Liquid baths are a temperature measurement and calibration tool in which a liquid, or in some cases a material that acts as a liquid, is heated or cooled to a specific temperature
and maintained. In many ways, liquids baths are similar to dry block calibrators except that they utilize a liquid as the calibration medium, rather than a metallic insert, which permits
easier calibration of oddly shaped or sized probes. Since liquid baths do not rely on drilled inserts, they are also capable of calibrating many more inserts at a time. Because the liquid
is being stirred, these baths do not suffer from vertical temperature differences experienced in dry blocks and by consequence usually will provide much better overall uncertainty.
Depending upon the model, liquid baths using an oil medium can achieve temperatures from about –30°C to 200°C. Specialized baths using sand, salt, or aluminum oxide particles instead of oil
can reach temperatures up to 700°C. These "fluidized" baths act as high temperature liquid baths.
Liquid baths offer greater precision and larger calibration volumes than dry block calibrators. They also offer excellent stability over the entire temperature range. Accuracy with liquid baths
can be as high as ≤0.2°C, better than that which can be achieved by most dry blocks. Liquid baths offer secondary-level calibrations on nearly any type of temperature sensor, including: RTDs,
thermocouples, thermistors, , PRTs, bi-metal thermometers, etc.
Temperature Calibration
Temperature is the most commonly measured parameter in commercial and industrial settings. Industries as diverse as food processing, pharmaceuticals, cold storage, paper manufacturing, and
others absolutely rely on process temperatures being within a certain range. To maintain accuracy, calibrating temperature monitoring equipment is necessary.
Measuring temperature involves a wide range of specialty sensors such as thermocouples, thermistors, resistive temperature detectors (RTDs), infrared (IR) sensors, bimetal thermometers, and
others. These sensors produces an electrical output such as resistance, millivolts or milliamps which correspond to the temperature. These output signals are sent to a readout or controller
where they are displayed or used to control a process function.
When discussing temperature calibration it's important to note that output of the temperature sensors, themselves, cannot be adjusted. Instead, the controller or readout is adjusted to account
for the inaccuracy of the sensor.
Calibration is a comparison between two devices. The first device is the unit to be calibrated, often called the unit under test. The second device is the standard, which has a known accuracy.
Using the standard as a guide, the unit under test is adjusted until both units display the same results while exposed to the same temperature. Typically, calibration of an instrument is checked
at several points throughout the calibration range of the instrument.
Not all standards are created equally. While all standards have a known accuracy, there are some—known as primary standards— that are the highest level of accuracy for a specific parameter. Primary
standards achieve their high accuracy by relying upon measurement technologies using fundamental physical constants that do not drift such as the triple point of water. These fixed values minimize
uncertainty, making primary standards the most accurate calibration tools.
ITS-90
Temperature calibrations are done in accordance with The International Temperature Scale of 1990. ITS-90 is the legal temperature scale that establishes a number of fixed point temperatures that
can be used as reference values. The purpose of which is to define procedures for calibrating temperature equipment in such a way that the values of obtained are precise and reproducible, while at
the same time approximating the corresponding thermodynamic values as closely as possible.
Traceability
To improve the quality of a calibration to levels acceptable to outside organizations, it is generally desirable for the calibration and subsequent measurements to be traceable to internationally
recognized standards. Establishing traceability is accomplished by a formal comparison to a standard which is directly or indirectly related to national standards ( such as NIST in the USA),
international standards, or certified reference materials.
Temperature Calibrator Specifications
There are three specifications that are of extreme importance when selecting a temperature calibrator. Understanding these specifications and their implications will go a long ways towards helping
you select the best calibrator for your needs.
Accuracy: An expression of how closely a measured value agrees with the true or expected value of the quantity of interest (NCSL glossary). For temperature calibrators, accuracy
is the relationship between the instrument's display temperature and the actual temperature of calibration well. Accuracy is improved by regular calibrations to a traceable standard.
Stability: The tendency of an attribute to remain within tolerance (NCSL glossary). When a calibrator reaches its set-point, there is some fluctuation in temperature as the unit
tries to maintain that temperature. That fluctuation can influence calibrations. Stability adds to accuracy in determining overall system uncertainty.
Uniformity: Temperature homogeneity of the heat source throughout the test zone. All calibrators have slight temperature differences from the bottom of the of the test well to
the top as well as from the middle of the test well outward. A few simple strategies allow users to place probes in such a way as to minimize uniformity errors.
Things to consider when selecting a temperature calibrator:
- What type of electrical signal does it output?
- Do I need to calibrate sensors or just temperature displays?
- Which is more important? Speed to change temperatures or doing multiple probes at a time?
- How deep does the dry block or liquid bath need to be?
- What are the physical characteristics (size, shape) of the sensor?
- Over what temperature range is it used?
- And what accuracies are relied on over those temperatures?
- Must the calibrations occur in a laboratory or can they be done in the field or even in-situ?
If you have any questions regarding temperature calibrators please don't hesitate to speak with one of our engineers by e-mailing us at sales@instrumart.com or calling 1-800-884-4967.
