How Digital Thermocouple Calibrators Work
Temperature is such an important measurement in today’s complex process industry that many companies are now requiring routine validation of temperature measurements, as often as at the end of every shift. There are several ways to fulfill this requirement.
A company can use physical standards, i.e. ice baths and boiling water. Although this sounds like a simple process, this is the route most prone to error when used by inexperienced staff. (NIST has a nice video on how to construct an ice bath properly.¹)
A company can set up and staff a metrology lab. This may be the best route but can also be expensive to set up and maintain.
The direction many companies are taking is to buy a Digital Thermocouple Calibrator that comes with an ISO 17025 calibration certificate. The ISO 17025 calibration is often required to verify that the calibrator will meet the accuracy requirements of industry or company standards. A thermocouple calibrator can be used by most technicians in a quality department, is easy to maintain, and keeping its calibration up to date is easy with a subscription to a calibration service (often from the manufacturer) where they will contact you when the instrument is required to be sent back for a new calibration. Typical calibration periods are 1, 2 or 3 years.
Another bonus of using a Thermocouple Calibrator is that you have the manufacturer’s tech support team to help you and answer any questions regarding functionality or operations.
Today’s digital thermocouple calibrators have evolved from a simple thermocouple simulator to high-accuracy, multi-function diagnostic tools for thermocouple circuits. See Appendix A for more details regarding the display and functions available.
Some of the most important functions you should look for in a thermocouple calibrator include:
Support for multiple types of thermocouples – choose a model compatible with the thermocouple types that you use. Thermocouple calibrators are generally available in a basic model which will support Types J, Type K, Type T, and Type E thermocouple probes - which are the four most popular thermocouple types - and a higher priced model that will support 14 (or more) thermocouple types. If you are a heat treater or measuring high temperatures, you will need the model with more TC types in order to get the Tungsten and Platinum thermocouple types typically used in high-temperature measurements.
Precision Thermometer – a thermocouple calibrator should also be a precision thermometer. The TEGAM 940 and 945 thermocouple calibrator models now have 0.01° resolution so you can easily see the direction of drift or when the system has reached stability.
Precision Sourcing – simulating a thermocouple output. The thermocouple calibrator should easily and accurately supply (SOURCE) a small, stabile voltage that simulates a thermocouple signal. “Easily” is the keyword here as this is what you as the user will have to set up and adjust every time you use the thermocouple calibrator. “One hand” operation is important for the sourcing function as you are often holding something in your other hand or recording a reading when performing this operation.
Dual Displays – being able to see what the unit is SOURCING and what a system thermocouple is outputting to the calibrator (READ) at the same time is a must-have feature. Otherwise, you will have to switch between the SOURCE and READ functions (and wait for the reading to settle) to get both temperatures and you can never be sure the output is where you set it at the exact time to compare the two.
Cold Junction Compensation (CJC) – is a key component of any thermocouple measurement. Remember, a thermocouple is a differential device - creating a voltage that is equal to the difference in the temperature between its two ends: one end being your measurement point and the other end is the temperature at the connection to the temperature calibrator. The CJC is also a key component in the overall accuracy of the calibrator. Be sure to note that the calibrator’s accuracy specification includes the CJC. Some manufacturers claim high accuracy but use an external $5000 ice point reference as the CJC for their specifications……yielding unrealistic accuracy specifications that you will never achieve using the unit.
MAX, MIN, AVG, STD (Deviation) and Trend direction can be invaluable tools for understanding your readings.
The MAX function will capture and hold the maximum reading the unit reads once you start the function. This can be used to check for long-term stability of a system or to assure that nothing went wrong over an unobserved time period of a test (overnight).
The MIN function works in conjunction with the MAX function so you can immediately know the MAX and MIN temperatures that a system produced over a time period.
The AVG function does a mathematical average of the temperature over the time between resets. This function helps interpret real-world situations where the temperature will vary slightly due to heating/cooling systems or day/night temperatures and you need to balance the overall effect vs the temperature.
STD is the standard deviation over a period of time. This helps you understand how large the temperature swings of a system are and helps in tuning or dialing-in a control system.
Trend arrows are a feature that can tell you at a glance whether your system is increasing or decreasing in temperature. (Rather than watching the display bounce between 0.1 degree increments…)
Trouble Shooting Capabilities
The Open Circuit Detection function of a thermocouple calibrator is a very important feature. You can check your system’s thermocouple wire from end to end with this feature. Use it to find an intermittent connection, which is one of the hardest problems to find. Once connected, you can open and close doors, move wires, check connections etc. and the calibrator will tell you when the open circuit occurs. To do this, it has to inject a very small current into the circuit. You should also be able to turn this function off during calibration. This assures that no errors are introduced in your controls. Make sure the calibrator has this open circuit on/off feature.
Probe offset is a little-understood problem with thermocouples. Every piece of thermocouple wire has a slightly different output than the next. This goes for thermocouple probes also. One thermocouple probe may read +0.2 degrees at a true zero, another may read -0.3 degrees. In precision applications, these offsets are noted on a chart and the readings are adjusted. With a probe offset feature, the calibrator can either adjust for the offset or simulate the thermocouple with that offset to match its real -world performance. Using this feature, you can validate the probe readings and calibrate the system for various probes with various offsets.
Transfer Mode is another troubleshooting feature to look for in these calibrators. The function isolates the thermocouple from the rest of the system (removes static and ground loops) while still reading the thermocouple and retransmitting the exact signal up stream in the system. It allows you to determine if there is a system problem or a thermocouple problem.
Other useful functions
Range: A Type K thermocouple has a range of -250 to 2282°F. Many features of a thermocouple calibrator off step changes. 10 steps between –250 and 2282 is a 253° step which may not be useful to you. The range feature allows you to select a range that is important to you. You can set a range of 500 to 1000°F and the step functions are then in 50° increments.
Units: Most thermocouple calibrators offer Fahrenheit and Celsius units; top-end models will also offer Kelvin.
Presets are what converts your calibrator from an instrument to a precision tool. These allow you to preset various parameters such as range, offset, TC type, and temperature points. Once set up, they allow you to go from setup to setup and be ready to test with the push of a button.
The Ramp function continuously cycles between programmed limits to check temperature switches and other functions that occur in your temperature system. A good calibrator will have slow and fast ramp features and have the ramp work in conjunction with the range function - this way it covers only the range of your process, not the full range of the thermocouple.
The Step function is similar to the ramp function, only you can manually step it between 10% and 90% of range or cover the range in 4 steps in 25% increments. This function is used to quickly validate a system is reading properly and to verify that certain functions in your system are occurring in the expected range.
Remember this is a portable instrument
You will use your thermocouple calibrator on a production floor and near ovens, as shown below. For heat treating, you have to validate that the controller is reading properly, and then insert a reference thermocouple to validate that the system is performing properly.
You need the instrument to be accurate where you use it. Verify that it will hold its accuracy over an Operation Range that meets your real-world applications. A 10°C (18°F) operating range around ambient 23°C (73°F) should cover most plant floor applications.
The calibrator should also have mil-spec Shock/Drop/Vibration specifications. It is a working tool and tools occasionally get dropped and they need to keep functioning.
Backlite Display is important, as lighting on a production floor or inside cabinets may be insufficient to read instruments.
Battery Life is a very overlooked feature. Many thermocouple calibrators have paltry battery life. That means you need to keep a bag full of spare batteries close by, plus all those dead batteries go into landfills. Long battery life is more ecofriendly and is a sign that the designers paid attention to important features. Look for a temperature calibrator that has a 500 hour battery life. It will be ecofriendly and will work when you need it. TEGAM has a blog post that covers this subject in more detail – read it here.
Take a look at the new TEGAM 940 and 945 Thermocouple Calibrators. They have all the features described above plus one of the highest accuracies and broadest operating temperature range in their price range, as well as 500 hours of battery life on 3 AA batteries.
2 Source 5-Digit Display
3 T/C Types
4 Battery Indicator
5 Temp Scale
6 Active Digit Indicator
7 millivolt indicator
8 Read 5-Digit Display
9 Read % indicator
10 Read Std Deviation
11 Blue tooth indicator
12 Transfer Mode
13 Range is displayed
14 Average is displayed
15 10 Step Function elected
16 MAX reading is disp.
17 Slow Ramp is selected
18 MIN reading is disp.
19 Fast Ramp is selected
20 Pre-Set point number 0 to 19
21 Preset is active
22 Trend Indicator for Read
23 Minus Sign for Read
24 Read Channel Label
25 Minus Sign for Source
26 Source Channel Label