4-20mA transmitters
allow resistive sensor measurements to be extended long distances.
Resistance temperature sensors, such as thermistors & RTDs, provide the ability to make precise & accurate measurements; based on a predictable change in ohms, per degree of deviation from rated temperature. The way that the resistance changes with temperature depends on the type of sensor. Resistance temperature sensors can have; a NTC or negative temperature coefficient, where resistance decreases as temperature increases. Resistance temperature sensors can have; a PTC or positive temperature coefficient, where resistance increases as temperature increases. Resistance can change, with a constant number of ohms, per degree of deviation. Resistance can change, with a variable number of ohms, per degree of deviation. It is important to keep this in mind, when selecting a resistance temperature sensor, as the measurement system needs to be configured for the sensor connected to it to work accurately.
As the temperature measurement is a function of total resistance, it is important to remember that the wire that is used to connect the sensor to the measurement system also has a resistance associated with it, in ohms per foot (which can also change with temperature). This connection resistance is added to the resistance of the temperature sensor, affecting what the measurement system sees as total resistance.
Where is this important? When you need
accurate temperature measurements. How the wire a resistance temperature
sensor impacts your measurement is in the accuracy of
sensed value it can create in your measurement data. There are 2 types of
inaccuracy that you can see, depending on the type of sensor used; this can be seen as an error that can vary per installation (based on connection length), as well as being seen as an error that can vary for an installation (based on variation in connection wire temperature).
The ROI-XMA
is an improved way to read sensors. Multiple sensor types. Multiple signal
ranges. High linearity. High precision. Fast response. Long transmission
distances. True analog signal conditioning, not microprocessor based A-D
conversion.
