Glass Thermometer Selection Guide

Glass Thermometer Selection Guide

A guide for product comparisons!

Glass thermometers are accurate, economical instruments that measure temperatures of liquid or gas. All of our glass thermometers conform to the International Temperature Scale of 1990 (ITS-90). All Ertco® brand thermometers have individual serial numbers. ASTM thermometers vary from 5.5- to 8-mm diameter; most other thermometers have a 6- to 7-mm diameter. For PTFE-coated or tapered thermometers, call our Application Specialists for ordering information.

Types of Glass Thermometers

Choose your thermometer by the length that will be immersed in the liquid or gas.

  1. Partial Immersion thermometers are immersed in the fluid to the specified immersion depth. The remaining (emergent) portion of the stem is exposed to the air.
  2. Total Immersion thermometers need to be immersed to the liquid temperature mark on the thermometer. If you are unable to immerse properly, see "Definitions—Emergent Stem Corrections" below. Since the thermometer column is fully immersed, these thermometers are the most accurate.

The accuracy of a typical glass thermometer is approximately ±1 scale division. Precision ASTM- and SAMA-designated thermometers are manufactured according to strict performance specifications set by ASTM and SAMA. For accuracy ensured in writing, our ASTM and SAMA thermometers are available with NIST-traceable certificates.


Accuracy vs Precision: Accuracy is the difference between the measured value and the true value of a tested material. Precision is the repeatability of successive measurements under the same conditions.

ASTM (American Society for Testing and Materials): A not-for-profit organization that has developed standards for materials, systems, products, and services. The ASTM number refers to a thermometer's specific application.

NIST (National Institute of Standards and Technology): U.S. government agency that provides standard reference materials and calibration services. NIST-calibrated thermometers are calibrated at NIST. NIST-traceable with data thermometers are factory calibrated to NIST-calibrated standards and include a certificate with the calibration data.

SAMA (Scientific Apparatus Makers Association): Precision glass thermometers approved by ASTM subcommittee E20-05 and the American National Standards Institute (ANSI) as a national and international standard. Manufactured with guidance from NIST.

Emergent Stem Corrections. Avoid inaccurate readings when a total immersion thermometer cannot be properly immersed. Determine the approximate stem correction with these formulas:
  • For mercury Fahrenheit thermometers:
    °Correction = 0.00009°F x n x (T-t)
  • For mercury Celsius thermometers:
    °Correction = 0.00016°C x n x (T-t)
  • For spirit-filled Fahrenheit thermometers:
    °Correction = 0.0006°F x n x (T-t)
  • For spirit-filled Celsius thermometers:
    °Correction = 0.001°C x n x (T-t)
where T is the bath temperature (the temperature indicated on the thermometer), t is the average temperature of the emergent stem, and n is the number of emergent thermometer degrees. Hold an auxiliary thermometer next to emergent stem to determine t.