Welding a thin thermocouple with cylindrical junction under a microscope

Thermocouple is a standard to measure temperatures and is widely used in industry and academia. Thermocouples are made by welding two wires with different materials at one end. The temperature of the welded junction is obtained by measuring the voltage between the two open ends. The physical principle of thermocouple is based on thermoelectric effect (Seebeck effect) where the temperature difference can be converted to electricity. The temperature and voltage conversion are tabulated into standard tables for various types of thermocouples made with different materials. These tables (or sometimes polynomial fitted equations) are often used to refer temperature from a measured voltage.

Thermocouples only perform point measurement of the junction temperature. The junction of the most commercially available thermocouples are mostly sphere-like but the geometry is not in a well-defined shape. The junction is also normally bigger than its adjoining wires.

This could introduce two problems: 

1. When thermocouple is used to measure fluid temperature such as hot gas stream, there is more uncertainty in calculating heat transfer when the junction does not have a well-defined geometry; 

2. The junction may have different temperature of its adjoining wires due to different radiation loss.

The merits of making a thermocouple with cylindrical junction is to solve the two problems to improve the accuracy of temperature measurement.

The following video shows how I welded the thermocouple under a microscope using a micro-torch.

The video is also available on youtube:

http://www.youtube.com/watch?v=HUriLTnkCQw

Spectral response of Nikon DSLRs (D90 and D300s)

Relative spectral response of two Nikon DSLR cameras were measured and shown in the post.

Background on spectral response:

Spectral response of a detector describes the efficiency of transforming photons to digital signals (i.e. CCD counts). It is related to quantum efficiency QE by the following equation

SR(lambda) = Const*QE(lambda)./lambda

where SR stands for spectral response, QE stands for quantum efficiency and lambda is the wavelength. 

Bayer pattern filter array employed in DSLR cameras

For a DSLR camera, a Bayer pattern filter array with RGB channels are normally employed to yield a vivid color image. The spectral response of each channel was measured and shown below.

Setup:

A stable light source is used to provide the illumination. A calibrated spectrometer is used to disperse the light and provide a known spectrum on the ground glass. The spectrum is then imaged by the camera. The ratio of the real spectrum (on the ground glass) and the imaged spectrum (by the camera) is a direct measure of the relative spectral response.

Setup to measure the spectral response of a DSLR camera

Images are taken in raw format (.NEF for Nikon) to preserve quantitative information. The subsequent image decoding and processing is performed by an open source software OMA that runs on Mac OS.


Results:

The spectral response of two Nikon DSLR cameras are shown here. Despite that D300s is a more advanced model than D90, the two employ the same CMOS sensor. The measured spectral responses of the two are quite similar. The Y-axis shows the relative signal response.

Spectral response of Nikon D90

Spectral response of Nikon D300s