Since the thermal stresses can arise both on the supply line as shown on the return line be assumed usually assume that they cancel each other out. In unfavorable cases, however, for example because of irregular heat transitions occur on thermal stresses that can be distinguished from the evaluating electronics only from the voltage drop across the sense resistor, if it applies the polarity for each measurement.
The electric current produces a power loss at the measuring resistor, which is converted into heat and is dependent on the base value of the measured temperature, the type and the heat conduction and heat capacity. Since, in general, a measuring current of 1 mA is not exceeded, this power loss is at a Pt100 in the range of a few tenths of a milliwatt, normally produces no appreciable measurement error. Only in rare cases, the self-heating must be considered and determined for the particular application under operating conditions.
The hysteresis can become evident that the thermometer for large temperature changes no longer the same value measures as before. It is due to mechanical stresses in the sensor element, by different coefficients of expansion of the platinum and the support material or with glass sensors of the enclosure formed. This deviation caused by the pre-treatment must not for platinum measuring resistors according to one specified in the relevant standard test methods be greater than permitted by the limiting error at the test temperature for each accuracy class.