Precision Measurement Engineering, Inc.
Precision Measurement Engineering, Inc.
The 5199 Accurate Conductivity Temperature sensor consists of a conductivity sensor, a thermistor, and a compensating resistor. The ceramic substrate and thermistor housing are molded thus causing a more reliable seal. Once the assembly is inserted into the stainless steel shaft, shrink tubing is used to cover this interface.
The mechanical
drawing shows that this sensor is designed to be mounted on the SCAMP, but has a
straight shaft and will not come into close alignment with the other sensors.
The thermistor is a Thermometrics T1201/B3A-B07KA104N. 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 20K 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, 4243.
The thermal time constant of this sensor is approximately 0.2 seconds.
PME allows a range of +/- 20% 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 measures every thermistor when received and determines a compensating resistance. This resistor is included with each sensor.
The conductivity sensor is manufactured by PME. The sensor consists of an alumina substrate on which 4 electrodes, two on each side, have been deposited by a high temperature thick film process. These electrodes are made of platinum and are platinized. The sensor functions by being a dimensionally stable contact between the salt water and the electronic circuit connected to the sensor. The electronic circuit makes a 4 terminal A.C. measurement of the conductance of the salt water. The Accurate Conductivity diagram shows the sensor's wiring and wire color definition.
The measurement made by the combination of the conductivity sensor and its electronic circuit can be shown to result from a volume weighted average of the conductivity of the salt water near the sensor and within the sensor cylinder. This sensor has a spatial response that is effectively a low pass filter in space. The roll off point is presently unknown but can be estimated as a running boxcar average of conductivity over the length of the cylinder, 1.5 cm. There is also a time related response, which is presently unknown.
The purpose of the Accurate CT sensor is to provide a convenient calibration reference for the faster sensors, especially the Fast Conductivity sensor. Because of its small size, the Fast Conductivity sensor suffers from drift induced by fouling. The Accurate Conductivity sensor is much larger, has much larger electrodes, and resists fouling. Information from this sensor can be used to determine if the Fast Conductivity sensor is reporting correct values during a profile and to provide information to correct the Fast Conductivity sensor if it is not.
The accurate temperature channel circuitry is the same as that used for fast temperature. This circuitry is located on the 5132 Conductivity-Temperature circuit board. The accurate conductivity channel circuitry is the same as the fast conductivity circuitry with the exception of the scale selection resistor value and type and is located on the 5132 circuit board also.