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Measurement Articles

The Measurement Standards Laboratory publishes practical, educational articles on measurement in a number of industry publications.

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General

Changing the SI (PDF 123KB)

The International System of Units (SI) has been an outstanding success since its introduction in 1960, meeting well the needs of science and commerce. By relating all our measurements to this shared set of standard units, such as the metre, kilogram and second, it is possible to trade and communicate throughout the world without misunderstanding. The SI is based on the very best scientific knowledge, and it is extremely rare for measurement difficulties to be traced to the SI system itself. Yet there is a misconception that the SI measurement system is cast in stone… (Click the title above for the full article.) (reproduced with permission from Electrical + Automation, December 2007-January 2008 issue.)

Electricity

Digital Doubt (PDF 763KB)

We are spoiled for choice when it comes to making electrical measurements; digital multimeters (DMMs) displaying anything between three and nine digits are readily available. However despite the accomplishments of these digital devices we must keep in mind that they are complex instruments with imperfections and often subtle but significant faults. Our experience is that many of the high-level (DMMs) submitted to us for calibration have faults (or even just odd patterns of behaviour) of which the owner is unaware. Calibration of a DMM not only provides for traceable measurements but also gives us assurance that any such faults will have been identified... (Click the title above for the full article.) (reproduced with permission from Electrical + Automation, February-March 2006 issue.)

Following the Flow (PDF 1.93MB)

Ohm’s law allows the determination of current by measuring the voltage across a series resistance of known value (I=V/R). Specially-designed resistors, called current shunts for historical reasons, are used to measure high DC currents (say above 1 A). This article describes the key design features of these devices and how to make good measurements with them…(Click the title above for the full article.) (reproduced with permission from Automation & Control, June-July 2005 issue.)

Keeping Your Cool (PDF 1.58MB)

Modern measuring instruments can give you accurate results in a wide range of environments, their use being limited as much by operator comfort as engineering design. Starting work on a nice icy winter morning, you might turn on the heater, pick up your hand-held meter up from the bottom shelf in the workshop, and then wonder why that ac signal is low by 4 %. You were sure it was set up well inside 2 % the night before, but a quick adjustment is easy. Coming back that afternoon after a call out, you realise that you left the meter on the seat in the van in full sun. On retrieving the meter, you check that signal again to find it’s now high by a few percent. What’s going on?... (Click the title above for the full article.) (reproduced with permission from Electrical + Automation, September-October 2009 issue.)

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RF Connector Care (PDF 4.69MB)

When working at frequencies above a few tens of megahertz, more attention should be paid to the quality and reliability of connections than at lower frequencies. A good connection at low frequencies simply means that the conductors should make contact. At radio frequencies (RF), and above, the integrity of the transmission line must be maintained right through the connection, which means that electrical and mechanical considerations are more important. Damaged or dirty connectors and cables can significantly degrade measurements of RF power and other quantities…(Click the title above for the full article.) (reproduced with permission from Electrical + Automation, December 2008-January 2009 issue.)

Transforming Current Into Dollars (PDF 1.06MB)

Commercial, industrial and the larger electrical energy metering sites can carry hundreds of amperes of current, well beyond the 5 A or 1 A typical input current rating of electrical energy meters. Current transformers (CTs) are required to accurately scale these large line currents to smaller acceptable values with a known phase relationship between the input current and the scaled output current……?... (Click the title above for the full article.) (reproduced with permission from Electrical + Automation, October-November 2008 issue.)

Length

No measurement articles at present

Photometry and Radiometry

Making Light Work (PDF 105KB)

Article by John Clare

Light meters (more correctly illuminance meters or luxmeters) are typically used to measure interior light levels of a few hundred lux.  The human eye has a very wide dynamic range; one can see objects illuminated with 0.005 lux, at which level outlines can just be perceived, through to the 100,000 lux of direct sunlight.  Light meters are commonly used over the range from 1 lux, the nuisance level from street lighting, to the 10,000 lux required in some surgical situations.  Specific requirements are frequently imposed by legislation, regulations, or contracts; for example 750 lux is required for the inspection of exported meat... (Click the title above for the full article.) (reproduced with permission from Automation and Control, October-November 2004 issue.)

How Flat is Flat? (PDF 5,102KB)

Article by David Cochrane

The Measurement Standards Laboratory (MSL) maintains New Zealand’s national standards of measurement and is part of the government owned Crown Research Institute, Industrial Research Limited. One of the most common requests of the Measurement Standards Laboratory’s optical workshop is "would you please polish this flat". Very often the person asking the question has no idea how flat they want the work or indeed what the word flat means to an engineer. Shiny (specular) means flat to them and quite often that is all they really need.... (Click the title above for the full article)

Mass and Pressure

Weighing with Confidence (PDF 1032KB)

Article by Chris Sutton

Weighing in trade, in industry, and in the laboratory is performed almost exclusively using electronic balances.  These balances are convenient, quick to use and can be remarkably precise.  For example, the resolution of relatively inexpensive laboratory balances can be less than one-millionth of the maximum load.  (Click the title above for the full article.) (reproduced with permission from Automation and Control, December-January 2004/5 issue.)

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Temperature and Humidity

Humidity Hassles (PDF 665KB)

Article by Jeremy Lovell-Smith

Control of relative humidity is difficult and rarely managed well.  It is generally not well understood by most users and specifications can easily impose contradictory criteria. (Click the title above for the full article.) (reproduced with permission from Automation and Control, June-July 2004.)

How good is Quantitative Thermal Imaging (PDF 1,380KB)

Article by Peter Saunders

Infrared thermal imaging cameras, or thermal imagers, are devices that detect the infrared radiation emitted by all objects and generate a detailed two-dimensional temperature map of a surface.  They are becoming increasingly important in many industries for inspection and maintenance of plant and buildings. (Click the title above for the full article.) (reproduced with permission from Automation and Control, February-March 2005.)

Reliable infrared temperature measurements of food products (PDF 249KB)

Article by Peter Saunders

Infrared thermometers are becoming increasingly widespread in the food processing and storage industries.  Their popularity is not surprising.  They are compact and easy to use, they respond quickly, enabling users to make many measurements in a short period of time, and they measure temperature without physical contact, eliminating the possibility of contamination and allowing measurements to be made at distance... (Click the title above for the full article.) (reproduced with permission from Automation and Control, April-May 2004 issue.)

Non-contact Temperature Measurement in the Food Industry (PDF 40KB)

Article by Peter Saunders

The safe storage of all perishable food products is critically dependent on storage temperature. Health regulations govern the temperature range required to avoid the risk of spoilage or disease through bacterial growth. For example, meat products must be stored from 4°C to 7°C, fish products below 4°C, and shellfish from 10°C to 15°C. These regulations have a major impact, for example, in the hundreds of supermarkets throughout the country. Accurate temperature measurement is crucial not only for health reasons but also for profitability through the reduction in wastage that arises from bad measurements.... (Click the title above for the full article)

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An Imaging Radiation Thermometer (PDF 38KB)

Article by Peter Saunders

For many years radiation thermometers have been used to make non-contact temperature measurements of hot objects, particularly in industrial situations such as reformer furnaces, steel reheat furnaces, and ceramics kilns. They occupy an important niche in temperature measurement since any other type of thermometer would have very slim chances of surviving in these and other such hostile environments.... (Click the title above for the full article)

In Hot Water (PDF 429KB)

Article by Peter Saunders  

Many thermometer users will be aware that crushed or shaved ice makes a very good and cheap temperature reference; properly made it has a temperature of 0 °C and a potential accuracy of about 0.001 °C.

Since water boils at 100 °C, one might expect that boiling water would also be a good temperature reference. Unfortunately this is not so.... (Click the title above for the full article)  (reproduced with permission from electrical + automation technology April /May 0 7 issue)

Leading Questions (PDF 658KB)

Article by Rod White

Lead resistances have a large impact on the accuracy of resistance temperature detectors (RTDs).  With typical 100 Ω platinum RTDs, lead-wires cause an error of about 2.5 °C for every ohm of lead resistance and for large installation, errors of several degrees are common.  Since platinum RTDs are preferred for applications such as food and chemical processing where accuracies in the range 0.1 to 1.0 °C are required, this is a serious problem... (Click the title above for the full article.) (reproduced with permission from Automation and Control, April-May 2005 issue.)

Making Sense of Thermocouples (PDF 88KB)

Article by Rod White

Thermocouples are the most widely used temperature sensor.  The simplicity of two wires connected to a meter has an obvious appeal.  However, when high confidence in measured temperature is required, thermocouples can be a liability.  Thermocouple literature often mistakenly states the junction is the source of the voltage.  In fact, the voltage is generated along the length of the wire, and in a well designed thermocouple installation the junction does not contribute at all! (Click the title above for the full article.) (reproduced with permission from Automation and Control, February-March 2004 issue)

Time and Frequency

Testing Times (PDF 187KB)

Article by Tim Armstrong

Stopwatches and timers are used in a wide variety of laboratory and industrial measurements such as flow measurement, process timing, and chemical and radiological exposure control. (Click the title above for the full article.) (reproduced with permission from Automation and Control, August-September 2004 issue.)

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