Null vs. Deflection Type Instruments

Null Instrument

The null technique is one of the operational modes for a measuring instrument. A null instrument employs the null method for measurement. In this technique, the instrument exerts an influence on the measured system so as to oppose the effect of the measurand. The influence and the measurand are balanced until they are equal but opposite in value, yielding a null measurement. Usually, this is achieved by some type of feedback operation that allows the comparison of the measurand against a known standard value. Basic features of a null instrument include: an iterative balancing operation using some type of comparator, either a manual or automatic feedback used to achieve balance, and a null deflection at parity.

A null instrument has certain intrinsic advantages over other operational modes of instrumentation for instance deflection instruments. By balancing the unknown input against a known standard input, the null method minimizes interaction between the measuring system and the measurand. As each input comes from a separate source, the significance of any measuring influence on the measurand by the measurement process is reduced. As a result, the measured system sees very high input impedance, thereby minimizing loading errors. This is particularly effective when the measurand is a very small value. Therefore, the null operation can achieve a high accuracy for small input values and a low loading error. In practice, the null instrument will not achieve perfect parity due to the usable resolution of the balance and detection methods, but this is limited only by the state of the art of the circuit or system being used.

A shortcoming of null instruments is that an iterative balancing operation requires more time to execute than simply measuring sensor input. Hence, this technique might not offer the fastest measurement possible when high-speed measurements are needed. Nevertheless, the user should weigh achievable accuracy against needed speed of measurement when considering the operational modes. Besides, the design of the comparator and balance loop can become involved such that highly accurate devices are generally not the lowest cost measuring alternative.

An example of a manual balance feedback null instrument is an equal arm balance scale illustrated below:

This scale compares the unknown weight of an object on one side against a set of standard or known weights. Known values of weight are iteratively added to one side to exert an influence to oppose the effect of the unknown weight on the opposite side. Until parity, a high or low value is noted by the indicator providing the feedback logic to the operator for adding or removing weights in a balancing iteration. At true parity, the scale indicator is null; i.e. it indicates a zero deflection. Hence, the unknown input or measurand is deduced to have a value equal to the balance input, the amount of known weights used to balance the scale. Some of the factors that influence the overall measurement accuracy include the accuracy of the standard weights used, the resolution of the output indicator and the friction at the fulcrum. Null instruments are used in the measurement of most variables. Other common null instruments include bridge circuits, often used for highly accurate resistance measurements and found in load cells, temperature-compensated transducers, and voltage balancing potentiometers used for highly accurate low-voltage measurements.

The iteration and feedback mechanism in a null instrument is a loop that can be controlled either manually or automatically. Basically we have two inputs to the null instrument: the measurand and the balance input. The null instrument includes a differential comparator that compares and computes the difference between these two inputs as demonstrated in the figure below:

A nonzero output from the comparator provides the error signal and drives the logic for the feedback correction. Repeated corrections provide for iteration toward the eventual parity between the inputs and results in the null condition where the measurand is exactly opposed by the balance input. At parity, the error signal is driven to zero by the opposed influence of the balance input and the indicated deflection is at null, hence the name null for this method. The magnitude of the balance input drives the output reading in terms of the measurand.

Related: Types of Sensors used in Measurement and Process Control

Deflection Instrument

The deflection technique is another possible operational mode for a measuring instrument. A deflection instrument employs the deflection technique for measurement. A deflection instrument is influenced by the measurand so as to bring about a proportional response within the instrument. This response in an output reading that is a deflection or a deviation from the initial condition of the instrument. In a typical form, the measurand acts directly on a prime element or primary circuit so as to convert its information into a detectable form. The magnitude of the deflection of the prime element brings about a deflection in the output scale that is designed to be proportional in magnitude to the value of the measurand.

Actually, the most common measuring instruments are of deflection type. The relationship between the measurand and the prime element or measuring circuit can be a direct one, with no balancing mechanism or comparator circuits used. The proportional response can be manipulated through signal conditioning methods between the prime element and the output scale so that the output reading is a direct indication of the measurand. Effective designs can achieve a high accuracy.

Deflection instruments can be designed for either static or dynamic measurements or both. When used for dynamic measurements, the deflection instruments offers advantage of high dynamic response. A major disadvantage of deflection instruments is that by deriving their energy from the measurand, the act of measurement will influence the measurand and change the value of the variable being measured. This change is termed to as a loading error. For this reason, the user must ensure that the resulting error is acceptable. This normally involves a careful look at the instrument input impedance for the intended measurement.

For deflection instrument like a spring scale (figure above), the input weight or measurand acts on a plate spring. The plate spring serves as a prime element. The original position of the spring is influenced by the applied weight and responds with a translational displacement, i.e. a deflection x. The final value of this deflection is a position that is at equilibrium between the downward force of the weight, W, and the upward restoring force of the spring, kx. That is to say, the input force is balanced against the restoring force. A mechanical coupler is connected directly or by linkage to a pointer. The pointer is mapped out on a corresponding scale that serves as the readout scale.

The flow diagram logic for a deflection instrument is shown below is somewhat linear:

The input signal is sensed by the prime element or primary circuit and thereby deflected from its initial setting. The deflecting signal is transmitted to signal conditioners that act to condition the signal into a desired form. The signal conditioning circuit can multiply the deflection signal by some scaler magnitude or transform the signal by some arithmetic function. The conditioned signal is then transferred to the output scale, which provides the indicated value corresponding to the measured value.

Related: The Principle of an Optical Pyrometer Temperature Sensor