Question

** My system is RTD

Question 4: System Installation and Testing

Describe how you would install your system to minimize environmental effects on the sensor operation

Discuss the main error sources (not types: system & random) that would affect your system measurement accuracy.

Describe how you would test and calibrate the measurement system designed. Show details for one sensor to verify operation according to specifications

Answer 1

RTD- Resistance Temperature Detector

A most common type of temperature measuring device which is almost stable and accurate. It is based on the fact that the electrical resistance of any material will vary depending on the temperature of the material.( In positive temeprature co-efficient devices , the tempterature increases with the increase in the resistence).

How to install RTD to minimize Environmental effects on the sensor?

Alhough the RTD may be accurate, the noise from the external sources ie from the environment can create an additional error in the measurement. The errors in the measurement due to environmental effects can be minimised by using differential, ungrounded and shielded elements.

Sensors also receive conductive, convective and radiative inputs that contribute to the measurement accuracy.These effects can also be limited by usage of optional electronics that performs 10% duty cycle measurements to limit the self heating power but without reducing the signal strength.

Another common problem in RTD is the lead wire corrosion problem which can be avoided by the usage of 3 or 4 wire system which effectively measures the resistance of the sensing element

and If the atmosphere is at a different temperature or the measuring surface is moist then the heat flow assoiated with the convection must bve considered as a heat source or sink and the corresponding compensation in the measurement must be made accordingly.

Main error sources:

some of the main sorces of the error in the measurement of the temperature by using RTD's are:

a) Location errors:

location error is a direct result of entire sensor being displaced from its desired location and the orientation of the sensor will also contribute to the location error. The size of the sensor will also be the deciding factor for the location error. larger the size of the sensor , the lesser the accuracy of the measurement.

b) Transient errors:

transient errors are dynamic thermal errors which cannot be compensated since each and every material within the thermal system will have its own thermal conductivity which is unique.

c) Heat Transfer error:

Sensors also receive conductive, convective and radiative inputs that contribute to the measurement accuracy. This error occurs along the sp[ecific path aloing the orientation of the sensor whenever a sensor is heated by a nearby heat source. ie when electrical resistance produces the heat inside a sensor, it causes a false reading . that is known as the sself-heating error.

d) Mechanical effects:

Mechanical effets such as the motion , vibration and high acoustics can cause errors in the measurement . hence small wires and fragile sensors muat be avoided in the construction or the selection of the sensors.ie in simple wors the wire failure most commoinly occurs in the connection point where the mechanical stress is high.

How to Test and Calibrate RTD system?

the following are the steps involved in testing or calibrating the temperature masurement system,

1. Isolate the sensor from the overall system.

2.now we need to immerse the sensor into the precision temperature source such as the dry well which is capable of providing the required temperature range.

3.also uase a temperature standard into the dry well and then compare the working of the sensor.

4.Then disconnect the RTD from the systema nd connect the RTD to a precision instrument which is capable of measuring the resistance of RTD.

5. then adjust the temperature of the dry well for each of the test points and record the readings of the RTD and the sensor.

6. finally compare the reading on the instrument display with the reading of the temperature standard.

For Example,