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Calculating Total Uncertainty of Temperature Calibration With a Dry Block
In Calibration World magazine (Spring/Summer 2011) there was an article on temperature and the calibration of temperature instruments. In this article we continue with temperature-related articles. We will discuss the various uncertainty components related to temperature calibration using a temperature dry-block. Also, we will discuss how to calculate the total uncertainty of a calibration performed with a dry block.
First, let's discuss what a temperature dry block is:
- consists of a heatable and/or coolable metallic block, controller, an internal control sensor and optional readout for external reference sensor. This article will focus on models that use interchangeable metallic multi-hole inserts.
- There are fast and lightweight dry blocks for industrial field use as well as models that deliver near bath-level stability in laboratory use.
- There are also some work safety issues that favor dry blocks in preference to liquid baths. For example, in temperatures above 200 °C liquids can produce undesirable fumes or there may be fire safety issues. If a drop of water gets into hot silicon oil, it could even cause a small steam explosion which may splash hot oil on the user.
- Dry blocks are almost without exception meant to be used dry. Heat transfer fluids or pastes are sometimes used around or inside the insert, but they don't necessarily improve performance. They may actually even impede the dry block's performance and damage its internal components.
Let's have a look at the various uncertainty components that are related to temperature calibration done using a dry block. These components are relevant to all manufacturers' dry blocks. Some manufacturers specify these components and some do not.
It is possible to use a dry block with the block's internal measurement as the reference (true value), or you can also use an external reference temperature probe inserted in the block as a reference measurement.
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