Precision Measurement Engineering, Inc.
Precision Measurement Engineering, Inc.
The fast temperature channel consists of the 5316 Fast Temperature sensor and associated circuitry. These circuits are located on the 5264 uDO & Dual T circuit board.
The 5316 Fast Temperature sensor consists of a fast thermistor and a compensating resistor. The image below shows the sensor. The compressed seal fitting uses the nut shown to compress an internal Teflon ferrule.
Completed sensors are extensively thermal-cycled and pressurized before being electrically tested. Our electrical test will detect any cracks that occur anywhere in the assembly.
The sensor is designed to be mounted on the SCAMP and is bent such that it can be closely aligned with other sensors.
The fast thermistor is a Thermometrics FP07DB104N. For more information, view the manufacturer's data sheet. This thermistor is specified as having a resistance at 25 deg C of 100K ohms, plus or minus 25K ohms. The resistance of this type sensor changes in response to the temperature of the sensor. The change is not a linear function of temperature and can be described by
R(T) = R(To) * exp [Beta * (1/T - 1/To))]
where
T - sensor temperature, deg K,
To - reference temperature, 296.15 deg K (25 deg C),
R(To) - sensor resistance at reference temperature,
R(T) - sensor resistance at temperature T,
Beta - thermal sensitivity of thermistor, 4015.
The thermal time constant is given by the manufacturer as 0.007 seconds giving an approximate response frequency of 22 Hz. At the SCAMP travel rate of 10 cm/sec, this response is 2.2 cycles/cm, 220 cycles/meter.
PME allows a range of +/- 25% variation of R(To) that makes this part much easier, and therefore less expensive for the manufacturer to build. Unfortunately, this tolerance is too wide to allow a single circuit to operate all possible thermistors. PME therefore measures every thermistor when received and determines a compensating resistance. This resistor is included within the 5316 sensor shaft. The electrical wiring diagram shows the wire color definition.
See REFERENCES for more information.
The electronic circuit applies Vref reference voltage
to R1. This voltage is divided by R1 and the thermistor resistance to produce a voltage
that is a quasi-linear function of temperature. The circuit amplifies this voltage,
filters it, and transmits it to the multiplexor and also to the gradient circuit.
Fast T - example of a typical calibration of
this channel. Since a compensating resistor is selected for each thermistor, this
calibration will vary only slightly from sensor to sensor.