In order that I can understand why the boiler is not quite working as expected I first need to setup boiler instrumentation for testing. The instrumentation is all around temperature measurements. For this setup I’m using:
- Pico Technology thermocouple logging board
- RS K-type thermocouples
- Akasa silver based thermal compound
- Dell Inspiron laptop
I used 3 thermocouples, these K-type thermocouples are ok up to ~250°C. They consist of a twisted pair of wires with a tiny bead formed at the end from 2 different metals, Chromel and Alumel.
The 3 temperature points were:
- Local Air Temp – this is the thermocouple shown to the left of the picture with the wire clamped to the tray and the thermocouple itself point up into the air around 75mm (~3 inches) from the workbench surface.
- Lower Boiler – this is the measurement in the lower point on the boiler with the thermocouple pushed 10mm into the brass blanking plug.
- Top Boiler – this was to be in the steam area at the top of the boiler, again the thermocouple is pushed 10mm into the blanking plug.
The blanking plugs were drilled 2mm diameter to a depth of 10mm. This meant that the thermocouple could be situated close to the end of the blanking plug and so more closely see the temperature at that point.
This is not perfect, but a good compromise to maintain the integrity of the boiler.
The thermocouple had a bead of Asaka silver conductive paste put onto it before it was pushed into the hole. This would improve the thermal connection to the brass plug.
The local air temperature probe is just to give me a good reference of the local environment. This would help me with the calculation of heat loss. This is quite stable though and so could just have been an initial recording of the air temperature.
A thermocouple consists of two wire legs made from different metals joined together to form a junction. Two of these are used to create a hot and a cold junction. The hot junction is connected to the body whose temperature is going to be measured. The cold junction or reference junction is connected to a body of known temperature, in this system the reference cold junction is provided by the Pico Technology measurement box. When the measuring junction is placed on something hot a voltage or potential difference between this and the reference junction occurs. This voltage is converted into a temperature measurement using thermocouple reference tables.
The heart of the measurement system is the two-tone blue box on the right of this image. The measurement thermocouples are plugged into this box. In this case three thermocouples are used. This is a Pico Technology TC-08 Data Logger.
The reference cold junction is provided internally by the measurement box.
This box is then connected via USB cable to the laptop.
The measurement software, PicoLog 6, is a free download from Pico Technology. Well effectively this is paid for when you buy the measurement box. The software is available for Windows, Mac and Linux.
This software is very easy to use. You can name the thermocouples, making it easier to relate to the data afterwards.
You can plot the data as the measurement is occurring. You can see the data in tables. You can export the data as a PDF file or as CSV. This software is just at the right level of complexity.
You also get control over the measurement sample rate. I selected 1 second.
I very carefully measured the quantity of water in the boiler.
This is the working fluid and is what all of the data and calculations will be based around.
This vertical boiler is tiny and holds just 75ml of water.
I now have the Boiler instrumentation for testing.