Measuring Temperature
Understanding the advantages,limitations and specifications of different temperature sensors

There are several types of sensors you can attach to Mosaic's A/D boards to measure temperature, each with their particular advantages. This page describes the considerations you will use in choosing which type of temperature sensor you need. To go directly to instructions for using particular types of sensors, choose from the following:

• Using Thermistors
• Using RTDs
• Using Thermocouples
• Using LM335 IC Temperature Sensor

Briefly, you can easily use the following sensors with Mosaic's controllers:

• Thermistors
  • Good sensitivity, easily measured, various accuracies available
  • Good for low to moderate temperatures (-55 to +180°C standard)
  • Excellent interchangeability
• Semiconductor devices
  • Good sensitivity and very easily measured
  • Good for low to moderate temperatures (-40 to +100°C)
• RTDs
  • Not very sensitive but extremely stable
  • Wide temperature range (-200 to 850°C)
  • Better accuracy than thermocouples
  • Good interchangeability
• Thermocouples
  • Least sensitive, difficult to measure
  • Ideal for very high temperature, to 2300°C

The basic characteristics of the various types of sensors are summarized in the following table, for specific exemplar sensors of each type.

A thermistor is a resistive device whose resistance varies strongly with temperature, roughly exponentially with inverse temperature. Their response is very easy to measure. They differ from resistance temperature detectors (RTDs) in that they are made of a semiconducting ceramic (with an exponential temperature dependance), while RTDs use pure metals (with a linear temperature dependance). Thermistors now come in many different accuracy grades. While traditionally they were limited to a temperature range of -50 to +150°C, you can now obtain accurate devices for use to +350°C.

Use thermistors if you need high accuracy at low to moderate temperatures and simple measurement. See Using Thermistors for details of their use.

Integrated circuit sensors, sometimes called diode temperature sensors, are small active electronic devices good for measuring temperatures below 150°C. The popular LM335A¹⁾ is a precision, easily-calibrated, integrated circuit temperature sensor. Operating as a 2-terminal zener, it has a breakdown voltage directly proportional to absolute temperature at +10 mV/°K. Its uncalibrated temperature error is ±1°C at 25°C and within ±2°C over a range of -40 to +100°C.

The sensor's low impedance and linear output make measurement easy. Use it for ambient temperature measurement, particularly if you prefer simple measurement circuitry.

The resistance of most metals is directly proportional to absolute temperature – RTDs rely on that characteristic to make very reliable and stable temperature sensors. They are simple resistors, usually made from platinum wire or thin films. With a temperature range up to 850°C, RTDs can be used in all but the highest-temperature industrial processes. When made using metals.

There are two resistance tolerances specified in the DIN/IEC751 standard for RTDs:

• Class A: ±(0.15 + 0.002*T) °C, or 100.00 ±0.06 Ω at 0ºC
• Class B: ±(0.3 + 0.005*T) °C, or 100.00 ±0.12 Ω at 0ºC

The combination of resistance tolerance and temperature coefficient define the resistance vs. temperature characteristics for the RTD. The larger the element tolerance, the more the sensor will deviate from the ideal, and the more variation there will be from sensor to sensor, affecting its interchangeability. The following table provides the uncalibrated accuracy (or interchangeability tolerance) for Class A and B RTDs.

You should consider using an RTD if you need a wide temperature range (approximately -200 to 850°C), better accuracy than you'd get from thermocouples, good interchangeability, and good long-term stability when measuring high temperature.

See Using RTDs for detailed instruction on measuring RTDs.

Thermocouples comprise two wires made of different metals and welded together at the temperature measurement end. When the hot junction is heated a small voltage is produced that can be measured. For small temperature differences the voltage produced is linear in temperature, but for larger temperature differences there are significant nonlinearities. Because the thermocouple is made of high melting point wire, you can use it at very high temperatures. For example, Type K thermocouples are good to 1260°C, and other types are good to 2300°C.

You should use thermocouples for high temperature measurement. See Thermocouple Types for details of their use.

The following diagram illustrates connections of the different types of temperature sensors to the 24/7 Data Acquisition Wildcard.

Briefly, each of the sensors shown are read by the A/D as follows:

• The LM335A produces an output voltage of 2.33V to 3.73V for a temperature range of -40 to +100°C, that is, 10 mV for each degree Kelvin. It requires a bias resistor which does not need to be precise. Although the A/D reference voltage is 2.500 volts, the 3V signal is can be read because it lies within the 0 - 5V common mode range of the A/D. It is read using differential channel pair CH_4 (the + input) and CH_5 (the – input) with unity gain. CH_5 is connected to the 2.5V reference so that the input signal stays within the common mode range of the A/D, and so that the difference signal remains less than the 2.5V maximum.
• The RTD is placed in series with a 1kΩ reference resistor and driven from the 2.5V reference voltage. It produces a voltage of approximately 0.045V to 0.7V for a temperature range of -200 to +850°C. The voltage is measured in pseudo-differential mode between CH_3 and CH_7. Depending on the temperature range of interest, the A/D may be configured with a gain of 1, 2, 4 or 8. The RTDs require a precision reference resistor which should be a 0.1% resistor.
• The thermistors, including the cold junction compensation thermistor, are assumed here to be standard 10kΩ at 25°C thermistors. They are placed in series with precision 10kΩ ±0.1% resistors and driven from the 2.5V reference voltage. They are measured in pseudo-differential mode with a gain of unity, between their respective input channel (CH_1 or CH_2) and the signal return channel, CH_7.
• The thermocouple is connected between CH_0 (the + input) and CH_7 (the – input), with CH_7 grounded to the analog ground. The thermocouple output varies from approximately -10 mV to +20 mV for a temperature variation from -200 to +500°C. The common mode range of the A/D extends to 30 mV below ground, so even when the thermocouple produces a slightly negative output it can be read by the A/D. The thermocouple is read using bipolar mode with a gain of 128, allowing an input range up to 19.5 mV.

To go directly to instructions for using particular sensors, choose from the following:

• Using Thermistors
• Using RTDs
• Using Thermocouples
• Using LM335 IC Temperature Sensor

You can also learn more about temperature measurement, and view specifications for many specific sensors, at the Omega website.