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TPMI

Thermopile Sensor Module With Integrated Signal Processing

PerkinElmer Inc 

TPMI™ - Thermopile Sensor Module With Integrated Signal Processing

Ambient Temperature Compensation

The thermopile sensor generates a voltage UTP, which is

proportional to the incident net radiation Prad:

with S = sensitivity, K = instrumentation factor containing

the Stefan-Boltzmann constant and the view angle,

ε = emission factor.

The equation shows that for a fixed ambient temperature,

the thermopile output voltage UTP is proportional to Tobj4.

As also seen from the equation, the output signal will vary

when the ambient temperature changes and, therefore,

the TPMI will need to compensate this effect. Because the

ASIC has good thermal contact with the thermopile

sensor, it measures exactly the ambient temperature with

its integrated linear temperature sensor. In order to

achieve a best fit to the thermopile T4 characteristics, the

linear temperature signal will be further processed into a

polynomial behavior, which approximates the

characteristic to the required T4 behavior over a large

temperature range.

This voltage is added to the thermopile signal in the

compensation stage. It has to be noted that the ambient

temperature compensation needs to be adapted to each

individual application. In particular it is determined by:

• The viewing angle, which depends on the optics, i.e. the

aperture, the optical lens or mirror,

• The emission factor, which is dependent of the material

and surface of the measured object,

• The sensitivity of the thermopile chip and its temperature

coefficient, which should be as small as possible (note:

PerkinElmer thermopiles are designed to have an

extremely small temperature coefficient),

• The total temperature range to be measured

These factors need to be determined first in order to

assess the needs of the customer. PerkinElmer can

provide assistance in finding the optimum conditions for

an individual mass production part.

Measurement Results and

Achievable Accuracy

Figure 4

In mass production, the TPMI Modules are

calibrated under controlled ambient

conditions. Different black body temperatures

are used for calibration and testing. This

procedure ensures a precise adjustment of

the output curve and an excellent

reproducibility from batch to batch. The

PerkinElmer TPMI is indeed a consumer type

pyrometer head, but due to the sophisticated

adjustment procedures for an device with

100°C range an absolute accuracy of better

than ±2.5 K over the whole ambient

temperature range of 10 to 60°C is

achievable. In fact, an accuracy of ±1 K at

the calibration point is accomplished. For the

TPMI, the achievable measurement accuracy

depends mainly on the measurement range

(higher range induces lower accuracy) and the ambient

temperature span (below 10°C and above 70°C deviations

occur). It has to be noted, however, that under dynamic

conditions, i.e. object or ambient temperature changes,

the specified values might temporarily not be met due to

temperature inhomogeneities over the sensor module.

The curve in Figure 4 shows a typical deviation of the

measured temperature as a function of ambient

temperature variation. For temperatures between 10°C

and 80°C the VTObj output is relatively independent from

Typical measurement error as a result of ambient

temperature variation. Note the wide interval ranging

from 10°C to 80°C, where the object temperature

deviation is smaller than 0.5°C.

changes of Tambient. Above 80°C the VTObj characteristics

becomes nonlinear related to Tambient, which is caused by

a decreased approximation of the T4 by the ambient

temperature measurement. The same effect occurs at

ambient temperatures below 10°C.

www.perkinelmer.com/optoelectronics

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