Development of a Oxygen Sensor for Marine ... - DTU Nanotech

36 CHAPTER 4. SYSTEM DESIGN
the resistance that I want to assign). This in itself may seem straightforward,
however neither Rc nor Rsp can be calculated precise, so as a result it isn’t
possible to measure Rs accurately.
The 4-contact approach solves this problem however; 2 carry the current
while the other 2 are used to measure the voltage. While Rc and Rsp is still
there, they are now negligible due to the very small current that flows through
them, as the voltage is now measured with a potentiometer that draws no
current, or with a high impedance voltmeter that draws little current.
Temperature Calculated Width Effective Heigth
Sensor Resistance (Ω) (µm) Length (µm) (µm)
Pt50 47.8 20 18019 0.2
Pt300 287 10 54170 0.2
Pt600 590 10 113207 0.2
4.4 Packaging
Table 4.1: Overview of the temperature sensors.
Shielding the interface electronic part of the chip from the corrosive effect
of seawater, is as mentioned earlier the ’O’-ring and packaging. How is this
built?
The package or casing consists of three different layers (hereafter referred
to as the lower, middle and upper layer) of laser 2 cut PMMA (PolyMethyl-
MethAcrylate), that each have their own structure. While there are materials
that are not as porous, and have better chemical properties, PMMA
have the advantage that several can be fabricated within minutes once the
actual structure is in place. Which have a higher priority than long term
usage as far as testing purposes goes. Also PMMA can generally be used in
the temperature area from -40 o C to 70 o C, as well as being electric isolating.
So what do these layers look like, and what purpose does each serve?
2 Laser used is a SYNRAD laser