EET 273 Spring 2017 – HW3

  1. Suppose you wish to calibrate a current-to-pressure (“I/P”) transducer to an output range of 3 to 15PSI, with an input range of 4 to 20 mA. Complete the following calibration table showing the proper testpressures and the ideal input signal levels at those pressures:

Input signalapplied (mA) / Percent of span / Output pressure (%) (PSI)
35
80
95
  1. Sketch a circuit whereby this loop-powered pressure transmitter sends a signal to an analog voltagemeter (acting as a remote pressure gauge). Be sure to route all wiring and attach any necessary componentsto terminals on the terminal strip:

  1. An important part of performing instrument calibration is determining the extent of an instrument’s error. Error is usually measured in percent of span. Calculate the percent of span error for each of thefollowing examples, and be sure to note the sign of the error (positive or negative):

• Pressure gauge

• LRV = 0 PSI

• URV = 100 PSI

• Test pressure = 65 PSI

• Measured pressure = 67 PSI

• Error = % of span ______

• Weigh scale

• LRV = 0 pounds

• URV = 40,000 pounds

• Test weight = 10,000 pounds

• Measured weight = 9,995 pounds

• Error = % of span ______

• Thermometer

• LRV = -40oF

• URV = 250oF

• Test temperature = 70oF

• Measured temperature = 68oF

• Error = % of span

• pH analyzer

• LRV = 4 pH

• URV = 10 pH

• pH of test buffer solution = 7.04 pH

• Measured pH = 7.13 pH

• Error = % of span

Also, show the math you used to calculate each of the error percentages.

Challenge: build a computer spreadsheet that calculates error in percent of span, given the LRV, URV,test value, and actual indicated value for each instrument.

  1. For each of these 4 types of calibration errors, draw an input/output graph that shows the calibration error.
  2. Span error
  3. Zero shift error
  4. Hysteresis error
  5. Linearity error

Of these 4 error types, which may be possible to calibrate out of the system? Which type(s) of errors may indicate it is time to replace the instrument?

  1. Suppose an electronic pressure transmitter has an input range of 0 to 100 PSI and an output range of4 to 20 mA. When subjected to a 5-step up-and-down calibration test, it responds as such:

Applied pressure
(PSI) / Output signal
(mA)
0 / 3.5
25 / 7.5
50 / 11.5
75 / 15.5
100 / 19.5
75 / 15.5
50 / 11.5
25 / 7.5
0 / 3.5

Sketch this instrument’s ideal transfer function on the graph below, along with its actual transfer function graph based on the measured values recorded above. Then, determine what kind of calibration error it has(zero shift, span shift, and/or linearity):

Finally, identify how this calibration error might be corrected. What steps or procedures would youfollow to rectify this problem?

  1. You have a temperature sensor that can measure temperatures in a range of 50°C to 140°C. It produces a 4-20mA current output.

Create a table that shows some of the possible temperature input values, and which output current values they map to. You table should include at least 5 points.

Use your table to create a linear equation that maps the input x to the output y, using the linear equation formula:

y = mx + b

You can solve for m using the formula:

m = (y2 – y1) / (x2 – x1)where (x1,y1) and (x2,y2) are 2 data points