Instrument Calibration Experiment
Instrument calibration is needed to ensure accurate measurements and to transform
electrical signals from measurement instruments into meaningful data. The purpose
of this laboratory is to learn about temperature, pressure and flow measurement
devices and to calibrate commonly used instruments for measuring these properties.
In this experiment, you will calibrate a thermocouple for temperature measurement, a
strain-type pressure transducer for pressure measurement and a turbine-wheel flow
meter for volumetric flow rate measurement.
Pressure
Pressure transducers are commonly used to measure pressures ranging from a less
than 1 Pa to over 100 MPa. Based on the pressure sensed, a voltage or current is
output from the device. This signal must be calibrated to transform it into meaningful
units of pressure. Typically, pressure transducers are calibrated with respect to a
high-accuracy gauge. As seen in Fig 1, the transducer and calibrated gauge are
exposed to the same pressure simultaneously. The pressure read from the calibrated
gauge and the voltage or current from the transducer are recorded. The pressure is
varied the process is repeated until a calibration curve is completed through the range
of the transducer.
Figure 1. Schematic of Pressure Calibration Setup
Temperature
Thermocouples operate on the Seebeck effect, whereby a temperature difference
between two junctions of dissimilar metals creates a voltage, as seen in Fig 2. Metals
A and B are joined at two locations, where T1 and T2 are marked. The temperature
difference between these two junctions, T2 – T1, creates a voltage, V. A reference
such as an ice bath at 0.0C is typically used for one junction and the other is used to
measure an unknown temperature. Modern data acquisition systems can also be used
to control the temperature of one junction so that the other may be used for
measurement. To calibrate a thermocouple, the temperatures of both junctions must
be known. Typically, one junction is held in an ice bath at 0.0C while the second
junction is exposed to a reservoir at predetermined temperature. The temperature of
the second junction is also measured independently with a calibrated thermometer.
The thermocouple voltage is then plotted against the temperature read with the
thermometer to create a calibration curve. Known references may be substituted for
the calibrated thermometer. For example, the boiling point of water at atmospheric
pressure, which is well known, may be used as a calibration point.
Figure 2. Schematic of a Thermocouple [1]
Flow
Flow measurement devices can be calibrated with two methods, steady or unsteady.
A steady flow calibration process involves placing the flow meter to be calibrated
directly in line with a pre-calibrated flow meter. The output from the unknown flow
meter can be correlated with the calibrated flow meter as described in the previous
two sections. If a calibrated flow meter is not available, the unsteady calibration
procedure may be appropriate for liquid flows. For this process, the fluid passes
through the flow meter being calibrated at a steady rate and is then stored in a tank.
After some time, the flow is stopped. The mass and/or volume of the fluid
accumulated in the tank is measured and divided by the time that elapsed to get the
mass or volumetric flow rate. In this calibration experiment, rotameters are used to
calibrate a turbine flow meter in a steady process.
Figure 3. Schematic of Flow Calibration Setup
ASSIGNMENT
1. Calibrate an Omega strain-gage pressure transducer over the range 0-2758 kPa (0-400
psig) using a calibrated bourdon gage as the reference. Plot the calibration curve in
psig vs. voltage and kPa vs. voltage. Provide a best curve fit for the pressure as a
function of voltage the R2 relationship between the regression line and the data.
2. Calibrate two thermocouples (Type J and Type T) over the range 0-260 C (32-500
F) using standard apparatus for the ice and boiling points, room temperature air, a
container of hot water, and the calibrated hot-point cell. Use a mercury-in-glass
thermometer as the reference standard, except for the 260 C (500 F) point. Since
the thermometer will not fit into the cell, use the cell as the reference. Plot voltage vs.
temperature, a best curve fit and the R2 value for the regression line.
3. Calibrate the turbine flowmeter by comparison with the rotameters installed in the
flow loop. The rotameters have been calibrated by direct measurement of quantity
and time. The flowmeter output can either be voltage or frequency. Calibrate both of
these parameters. Plot the voltage and frequency vs. flow rate, a best curve fit and the
R2 value for the regression.
REFERENCES
1. Figure courtesy of Wikimedia Commons.