XIDIBEI temperature sensor: advantages and disadvantages

Perfect temperature sensor：

• No effect on the measured medium
• Very accurate
• Immediate response (in most cases)
• Easy adjustment of output

 

Regardless of the type of sensor, all temperature sensors should consider the above factors.

Whatever is measured, the most important thing is to ensure that the measuring equipment itself will not affect the measured medium. This is particularly important when measuring contact temperature. Selecting the correct sensor size and wire configuration is an important design consideration to reduce "rod effect" and other measurement errors.

After minimizing the impact on the measuring medium, how to accurately measure the medium becomes critical. Accuracy involves the basic characteristics of the sensor, measurement accuracy, etc. If the design problem of the "rod effect" is not solved, no matter how accurate the sensor is, it will not help.

The response time is affected by the quality of the sensor element, and also by some wires. The smaller the sensor, the faster the response.

Characteristics

	NTC Thermistor	Platinum RTD	Thermocouple	semiconductor
Sensor	ceramics	Platinum wound type	thermoelectricity	semiconductor
	Metallic oxide spinel	Or metal film		Connection point
Temperature range (standard)	-100 ~ +325˚C	-200 ~ +650˚C	-200 ~ +1750˚C	-70 ~ 150˚C
Accuracy (standard)	0.05 ~ 1.5 ˚C	0.1 ~ 1.0˚C	0.5 ~ 5.0˚C	0.5 ~ 5.0˚C
Long-term stability @100˚C	0.2˚C/year（epoxy）		Variable, some types will change with age	>1˚C/year
	0.02˚C/year（Glass）	0.05˚C/year（film）
		0.002˚C/year（wire）
Output	NTC resistance	PTC resistance	Thermal voltage 10µV ~ 40µV/°C	Digital, various outputs
	-4.4%/˚C（standard）	0.00385Ω/Ω/°C
Linearity	exponential function	fairly linear	Most types are nonlinear	linear
Required power supply	Constant voltage or current	Constant voltage or current	Self-powered	4 ~ 30 VDC
Response time	0.12 ~ 10s	1 ~ 50s	0.10 ~ 10s	5 ~ 50s
Sensitivity to electrical noise	Quite insensitive,	Quite insensitive	Sensitive/cold junction compensation	Largely depends on the layout
	Only sensitive to high resistance
Influence of wire resistance	Only low resistance parts	Very sensitive.	No impact on short-term operation	Not applicable
		Requires three-wire or four-wire configuration	TC extension cable required
Cost	Low to medium	Wound type——high	Low	Medium
		film——low

 

 

The basic operating theory of each of the above main types of sensors is different.

The temperature range of each sensor is also different. The thermocouple series has the widest temperature range, spanning multiple thermocouple types.

The accuracy depends on the basic sensor characteristics. All sensor types have different accuracy, but platinum elements and thermistors have the highest accuracy. In general, the higher the accuracy, the higher the price.

The long-term stability is determined by the consistency of the accuracy of the sensor over time. Stability is determined by the basic physical properties of the sensor. High temperatures usually reduce stability. Pt and glass encapsulated wound thermistors are the most stable sensors. Thermocouples and semiconductors have the worst stability.

The sensor output varies by type. The resistance change of the thermistor is inversely proportional to the temperature, so it has a negative temperature coefficient (NTC). Platinum and other base metals have a positive temperature coefficient (PTC). The kilovolt output of the thermocouple is low and will change with the temperature. Semiconductors are usually adjustable with various digital signal outputs.

Linearity defines the condition that the output of the sensor changes uniformly within a certain temperature range. The thermistor is exponential nonlinear, and its sensitivity at low temperature is much higher than that at high temperature. With the increasing application of microprocessors in sensor signal conditioning circuits, the linearity of sensors becomes more and more problematic.

After power-on, both thermistor and platinum element need constant voltage or current. Power regulation is critical to control automatic heating in thermistors or platinum RTDs. Current regulation is not very important for semiconductors. The thermocouple produces a voltage output.

The response time, that is, the speed at which the sensor indicates the temperature, depends on the size and quality of the sensor element (assuming no prediction method is used). The response speed of semiconductor is the slowest. The response speed of wound platinum element is the second slowest. Platinum films, thermistors and thermocouples are available in small packages, so they have high-speed options. The glass bead is the most responsive thermistor configuration.

Electric noise, which may cause incorrect temperature indication, is a major problem when using thermocouples. In some cases, extremely high resistance thermistors may be a problem.

Wire resistance may cause error deviation in resistive devices such as thermistors or RTDs. This effect will be more obvious when using low-resistance devices (such as 100 Ω platinum elements) or low-resistance thermistors. For platinum components, use a three-wire or four-wire wire configuration to eliminate this problem. For thermistors, this effect is usually eliminated by increasing the resistance value. Thermocouples must use extension wires and connectors of the same material as wires, otherwise errors may be caused.

Although thermocouples are the cheapest and most widely used sensors, NTC thermistors are often the most cost-effective.

 

Advantages and disadvantages

	NTCThermistor	Platinum RTD	Thermocouple	Semiconductor
Sensor	Ceramic (metal oxide spinel)	Platinum wound or metal film	Thermoelectricity	Semiconductor connection point
advantages	Sensitivity Accuracy Cost Robust and durable Flexible packaging Sealing Surface mounting	Accuracy Stability Linearity	Temperature range Self powered No automatic heating Robust and durable	Easy to use Plate installation Robust and durable Total cost
Disadvantages	Nonlinear Automatic heating Wet fault(For non-glass equipment only)	Wire resistance error Response time Vibration resistance Size Packaging restrictions	Cold junction compensation Accuracy Stability TC extension cable	Accuracy Limited application Stability Response time

 

Each sensor has its advantages and disadvantages. The main advantages of thermistors are:

Sensitivity: The thermistor can change with very small temperature changes.

Accuracy: Thermistors can provide high absolute accuracy and error.

Cost: For the high performance of thermistor, its cost performance is very high.

Ruggedness: The structure of the thermistor makes it very rugged and durable.

Flexibility: Thermistors can be configured in a variety of physical forms, including minimal packaging.

Sealing: The glass package provides a sealed package to avoid sensor failure due to moisture.

Surface mounting: provide various dimensions and resistance tolerances.

Among the disadvantages of thermistors, only automatic heating is usually a design consideration. Appropriate measures must be taken to limit the induced current to a low enough value to reduce the automatic heating error to an acceptable value.

Nonlinear problems can be solved by software or circuit, and wet problems that can cause faults can be solved by glass packaging.

All sensors have specific advantages and disadvantages. To ensure the success of the project, the key is to match the sensor function with the application. If you need help in determining whether the thermistor is the best design option, please contact xdbsensor@gmail.com