R. Larson
15 January, 2014

MONTANA STATE UNIVERSITY  
DEPARTMENT OF MECHANICAL ENGINEERING                                

ETME 360 Spring 2014 – Measurements and Instrumentation Laboratory
Resistance-Based Temperature Measurement: RTD                                           EXERCISE 2

Description:
This lab experiment utilizes an array of laboratory apparatus to examine the electrical behavior of a resistance temperature detector (RTD.)
This experiment also introduces the Wheatstone bridge circuit, and uses it in a
null balance configuration

References:
A. Wheeler & A Ganji, Introduction to Engineering Experimentation, Chapters 3, 4, and 9 
ETME360 Lecture; Circuit analysis review & Temperature Measurement topics.

 Equipment:  

3 Precision decade resistors  
DC power supply  
Platinum resistance temperature detector  
Constant temperature circulator  
Digital Multi-meter
Digital Thermometer

Discussion:
The Wheatstone bridge circuit is one of the most commonly used electrical circuits in transducer-based measurement applications. As discussed in lecture, this circuit is typically comprised of four resistors arranged (schematically) in a diamond formation. Several bridge circuit configurations are possible. The “Null balance” or " zero balance" Wheatstone bridge circuit is commonly used to condition and control resistance-type transducer output.  A basic Wheatstone bridge circuit shown in Figure 1, below.

 

Figure 1.  Wheatstone Bridge Circuit 

The benefit of using a bridge circuit for measurements that utilize resistive transducers is that the resolution of  transducer resistance measurement can be set by the experimenter, by selecting values of the other circuit resistance elements. 

In this laboratory experiment, two of the four resistances are fixed. The RTD resistance will vary with temperature (as predicted by theory) and therefore at a given temperature the RTD will have a certain corresponding resistance value. The fourth resistor setting value can be adjusted manually to a value which "balances the circuit" at any given RTD temperature. This "Null Balance" technique of varying a resistance until  zero voltage or “null” output occurs permits the resistance of the RTD to be found since resistance  ratios for the null condition can be easily calculated.

For this experiment, we wish to determined RTD (Rb) resistance at any given temperature to within +/- 0.50 ohm, that is, a resolution of half an ohm: We'll assume the resistance Rc is fixed at a value of 10,000 ohms (10 K-ohm.) Rd is a variable resistor which can be adjusted in increments as small as 1 ohm (i.e., a 1 ohm resolution.)  For calculations,  the smallest 1-ohm resistance setting is used. (This is permissible since the calculation for resolution must apply to all possible cases including the simplest case where Rd is set at the single ohm setting.) Given these values and utilizing the null balance circuit described, we will calculate the resistance setting of Ra necessary to achieve the desired resolution for Rb. (Solved with instructor guidance at beginning of lab.)

Circuit current must be limited to avoid damaging the RTD: We will use the value of Ra from above, and compute the bridge excitation voltage (Ei) so that a current of 25 milli-amps in the RTD element is not exceeded.  The link to the Manufacturer's Data Sheet (below) is used to obtain specified RTD resistance values versus temperature. (Solved with instructor guidance at beginning of lab.)

Laboratory Procedure:

Results:

Manufacturer's Data Sheet