Making Radiometric Measurements with Thermopile Detectors

This article briefly shows how to make radiometric measurements using thermopile detectors. However, before taking measurements, the cold junction temperature of the thermopile has to be approximated. Dexter has included internally mounted thermistors on certain models as well as externally mounted thermistors on all of its models.

The DC-4005 Chip Thermistors can be internally mounted on all Dexter’s models such as the ST120, ST150, ST60, 2M, 2M Quad, 10-Channel, T34, TM34 and the SLA32 array to track the thermopile’s ambient temperature. Dexter supplies the DC-4007 Chip-in-Glass Thermistors that can be mounted externally by customers.

Customers can either supply thermistors to Dexter or select from the company’s regular range of thermistors. In addition, the ST60R and ST150R on detector die poly-silicon resistor/thermistors are also available.

Chip Thermistor DC-4005 (T4)

The key specifications of the Chip Thermistor DC-4005 (T4) (Dexter part number: DC-4005 (formerly MT04)) include:

Table 1. shows resistance versus temperature in 1°C increments, and Table 2 shows Curve E resistance versus temperature in 5°C increments.

Table 1. Resistance versus temperature in 1°C increments -- Curve E resistance at 25°C = 30,000Ω.

Table 2. Curve E resistance vs. temperature in 5°C Increments with manufacturing tolerance adders, resistance at 25°C = 30,000Ω ± 5%.

Definitions of Terms Used

The negative temperature coefficient (NTC) of resistance refers to the percentage of the resistance change with respect to the change in temperature, and is represented in units of -%/°C. One way to establish the thermistor’s approximate % resistance tolerance at a specific temperature is to multiply the specified NTC value by the temperature tolerance at that particular temperature.

In the case of thermistors that have a specified tolerance at a single temperature point the tolerance at other temperature points is determined using the manufacturing tolerance. Generally, thermistors are specified at 25°C temperature. Also the manufacturing tolerance can be added to the given point matched tolerance to establish the resistance tolerance at a different temperature point.

For instance, a thermistor having a ± 5% resistance tolerance at 25°C temperature exhibits a ± 6.4% tolerance at 0°C temperature. Typically point matched thermistors should not continuously work over a temperature of 105°C.

Chip-in-Glass Thermistor DC-4007 (T6)

The key specifications of the Chip-in-Glass Thermistor DC-4007 (T6) (Dexter part number: DC-4007 (formerly MT06)) are as follows:

Figure 1 shows the resistance of the DC-4007 thermistor as a function of temperature.

Figure 1. DC-4007 resistance as a function of temperature

Thermistor ST60R

The following are the key specifications of the ST60R thermistor (Dexter part number: ST60R):

Figure 2 shows the resistance of the ST60R thermistor as a function of temperature.

Figure 2. ST60R resistance as a function of temperature

Thermistor ST150R

The following are the key specifications of the ST150R thermistor (Dexter part number: ST150R):

Figure 3 shows the resistance of the ST150R thermistor as a function of temperature.

Figure 3. ST150R resistance as a function of temperature

This information has been sourced, reviewed and adapted from materials provided by Dexter Research.

For more information on this source, please visit Dexter Research.