Temperature Sensors

Temperature sensors are required at a minimum up at the outflow from the collector and towards the bottom of the hot water cylinder (usually between the coils), with a third temperature sensor sometimes installed at the top of the cylinder. There are two main types used, thermocouples (NTC & PTC) and resistance temperature detectors (RTDs). Because of higher accuracy and repeatability, RTDs have generally replace thermocouples in solar applications, some chinese controllers are still using thermocouples to sense temperatures at the cylinder. Normally 12V (but always a low voltage) is placed across the RTD or Thermocouple, and the current is measured to give the resistance reading and hence the temperature. RTDs are generally made of platinum, and come in three (main) flavours.

• PT100, 100 Ωs at 0°C
• PT500, 500 Ωs at 0°C
• PT1000, 1000 Ωs at 0°C

The PT1000 increases in resistance by 3.85 Ω per °C
At 30 °C, the resistance would be 1115.5 Ωs

Resistance of PT1000 extension leads

A very high 2Ω resistance on an extending lead (which is HUGE) would lead to a 0.5°C error in reading temperature using a PT1000. Earlier controllers also used PT100, which exhibit a small increase in resistance (0.385 Ω / °C). A 2Ω resistance here would lead to a 5.2 °C error, which is much more significant. As a result, earlier texts encourage a 0.5mm² leads up to 50meters, and 0.75mm² leads over this distance. If a PT1000 is used in the solar circuit, smaller diameter leads can be used, and indeed "bell wire" at €4.00 for 100 meters makes a very good low cost solution.

Another advantage in using a PT1000 is because the resistance is 10 times higher than the PT100, 10 times less current is used to measure the resistance, which in turn means lower parasitic electrical losses.