Microstructure Thermistor

The FP07-38 is a fast-response temperature sensor for oceanographic applications. The sensing tip is a  microbead thermistor (manufactured by GE Thermometics), mounted on a 3/8" diameter stainless steel sting, which can be conveniently attached to ocean microstructure instruments.

The FP07 consist of small diameter glass-coated thermistor bead, hermetically sealed onto the tip of a shock-resistant glass rod. The small bead thermistor has a very thin glass coating, which allows frequency response of ~23 Hz, or a response time of 0.007 s. The bead is exposed as much as possible at the tip of the glass rod to provide the fastest response times. These sensors are ideally suited for high speed measurement of fluid temperatures.

Specifications:

Range	-2... 32 °C
Resolution	0.0001 °C
Response	7 ms (nominal)
Nominal resistance 2 kOhm (at 25 °C)

Dimensions:
Glass bead max. 2.2 mm
(thermistor tip) ~0.2 mm
L (overall) 127 mm
W (in air) app 100 g

Diameter at base 0.375 inches (9.53 mm)

Materials:
Housing: SS316 stainless steel
Connector: brass
Bead: glass

Noise Level:
The FP07-38 thermistors in conjunction with the ASTP signal conditioning electronics is used to measure microstructure temperature fluctutions. A small amount of current must be passed through the thermistor in order to measure its resistance and, hence, to measure temperature. The electric current heats the thermistor and raises its temperature above the temperature of the ambient water. The concern for microstructure measurements is the "anemometer" effect of the self-heating because the temperature rise of the thermistor depends on the speed of the flow of water past the sensor. Thus, a thermistor is also sensitive to velocity fluctuations and this can contaminate the measurement of temperature fluctuations.

The noise level is specified in terms of the rate of dissipation of temperature variance, χ. Based on the characteristics of the ASTP board and the FP07 thermistor, and using a realistic values for the maximum velocity fluctuations in the ocean, we can estimate the measurement noise in terms of the rate of dissipation of temperature variance.

W [m s-1]	χN [oC2s-1]	χF [oC2s-1]
0.1	3 x 10-11	9 x 10-11
0.3	4 x 10-12	7 x 10-12
0.5	1 x 10-12	2 x 10-12
0.7	7 x 10-13	9 x 10-13
1.0	3 x 10-13	4 x 10-14

W is the speed of the thermistor through the water.
χN is the noise of the measurement of χ due to the Johnson noise.
χF is the noise of the measurement of χ due to oceanic velocity fluctuations.

From the above, it is clear that the Johnson noise of the thermistor is similar to the maximum velocity signal that the thermistor will sense. Thus, the FP07-3/8, in conjunction with the RSI ASTP signal conditionaing board, operates at the theoretical limit imposed by thermodynamics and is completely free from velocity contamination.

Example Data:

The graph below shows data from an experiment in which the FP07 was towed through a warm, turbulent plume, over a distance of 1500 mm. The sensor was towed at a speed of 0.1 m/s. The temperature spectrum reaches a noise floor at 100 cycles per meter.