The Five Sources of Sensor Error

Sensors, like all other devices suffer from certain errors. To maintain consistency, error is defined as the “difference between the measured value and the true value.” Although the full range of possible errors is beyond the scope of one publication, it’s possible to divide sensor errors into five basic categories:

Insertion errors occur when the act of inserting a sensor into the system changes the parameter of being measured. The problem generally occurs with electronic measurements, but can happen with all measurements. For example, a voltmeter that measures the voltage in a circuit must have an inherent impedance that is much larger than the circuit impedance, or circuit loading will occur, and the reading will be in significant error. Possible sources of this type of error include using a transducer that is too large for the system (e.g. pressures); one that is too sluggish for the dynamics of the system, or one that self-heats to the extent that excessive thermal energy is added to the system.

Application errors are operator-caused, which means that numerous types are possible. Errors seen in temperature measurements, for example, include incorrect placement of probe or erroneous insulation of the probe from the measurement site. Other application errors may include failure to purge systems of air and/or other gases. Application errors can also involve incorrect placement of a transducer, so that a positive or negative pressure head is erroneously added to the correct reading.

Characteristic errors are defined as those that are inherent in a device itself, which is the difference between the ideal, published characteristic transfer function of the device and the actual characteristic. This type of error may include a dc offset value (e.g. a false pressure head), and incorrect slope, or a slope that isn’t perfectly linear.

Dynamic errors. Many sensors are characterized and calibrated in a static condition, which means using an input parameter that is either static or quasi-static in nature. And many are also heavily damped, so they won’t respond to rapid changes in the input parameter. For example, thermistors tend to require many seconds to respond to a step-function change in temperature. As a result, a thermistor in equilibrium won’t immediately jump to the new resistance or an abrupt change in temperature. Instead, the device will change slowly to the new value. Consequently, if an attempt is made to follow a rapidly changing temperature with sluggish sensor, the output waveform will be distorted because it contains what is called a dynamic error. Factors to consider with respect to dynamic errors include response time, amplitude distortion, and phase distortion.

Environmental errors derive from the environment in which the sensor is used. They most often include temperature, but may also include vibration, shock, altitude, chemical exposure or other factors. These most often affect the characteristic errors of the sensor, so they are often lumped together with that category in practical application.