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Decontamination Process Validation

Thermocouples & Common Sources of Error

There are Some Common Sources of Error Found in Thermocouples and from the Way the Sensors are Used. Conformity, Interchangeability and Nonhomogeneous Regions are the Primary Cause of Error.

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There are some common sources of error found in thermocouples and from the way the sensors are used. Conformity, Interchangeability and Nonhomogeneous Regions are the primary cause of error.

Conformity errors address how closely the output of each thermocouple follows internationally accepted Standards as shown in the table below.  The ideal output of a Type T thermocouple is approximately 39 microvolts per °C change. Conformity errors from a single homogenous length of thermocouple wire can be eliminated from the results by calibration.

Conformity New

How accurately T/C follows International Standards standard output for Type T T/C is 39 uV/°C . Conformity error can be eliminated by calibration.

  • Interchangeability errors are the degree to which several thermocouples all have the same output at the same measured temperature. Outputs from thermocouples made from different production lots or different manufacturers can vary by as much as 5%. Interchangeability errors can also be terminated by thermocouple calibration.
  • Nonhomogeneous  Regions are the most critical type of error because they are random and not compensated in the calibration procedure. The only way to eliminate these errors is through the proper selection and installation of your thermocouple circuits. A nonhomogeneous region in a thermocouple circuit is an area where the purity and uniformity of the metals are not consistent. These errors are typically caused by the use of connectors, extension wire and a phenomenon known as Cold Working.

Although connectors and extension wire may be made from the same metals their outputs can vary by as much as 5% due to variations in composition and annealing. If these items are placed in an area where there is a temperature gradient or variation, such as at or near the wall of an autoclave, then serious errors can be introduced into your measurements.

Connector Errors - During Calibration

Error = (SW-SC) x (T1-T2) 2uV x 00C = No addition error from connection

Connector Error 1

The diagram above shows the errors which can be introduced by using connectors in the sensor chain. Connectors or junction boxes through the wall of the autoclave are often used to minimise the inconvenience of dealing with feedthroughs or compression fittings as well as minimising the cost and difficulties associated with replacing a damaged thermocouple.

Connector Errors - During Study

Error = (SW-SC) x (T1-T2) 2uV x 800C = 160uV or 4.0 0C error from connection

Connector Error 2

The diagram above illustrates the error introduced during calibration prior to the validation studies. This time the thermocouples are placed in a reference bath, and the vessel is not running. Since the vessel is not active, the temperature on the inside (T2) and outside (T1) of the vessel wall are relatively uniform, therefore the error due to the junction box is negligible . The user assumes all the errors have been compensated for in the calibration procedure. Next the user goes ahead with the validation studies. Since the vessel is now active the temperatures on the vessel wall have significantly changed. The temperature on the inside of the vessel wall (T2) is now calculated at 110°C and the outside of the vessel wall (T1) is 30°C . If we now calculate the error, we can see that the error introduced by the gradient across the juction box is approximately 4°C .

Error introduced using connectors or junction boxes can be mathematically calculated. In this example quality thermocouple wire (SW), has been connected from the measuring system (MS) to the junction box on the outside of the vessel and then from the inside of the vessel to the process. Although the junction box utilises matching material on the connectors the purity of the metals (SC) does not match the thermocouple (SW). The error introduced is equal to the difference in the output of the materials, multiplied by the temperature gradient across the junction box.

After completion of the studies the User performs a Post calibration to verify that the thermocouples have not varied since initial calibration. Since the vessel is now not running the gradient across the junction box is eliminated and the calibration matches that of the initial calibration and the study is assumed to be successful.

Since these errors are random and dependent on the composition of the junction and the gradient temperatures across them, they should be avoided. The only way to ensure the elimination of these possible errors is by utilising one continuous length of thermocouple wire from the measuring system to the inside of the vessel.

The sensor and circuit errors discussed can seriously impact the test data validity, however with good practices you can eliminate these errors and achieve accurate results.

Some Useful Recommendations to Minimise Sensor/Circuit Errors

  • Use Type T thermocouples. They provide more accurate, repeatable results in the validation environment and tare the most suitable in the temperature range of -100°C to 400°C.
  • Calibrate sensors prior to Validation Studies to eliminate any systematic errors such as Conformity and Interchangeability errors.
  • Use one continuous length of sensor wire, do not use connectors or extension wires, and terminate the sensor within the stable ambient of the logger.
  • Use stranded wire rather than solid wire to minimise cold working errors.