Samenvatting

This report describes the work that has been done on the water calorimeter project at VSL. The calorimeter determines the absorbed-dose-to water Dw in gray, by measuring the radiation induced temperature change. The temperature of the water will change due to irradiation of a beam ionizing radiation. The irradiation causes a temperature difference. The temperature change of the water is determined with thermistors.

Each thermistor is connected to a digital multi-meter (DMM) to measure the resistance of the thermistor. The resistance is determined by measuring the voltage across the thermistor. The temperature change of the water can be determined with the resistance of the thermistor. The DMM supplies a measurement current through the thermistor. Power is dissipated because of the resistance of the thermistor. Therefore heat will be dissipated by the thermistor. This effect is called self-heat.

This report describes various measurements that where done to determine the so called self-heat of the thermistors in the water calorimeter. It is important for the dose measurements to be able to describe the behavior of the self-heat. The behavior of the self-heat can be described by convection, conduction, absorption and radiation. The self-heat is described by means of two types of least square fits. The fitting parameters are determined with an uncertainty to understand the self-heat behavior of the thermistors. The uncertainty in the self-heat is determined with use of a comprehensive uncertainty budget for each fitting equation.

The DMM can measure the resistance change of the thermistor with or without use of the OCOMP on setting or the OCOMP off setting. OCOMP represents offset compensated ohms. The OCOMP setting has influence on the self-heat of the thermistor; therefore it is also important to determine the behaviour of the OCOMP setting.

The self-heat is determined by measuring the temperature of the thermistor at different current values with a constant water temperature. This is done by two different measuring methods. The first measuring techniques use an external constant (DC) current source. The dissipated power in the thermistor is changed by changing the input current of the source. The second techniques use parallel resistors to change the dissipated power in the thermistors. The second measurement technique uses the input current of the DMM’s; therefore it is possible to determine the self-heat with OCOMP on and OCOMP off.

The self-heat is determined for four different thermistors at OCOMP on and OCOMP off. The ratio between OCOMP on and OCOMP off is determined on the basis of the self-heat constants. The self-heat during a measurement with OCOMP on is less because of the offset compensation. The ratio between OCOMP on and OCOMP off is determined 16 times. The average ratio for the relevant thermistors between the self-heat at OCOMP on and OCOMP off is 0.86 ±0.10 (k=2). The self-heat constants have a value between 1.3 and 2.1 mK/μW. The uncertainty for the first fitting method is: 8.1 % and for the second fitting method: 6.5%.