Question

why self heating is a problem in RTD's and not in thermisters?

Answer 1

To understand the problem of the self heating first we will have to understand the working of these two

Thermistor

Thermistors are a type of semiconductor, meaning they have greater resistance than conducting materials, but lower resistance than insulating materials. The relationship between a thermistor’s temperature and its resistance is highly dependent .

Thermistors are made up of metallic oxides, binders and stabilizers, pressed into wafers and then cut to chip size, left in disc form, or made into another shape.   This allows the thermistor to change its resistive value in proportion to small changes in ambient temperature. In other words, as its temperature changes, so too does its resistance and as such its name, “Thermistor” is a combination of the words THERM-ally sensitive res-ISTOR. A graph between the temperature and resistance is made with the help of which the change in temperature is measured with the change in resistance.

There are two types of Thermistor

1. NTC Thermistor

An NTC thermistor is an accurate resistor that has a negative temperature coefficient. When the device’s resistance decreases, it is the direct result of an increase in temperature. Used as tempreture sensors.

2. P-type Thermistor

With PTC thermistors, resistance increases as temperature rises. PTC thermistors are commonly installed in series with a circuit, and used to protect against overcurrent conditions, as resettable fuses.

Self heating in Thermistor

Thermistors are frequently referred to as resistive temperature sensors. An NTC thermistor is known for having a greater sensitivity coefficient providing a high output resolution. thermistors are typically used in temperatures ranging from a low of 40°C to a high of 300°C..

The self heating is a phenomenon that occurs whenever a current is flowing through a thermistor. A thermistor is a resistor, which causes it to dissipate power as heat (when currents are flowing). The heat originates from the core of the thermistors. As more currents flow, the heat will grow. Besides currents, the second most crucial factor for causing the self-heating effect is the environment in which the current is flowing through. A ENVOIRMENT with more heat diffusivity would cause more self heating and vice versa.

As Thermistor resistance changes drastically with increased tempreture making them more accurate however the range of temperature of tempreture witin they work is narrow as stated above so self heating is considerably less.

RTD( Resistance tempreture detectors )

Resistance Temperature Detector is a device which used to determine the temperature by measuring the resistance of pure electrical wire. This wire is referred to as a temperature sensor. It has good linear characteristics over a wide range of temperature. As the current flows the metal core chages it's resistance and temperature. With with change in resistance the temperature is calculated.

In RTD devices; Copper, Nickel and Platinum are widely used metals. These three metals are having different resistance variations with respective to the temperature variations. That is called resistance-temperature characteristics. Platinum has the temperature range of 650oC, and then the Copper and Nickel have 120oC and 300oC respectively.

Platinum core is mostly used beause it operates for high temperature range (650degree C).

Self heating in RTD's

As RTD's gives lower rise in resistance as the tempreture is increased making them less accurate but operablity for high temperature. As the tempreture range of RTD' S is more so the self heating is more.