Calibration Spotlight: Hart Scientific 1590 Super-Thermometer II

What Is a Super-Thermometer?

Metrology laboratories around the world rely on super-thermometers for their reliably accurate, easy-to-take measurements.  For instance, the Hart Scientific 1590 Super-Thermometer II reads accurately to 0.00025°C or 1 ppm. This high degree of accuracy makes super-thermometers perfect for calibration of SPRTs (Standard Platinum Resistance Thermometers). Further, they are the best lab instruments to take advantage of SPRT accuracy. They read temperature directly, automate data collection, and calculate constants for ITS-90. Additionally, the large LCD screen of the 1590 Super-Thermometer II features a live graph which records temperature trends during calibration.  This allows PTS technicians to see when the temperature has stabilized enough to ensure an accurate reading.

The Hart Scientific 1590 Super Thermometer II features a large LCD with live readings for calibration technicians.

How Does PTS Use the Super-Thermometer II?

The typical benchtop thermometer has an error level twenty to forty times larger than the Super-Thermometer II. However, the resolution of the 1590 with a 25-ohm SPRT is 0.000125°C. Because of this,  the 1590 Super-thermometer II can easily calibrate Platinum Resistance Thermometers (PRTs), Resistance Temperature Devices (RTDs), thermistors, thermocouples, and related measurement devices. As a key provider in the scientific services calibration industry, PTS services all manner of temperature devices. As an ISO:17025 accredited organization, we meticulously catalog uncertainty data for our 1590. Customers may request this data as needed.

SPRTs interpolate temperature according to the International Temperature Scale of 1990 (ITS-90). In use, the AM1960 covers the range from -200 °C to 670 °C.

We paired our 1590 Super-Thermometer II with a quartz-sheathed SPRT Accumac AM1960. This allows accurate measurements up to 600°C.  This instrument gives PTS the ability to calibrate all temperature devices in-house. Because of this, we have more time to devote to serving our customers. This ensures our turn around time is quick and efficient to minimize the time you are without your instrument. You won’t wait six weeks (or longer!) to receive your PRT or temperature device back from calibration. You can rest assured knowing your temperature device will be calibrated to its greatest degree of accuracy from PTS.

Thermometer Calibration by the Comparison Method

When employing thermometer calibration by the Comparison Method, readings from a thermometer with unknown accuracy are compared to those from a standard device. The standard device is calibrated to meet the quality requirements of the National Institute of Standards and Technology (NIST) or a similar governing body.

Typically, this method of calibration is used for liquid-in-glass thermometers. This technique often applies to Standard platinum resistance thermometers (SPRT) and resistance temperature detectors (RTD) for industrial equipment as well.

Common Thermometer Types

Two types of liquid-in-glass thermometers exist:

  • Mercury thermometers contain a bulb filled with mercury attached to a narrow tube. Changes in temperature yield changes in the volume of mercury. These small volume changes drive the mercury up the tube or pull it down the tube.
  • Alcohol or spirit thermometers look and act like mercury thermometers. However, they use ethanol instead. Ethanol is less toxic than mercury, and cleans up more easily after breakage because ethanol evaporates quickly. Since ethanol costs less than mercury, replacing broken thermometers also incurs less cost.

The three categories of mercury and spirit thermometers include:

  • Complete Immersion Thermometers, which show temperature correctly when completely covering the entire thermometer with fluid (gas or liquid).
  • Total Immersion Thermometers, which show temperature correctly when covered by fluid except for a small portion of the column [6 to 12 mm (0.24 to 0.47 in )].
  • Partial Immersion Thermometers, which show temperature correctly when immersed to a specific depth. A line on the thermometer usually indicates required depth.

Calibration Procedures

Thermometer calibration by the Comparison Method includes the following steps:

  1. Review the device to be calibrated.
  2. Prepare the calibration bath.
  3. Test the device to be calibrated.
  4. Record any difference and reset if possible.

Next, let us take a closer look at the processes involved in each step.

1. Review the device to be calibrated.

  • Document the level of accuracy required for the tasks the device will be used to complete.
  • Look at the device to be calibrated. Ensure that the column and bulb are not cracked and the legibility of the scale.
  • Note any identification numbers. These numbers often trace back to manufacturer’s specifications that could help in making calibration decisions.

2. Prepare the calibration bath.

  • Set the calibration bath to the desired calibration temperature.
  • Ensure the immersion of the NIST standard thermometer in the calibration bath at the proper depth.
  • If calibrating more than one device, begin with the device with the lowest temperature.
  • Wait until the calibration bath stabilizes at the desired temperature.
    • Use the NIST standard thermometer to measure the temperature.
    • When the temperature remains unchanged for at least three readings, taken thirty (30) seconds apart, the bath has stabilized.

3. Test the device to be calibrated.

  • Insert the thermometer to be calibrated into the calibration bath at the proper immersion depth.
  • Allow the thermometer to be calibrated to achieve a stable temperature. The temperature is stable when the reading on the device has not changed for at least three readings, taken thirty (30) seconds apart.

4. Record any difference and reset if possible.

  • Use a magnifying glass to look at the tested device.
  • Identify the difference between the device’s output and the calibration bath.
  • Document the difference.
    • Perform any reset using manufacturer’s instructions.
    • Repeat steps 2-4 for remaining devices being calibrated at higher temperatures.

Closing Thoughts

Thermometer calibration by the Comparison Method enjoys popularity as one of the most widely used techniques. Restaurants employ this method via ice bath to ensure proper temperatures for food storage, and home chefs may use it in conjunction with oven thermometers to get the known offset of an oven’s settings.

As we’ve seen here, however, laboratory-grade thermometer calibration by the Comparison Method involves a much more rigorous procedure and complies with the standards of the appropriate governing bodies.

Thermometer Calibration by the Fixed Point Method

What is the Fixed Point Method?

The Fixed Point Method of thermometer calibration assures the quality and accuracy of a thermometer’s measurements. Typically, only national metrology laboratories use it. Thermometer calibration by the fixed point method focuses on instruments that must measure accurately within ±.001℃.

This method uses the ITS-90 international temperature scale developed in 1990. Based on the thermodynamic or absolute temperature scale, ITS-90 is not truly a scale. It is a set of fixed points that defines an international equipment calibration standard. It helps scientists know that a temperature measured in Asia will be the same as a temperature measured in Europe. Scientists can repeat results regardless of location with properly calibrated equipment.

In creating the ITS-90 standard, a body of scientists selected seventeen fixed points. These points are based on temperatures where various elements or compounds achieve equilibrium. These points include [1]:

The ITS-90 temperature scale defines seventeen fixed points to use during thermometer calibration by the fixed point method.

Although Freezing Point and Melting Point are common terms, Triple Point and Vapor Pressure Point might not be. The Triple Point is the temperature of a substance where all the phases of matter – solid, liquid, and gas – exist in equilibrium. The Vapor Pressure Point is the temperature of a substance where the gas and liquid phases are in equilibrium.

How is the Fixed Point Method used in thermometer calibration?

To achieve fixed point calibration, the thermometer to be calibrated is placed in a specially designed flask. This flask maintains a constant temperature through both heating and cooling. The technician selects a limited number of fixed points from the ITS-90 with the goal of using as few as possible. The actual procedure varies depending on the fixed points selected. [2]

The points of the ITS-90 temperature scale have been identified, documented, and globally accepted. As a result, technicians with proper training can reproduce standard calibration conditions.

[1] B.W. Magnum. “International Temperature Scale of 1990 (ITS-90).” John R. Rumble, Editor-in-Chief. CRC Handbook of Chemistry and Physics, 98th Edition, 2017-2018. CRC Press, Boca Raton, FL (USA): 2017. 1-17.

[2], last visited 01/18/2018

What Is Metrology?

Per Merriam-Webster’s online dictionary, metrology is “the science of weights and measures or of measurement.” [1] It can also refer to a system of weights and measures. [1]

Definitions are helpful, but application is more meaningful. Why are measurements so important? How does standardization work? What purpose does it serve in the real world applications of science, business, finance, and technology?

The Importance of Metrology

Metrology is important for several reasons:

  • Knowledge must be shared. [2] Without common measures, an event cannot be described and documented so that independent, unbiased observers can reproduce that event.
  • Measurements protect us. [2] From medication dosing to highway speed limits, numbers matter. Having a standardized number as a target, as well as having a standard measure for that target, protects the common well-being and potentially saves lives.
  • Contracts and transactions must have numbers as a foundation. [2] Everything from food portions to gas pump prices is affected by many measures. Without numbers, acceptable (and unacceptable) performance cannot be described and determined.
  • Measurements strengthen competition. [2] Once an industry knows both legal and customer requirements, they can analyze and determine how closely those are met. Then they can innovate and remain competitive by better meeting requirements.

Metrology in Operation

Obviously, having standard numbers is critical. But how is that achieved? How does society identify measures and implement them as standards?

First, it is critical to identify the what must be measured and the best measures to use. [3] Next, collaborate to generate approval for and acceptance of the measure. [3] Finally, codify how to use the measure [3] so others can use it.

Metrology in the Real World

Within metrology, there are three subfields:

  • Legal: This subfield focuses on numbers used in legal or legislative efforts. The documented measures may protect personal health, improve public safety, increase environmental protection, or ensure fair trade on behalf of consumers. [3,4] The numbers in these efforts help develop and enforce fair rules for all.
  • Applied or Industrial: People in this subfield concern themselves with numbers used to manufacture goods or to produce and refine natural materials. Once the numbers are established, people in this subfield also ensure proper calibration of instruments used to obtain numbers in the manufacture of goods or production of materials. [3,4]
  • Scientific or Fundamental: Focus on scientific metrology allows the establishment of new units of measure and the development of methods for repeatedly obtaining measurements in the new unit. Those in this field gain community support for new measures and methods of measurement and teach general users how to use the measures and related methods. [3,4]
An autoclave from the J. L. Mott Iron Works. This image dates to the 1920s, illustrating the historical need for precision measurements in industry. [6]
An autoclave from the J. L. Mott Iron Works. This image dates to the 1920s, illustrating the historical need for precision measurements in industry. [6]

Final Thoughts

Throughout history, man has looked for ways to measure what he produces and its value. As early as 1875, a global need for standardized measurements was recognized. [5] However, the standards vary from industry to industry. The science of measurement and related standards continues to grow in need and popularity.


[1], last visited 12/21/2017
[2], last visited 12/21/2017
[3], last visited 12/21/2017
[4], last visited 12/21/2017
[5], last visited 12/27/2017
[6], last visitied 12/27/2017