AVOID HEADACHE WHEN PURCHASING YOUR NTC
On what:
T1 is the temperature in Kelvin (K) of point 1
T2 is the temperature in Kelvin (K) of point 2
R1 is the resistance in ohms (Ω) of point 1
R2 is the resistance in ohms (Ω) of point 2
The Beta must always be accompanied with the reference of the temperature values used for the calculation, or they can generate significant errors, mainly when applied in processes, where the temperature is distant from 25 ° C. The most common references are:, and. From Equation 1 and the data from the resistance x temperature table of the NTCs 10K Type II, 10K Type III and 20K, it was possible to obtain their respective Betas references, presented in Table 1. It was considered that a variation of the Beta of ± 10 generates a negligible error, less than 0.1 ° C at high temperatures.
Table 1 - Betas calculated from the data of the resistance x temperature table of the NTCs 10K Type II, 10K Type III and 20K.
If the controller or manager allows the use of customized tables, it is perfectly possible to use thermistors of any Beta, but another point to be considered is the thermistor error. In general, manufacturers use two types of tolerance control in the manufacture of the thermistor:
Curved-matched measurement: tolerance is guaranteed over a temperature range. A common specification is 0.2 ° C @ 0-70 ° C, which means a maximum error of 0.2 ° C over the entire range between 0 and 70 ° C.
Point-matched measurement: tolerance is guaranteed at a single point, usually represented by sellers, for example, as 1% @ 25 ° C (or 0.22 ° C @ 25 ° C), which means tolerance 1% (in resistance) at 25 ° C.
Curved measurement is ideal for monitoring environments or processes that do not have a specific pre-defined temperature, which for some reason may vary their temperature over time. Single point calibration is indicated for processes that remain at the temperature at which the tolerance was defined or for those that are critical specifically at the calibration temperature.
The use of a thermistor with single point gauging in systems that have variations, can generate considerable errors in the acquisition of temperature. Figure 1 shows the temperature tolerance graph of a single point gauging thermistor with a specification of 5% @ 25 ° C. It is observed that at 25 ° C, the temperature at which the thermistor was measured, the tolerance is 5%. The further the temperature moves away from 25 ° C, either up or down, the tolerance increases significantly.
Figure 1 - Tolerance curve given by the manufacturer, with measurement at 25 ° C.
The use of the thermistor with specifications not ideal for your application can cause a reading error of several degrees. This can lead to loss of comfort and productivity in the control of environments, loss of efficiency in processes or excessive energy expenditure in refrigeration systems, which often reflect a monthly cost hundreds of times higher than the cost of a quality thermistor.
In the area of air conditioning, three types of NTCs are predominantly used, 10K Type II, 10K Type III and 20K. The standardization of these NTCs on the market allows interoperability between different brands, allowing the use of a sensor from one manufacturer and controller from another. These nomenclatures are already so ingrained in the area of air conditioning, that several basic definitions about thermistors have been lost and, with that, different companies are being offered offering 10K Type II or III NTCs with different curves. In the case of the 20K it is even worse, as no definition accompanies it, causing the impression that any 20K will do.
A good practice when buying a thermistor is to request its resistance table and compare it with the controller that will be used, in order to check the compatibility between the products. However, a more efficient way to reduce reading errors is to observe the Beta, constant of the thermistor that indicates the shape of its temperature resistance curve. Beta is widely used by the thermistor manufacturer, however, it is often not present in the final seller's manual. If it is not specified in the manual, the Beta value can be obtained by applying data from any two points, spaced at least 10 ° C, from the resistance x temperature table in the following equation:
Equation 1: