Your bad experience also raised several questions. How fast were you actually going? Was your speedometer set correctly? Was the police officer’s radar gun reading 10 mph too high? Is it legal to hide a small 30-mph sign behind a very large tree?
Aside from the last question, your concerns are about calibration. The response of a sensor can be relatively linear and stable, but its output is not scaled in engineering units. Sensors and transducers must be linearized and calibrated against a known reference for their outputs to be useful.
After calibration, the inherent stability and repeatability of the transducer determine how long it can be used without recalibration. Repeatability refers to how closely the sensor output returns to its original value after a large disturbance. For example, the platinum resistance temperature devices (RTDs) used as reference elements in some temperature calibrators indicate their original room-temperature within 0.05°C after heating to 700°C and subsequent cooling.
Stability relates to long-term performance under constant external conditions. Does the transducer drift or change the noise it produces just because an amount of time has elapsed? An extreme example of a stable environment is a stirred, constant temperature bath. Although not very portable, a large mass of liquid can be closely held to a required temperature. For field applications, the challenge is to maintain reasonable stability, but in a more practical, hand-held unit.
The Temperature Simulators/Calibrators Comparison Chart in this article lists a number of representative instruments. Some are hand-held, and others are intended for laboratory use. And, some are electronic while others are based on properties of basic materials. Before attempting to select an instrument, you need to determine your measurement requirements. Among the considerations are cost, precision, absolute accuracy, portability, functionality, traceability, and ease of use. For instance, if you are maintaining a factory, knowing pressure or temperature to an accuracy of a few percent may be fine.
In this case, a multifunction, hand-held recording calibrator/simulator would be a good choice. In a factory environment, 4-20 mA transmitters may be used, and your instrument could simulate the output from a transmitter for test purposes. It also could simulate or measure the output from thermocouples, RTDs, or pressure sensors to quickly confirm faulty devices in the network. The trade-offs you make compared with benchtop models typically are accuracy and stability.
At the other end of the scale are laboratory references. These instruments are directly traceable to NIST standards and achieve accuracy and stability up to 100 times those of portable units. For example, the ASL Model F900 Precision Thermometry Bridge has a basic resolution of 20 parts per billion (ppb) and imperceptible drift. It achieves this very high level of performance by using an AC bridge technique with tapped transformers. Bridge balance results from automatically choosing the combination of taps that gives the best null.
To use the bridge, both a calibrated and an uncalibrated reference RTD thermometer are immersed in a stable temperature bath. As an example, consider the Isotech Model 915 Parallel Tube Liquid Calibration Bath and its 7-liter capacity. The bridge measures the ratio of the unknown device resistance to that of the reference.
But a resistance ratio is not a temperature. The output of the bridge must be rescaled to produce accurate calibration values for the unknown device. By measuring the resistance ratio, accuracy is determined by the NIST-traceable reference RTD.
Don’t be in a hurry to make precise measurements. In describing the use of a water triple-point cell, Isotech refers to the small strain-induced error produced when the ice first forms. According to the company, the error begins to reduce after a few hours, and full accuracy will be reached after a day. The specification for achieving full balance in the ASL bridge is 20 s.
But what if you can’t use thermocouples or RTDs? Perhaps an infrared temperature meter is a solution. For field applications, Hart Scientific produces the hand-held Model 9131 Infrared Calibrator for temperatures from 33°C to 400°C. The price you pay for noncontact temperature measurement is less accuracy than most other means. On the other hand, 1% to 3% may be good enough for your requirements, especially if accessibility is poor.
Regardless of the calibrator you choose, review the specifications carefully. Accuracy usually is expressed as percent of reading plus some additional error. For example, the Fluke Model 724 Calibrator specifies voltage accuracy as 0.02% of reading + 2 least significant digits (LSDs). Most instruments are inaccurate near the lower end of their measurement scales, and the +2 LSD spec accounts for this. You should avoid measuring small values on insensitive scales because the added LSDs severely degrade accuracy.
Also, determine the likelihood that you will use uncommon thermocouples such as type B or U. Unless you really require the flexibility of an instrument that handles these types of thermocouples, choose a less expensive and simpler calibrator.
Similarly, check the types of RTDs that you will be working with. The wide ranges offered by Fluke, Extech, and Rochester Instrument Systems provide a degree of future proofing. But if you know that you will only be using Pt-100/385 units, for example, you will have more models to choose from.
Finally, look at the input-protection and battery-life specifications for your favorite hand-held model. Battery lifetimes range from 8 h to more than 50 for the models in the temperature comparison chart. Input protection typically includes a fuse and an overvoltage device, but capabilities do vary. For example, the ALTEK Model 235 Process Voltage Analyzer in the Voltage Simulators/Calibrators Comparison Chart provides 120-VAC protection on its inputs without using fuses.
Many of the considerations discussed for temperature calibrators apply to voltage calibrators. “The trends that have always been driving the design and development of calibrators are more accuracy and better long-term stability,” said Joe Inglis, sales manager at Krohn-Hite. “Most good calibrators specify an accuracy of 10 ppm to 20 ppm.“The reference used in the Krohn-Hite Model 522 DC Voltage and Current Calibrator/Simulator is a temperature-compensated, preconditioned zener diode with a temperature coefficient of <1 ppm/°C. It also is thermally stabilized,” he explained. “High-quality minirelays and resistors with low-temperature coefficients and thermally stable features are used to produce an instrument with long-term stability.”
Accuracy can be as good as 0.008% in a hand-held unit or 2.5 ppm in a benchtop unit intended for 8½-digit DMM calibration—a factor of about 30. Stability also varies between the two classes of instruments.
Benchtop models are specified more completely and provide detailed answers to the question: Sure, it’s accurate today, but how much can I trust it after 90 days or a year? In contrast, hand-held calibrators generally are priced attractively and feature toolbox toughness, an ALTEK term. Stability is important, but it only needs to meet the requirements of field applications. A hand-held calibrator usually is not the best choice for a calibration laboratory.
Another general trend is to combine many functions into one instrument. For example, although the Fluke Model 5520A and Gossen-Metrawatt METRAtop® 53 instruments are listed as voltage simulators/calibrators, they also provide current, resistance, RTD, and thermocouple outputs.
Multiproduct calibrators are a good choice for companies that have several types of instruments to calibrate. In a single box, you get very good performance at a reasonable price, but generally not the best possible performance.
As was the case with the temperature calibrators, specialized features are available in some of the voltage calibrators. For example, the Krohn-Hite Model 522 Calibrator includes a 10-nA to 100-mA DC current output accurate to 0.005% and with a compliance voltage of up to 100 V. This product also has typical noise as low as 9.0 µVrms on the 100-mV range.
For accurate AC measurements up to 100 MHz, consider the Ballantine Laboratories Model 1605B NIST-Traceable Thermal Transfer Standard. This instrument boasts a 100:1 crest factor and a frequency range of greater than 100 MHz. You use it to determine the DC voltage that produces the same heating effect as an unknown AC signal. On its 0.5-V scale, accuracy is better than 15 ppm of the reading.
And there are new technological developments changing the fundamental design of calibrators. As Warren Wong, the engineering manager at Fluke Precision Measurements, said, “Fluke uses direct digital synthesis (DDS) extensively in multiproduct calibrators to generate not only sinewaves, but also more complex waveforms such as square, triangle, and truncated sinewaves. DDS provides synchronized dual outputs for power calibration as well.
“A patented shared wavetable memory scheme results in 0.01° phase resolution between the two outputs,” he explained. “The 40-bit DDS is implemented in a complex, field-programmable gate array.”
Even the Internet has become involved in calibration. Charles Yen, Jr., a senior service technician at Keithley Instruments, said that the company has worked with NIST to develop a process of remote standards measurements.
“The concept is simple: NIST sends a digital multimeter (DMM) to the lab needing certification. The lab’s metrologist connects the DMM to the computer bus, and the actual standards measurements are taken over the Internet. This process allows NIST to certify calibration standards without the standards ever leaving the owner’s lab,” he explained.
Once you have determined the calibration and simulation functionality you require, you can be confident there probably is equipment available to meet your needs. Temperature and voltage have been considered in detail because they are the two most frequently measured parameters; however, you may be able to use instruments listed in the comparison charts for other purposes.
For example, strain gages change resistance when flexed, much like RTDs change resistance when heated. Pressure measurements often are made using transducers with four piezoresistive elements arranged in a Wheatstone bridge configuration. This type of pressure transducer can be used with some multiproduct calibrators that also provide the required bridge excitation.
Several hand-held instruments intended for process-control applications have been included in the comparison charts. But, if their specifications are adequate for your calibration or simulation work, they may be good alternatives to larger, more expensive instruments.
If a hand-held model appears suitable, read all the fine print. Ask the manufacturer for more specification details as well as a demo of the product. Better yet, borrow an instrument and use it in your environment. Make sure you understand what is and is not guaranteed. Too many typical specifications may add up to an instrument that might solve your problem rather than one that definitely will.
Benchtop calibrators/simulators focus more on performance and less on portability and cost. More parameters are tested and specified so it may be easier to determine which ones are suitable solely from their datasheets.
But hand-held or benchtop, the purchase of a calibrator is not a time to save money. Rather, buy the best product you can afford. The accuracy of all the measurements you will be making depends upon it.
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