How do you know if what you are seeing is the truth or you’re seeing what you want to see? Let’s use an analogy: carpenters and the tools they use. If a carpenter wants to see if a building is straight he uses a plumb line, essentially a string with a weight at the bottom. Gravity pulls the string taught and offering a vertical reference. Hold that string up in reference to the Leaning Tower of Pisa, you can confirm it is indeed leaning. The plumb line is a reference.
How do you make sure the floor you just installed is flat? You can use a bubble level or laser level. Other references include a square for ensuring a 90-degree corner, a tape measure for ensuring the proper length, etc.
Like the carpenter, engineers use references to examine quantities like temperature. Our tool belt looks a little different. The tools we use tend to be a mixture of portable equipment that goes out to the field and stationary equipment that stays in the shop.
Calibration is essentially comparing a process value against a known standard. In temperature, a temperature standard can be used to generate a physical temperature by heating up or cooling down from ambient. The most common type would be the dry block. Named because the well has no fluid.
The dry block contains an insert with one or more holes matching the size of the probe you are testing. The other type is a temperature bath. Think mini hot tub. OK, bad analogy, but instead of using a drilled insert, the temperature bath has a reservoir of fluid where the temperature element is immersed. Immersion depth of a probe typically allows for a more accurate measurement. A bath will more often be found in a high end metrology lab.
The other side of temperature standards is the actual simulation of temperature. Wait…what?!? How do you simulate a temperature? Let’s look at an resistance temperature detector (RTD). An RTD temperature probe outputs resistance. That measured resistance changes as temperature deviates and is directly proportional to the temperature change. Resistance increases with the increase in temperature.
A Pt100 is a common RTD type. The Pt means it has a platinum element and the 100 indicates the resistance at water freezing. So, you could simulate zero degrees Celsius by generating 100 ohms. If you have a temperature transmitter you could disconnect the RTD connection and source 100 ohms to give a zero degree C reading to the DCS or control system. That will test the output of the transmitter and verify the wiring all the way back to the DCS display in the control room.
Roy Tomalino has been teaching calibration management for 14 years. Throughout his career, he has taught on four different continents to people from over 40 countries. His previous roles include technical marketing engineer and worldwide trainer for Hewlett-Packard and application engineer with Honeywell. Today, Roy is responsible for all Beamex training activities in North America.
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