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National Conference on Building Commissioning: April 19-21, 2006

Sensor Accuracy and Calibration Theory and Practical Application
Karl Stum, P.E. Summit Building Engineering

Synopsis Some construction specifications dictate sensor accuracy and calibration tolerances and some projects have no direction other than “sensors should be calibrated.” Some controls contractors follow therequirements whether reasonable or not. Others argue that the sensors are “factory calibrated” and therefore don’t need field calibration. The real accuracy of test instruments is also not well understood. Some instrument companies advertise the accuracy of their instruments, but fail to highlight that the accuracy only includes the transducer and not the inaccuracy of the instrument sensor and wire. Evenif the user understands the real accuracy of their calibrating instrument, how much more accurate than the sensor being calibrated, does the calibrating instrument need to be? How does the calibration tolerance value used in the field come into play? Do design engineers and commissioning providers fully understand that specifying sensor accuracy is not the same as specifying end-to-end accuracy?This paper will answer the above questions and provide useful information on the issues surrounding sensor accuracy and calibrating instrument accuracy. Look-up tables will be provided for typical sensor and tolerance types.

About the Author Karl Stum is registered professional engineer and has been actively commissioning for over 13 years. He specializes in larger, complex facilities and incontrols. He is a past recipient of the Benner Award and is a frequent speaker at NCBC and ASHRAE. He is the owner of Summit Building Engineering in Vancouver, Washington.

Introduction
This paper is not about factory calibrating methods, full field calibration instructions or methods for calibration certification services. The information presented is what the commissioning provider should knowand be proficient at in the field. The paper applies primarily to HVAC applications and focuses on temperature measurement, but in various locations in the paper taking and calibrating other types of measurements are discussed.
Stum: Sensor Accuracy and Calibration … 1

National Conference on Building Commissioning: April 19-21, 2006

Fundamentals
There are a few fundamentals that warrantdiscussion including: sensor physical characteristics, accuracy theory and sources of error and field calibration methods.

Sensor Characteristics
The primary temperature sensor types are thermocouple, thermistor and resistance temperature detector (RTD). A brief description of the characteristics of these types of sensors is given in Table 1. Table 1. Temperature Sensor Characteristics
SensorType Thermocouple How It Works
Two connected wires of dissimilar metals cause a voltage which is related to temperature. Resistance of a small semi-conductor is related to temperature. Resistance of a fine wrapped wire is related to temperature.

Application
Hand-held instruments and rugged industrial and high temperature process applications. Hand-held instruments, HVAC systems

AdvantagesDurable, very high temperature, inexpensive; fast response High sensitivity; accurate

Disadvantages
Low accuracy (see below)

Thermistor

Moderate cost; moderate response time. Narrow temperature range (but plenty wide for HVAC). Moderate cost; moderate response time. Requires a transmitter or lead wire compensation which adds another source of error.

RTD (resistance temperaturedetector)

HVAC systems and equipment, hand-held instruments

Very stable, wide temperature range; accurate.

Source: PECI 3.1; Omega

Sensor Accuracy The accuracy (to be fully defined later) of the above sensors varies with the type, purpose and manufacturer. Table 2 provides some representative accuracies. It should be noted (and will be explained later) that the values below are for the...