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When you get to know Negative Temperature Temperature (NTC thermistor), it is inevitable that you will come across a series of parameters related to it. Imagine how unfortunate it would be if the thermistor does not work properly or even malfunctions because the parameters do not correspond to each other. Among these various parameters, B-value is one of the key ones.
B-value is the key factor in understanding and using NTC thermistors. In general, this parameter is the material coefficient. The B-value of a thermistor is defined as the ratio of the difference between the natural logarithm of the zero-power resistance value at two temperatures to the difference between the inverses of those two temperatures. NTC thermistors typically have B-values in the 2000K-6000K range, which varies depending on the application.
This article will look at what the B-value of an NTC thermistor is, why it is important, and how to calculate it. The rest of the parts will also derive the formula for deriving the B-value as well as ways to get the B-value wrong.
What Is The B-Value Of Ntc Thermistor
Thermistor B-value is a key parameter that affects the performance of NTC thermistors. It is a material constant (thermal index) and can be calculated by measuring the resistance of an NTC temperature sensor at 25°C and 50°C (or 85°C). Understanding the B-value of a thermistor is key to achieving accurate temperature measurements and selecting the right sensor for your application.
Chart Of B-value
The B-value is expressed in Kelvin temperature (K). Different temperatures exhibit different resistance values, with negative temperature coefficient thermistors (NTCs) having lower resistance values at higher temperatures.
Thermistor B value is expressed mathematically as follows:
Why Is B-value Important For Ntc Thermistors
Determining a B value involves measuring resistance at 25°C and 50°C (or 85°C). A higher B-value means that the resistance drops faster as the temperature increases, indicating greater sensitivity. Conversely, a smaller B-value means a slower response. It is important to note that the B-value is not a constant in practice; it increases slightly with increasing temperature.
The Importance of Temperature Sensing Accuracy
Accuracy is an important factor in the merit of a temperature sensor. From automotive and industrial systems to healthcare and consumer electronics, wherever temperature sensors are used, accuracy needs to be ensured.
Accurate temperature monitoring ensures that systems operate within safe and optimal limits, preventing overheating, inefficiency and even catastrophic failure. For example, in medical devices such as thermometers and imaging equipment, accurate temperature data is critical for reliable diagnosis and treatment. High-accuracy sensors improve energy efficiency, safety and performance. And accuracy is intrinsically linked to the thermistor’s B-value, a key parameter that determines how a thermistor’s resistance changes with temperature.
How B value affect accuracy of NTC thermistor
As you can see from the previous section, B-value and accuracy are closely related. A higher B-value NTC thermistor increases sensitivity, resulting in more accurate temperature readings.
E.g B25/50=3435K±1.0%
B value 3435K at T1=25℃, T2=50℃, accuracy 1%.
The following factors demonstrate how B-value affects accuracy:
Sensitivity
The B-value of an NTC thermistor affects the sensitivity of the measurement. In general, higher B-values enable more accurate temperature measurements.
Linealidad
The thermistor B-value also affects the linearity of the resistance-temperature relationship. Linearity and accuracy can be improved by stabilizing the NTC thermistor B value over the entire operating range.
Rango de temperatura
The accuracy of the mapping between resistance and temperature is highly dependent on the B-value of the thermistor. An incorrect B-value may:
- Result in accurate readings at the calibration point but different readings at other temperatures.
- Result in non-linear readings.
This error can lead to serious errors, especially in critical environments where accuracy is essential.
As for products designed for temperature measurement, compensation, and surge suppression resistance, the larger the B value, the better.
How To Calculate The B-value Of Ntc Thermistor
At this point, you know that the thermistor B value is a key parameter that affects the accuracy of temperature measurement. Then, it is necessary to understand how to calculate the NTC thermistor Beta value. Therefore, a method is needed to specify not only the resistance at a specific temperature (i.e., the nominal resistance), but also the shape of the curve through that point. Most datasheets provide Beta (β) values in K. The B values of common NTC materials range is from 3000K to 5000K. But sometimes these B values don’t seem obvious. For example, is a 3500K thermistor a suitable replacement for a 3650K thermistor?
This article provides 2 generalized algorithms for measuring B-values.
1. Basic two-point method
The B-value of an NTC thermistor is calculated using resistance measurements at two different temperatures:
Notes:
T1 is the first temperature point in Kelvin=Temperature 1 (K)
T2 is the second temperature point in Kelvin=Temperature 2 in (K)
Rt1 is the thermistor resistance at temperature T1 in ohms
Rt2 is the thermistor resistance at temperature T2 in ohms
This calculation method is relatively simple. However, it assumes that the NTC thermistor Beta value is constant over the entire temperature range, which can lead to inaccurate measurements and ultimately to incorrect accuracy.
2. Steinhart-Hart Equation
What’s better than knowing the resistance of two points on a curve? Well, obviously, it’s better to know the resistance at 3 points, and that’s exactly what the Steinhart-Hart coefficients (A, B, and C) are for. With these parameters, it is possible to more accurately approximate the curve at the entire operating temperature. This equation is given below:
This method allows for a more accurate calculation of the NTC thermistor Beta value and gives a better indication of the thermistor’s behavior at different temperatures.
Conclusión
The thermistor B-value is critical for temperature measurement of NTC thermistors. It affects accuracy, sensitivity and linearity, so measuring an accurate B-value is critical to selecting the right thermistor for a particular application.
The B-value of an NTC thermistor determines the thermistor’s performance under various conditions. There are generally two ways to make this measurement. The simpler one allows easy calculation of the result. However, when it comes to parameters that are currently constant, a more precise method (such as the Steinhart-Hart equation) is required to ensure accurate readings over a wide temperature range.
Understanding calculation methods and B-value variations can help achieve accurate temperature readings and improve overall system performance. For high-precision applications, understanding and selecting the correct thermistor B-value is critical to achieving reliable and accurate temperature measurements.
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