Temperature sensing with OpenEnergyMonitor

Temperature sensors for monitoring heat pumps

I have been using OpenEnergyMonitor.org energy monitors with heat pumps for a few years now and thought I should briefly share my experience.  This covers the practical side of temperature sensors, and is based on my experience.

(If you just want advice about fitting temperatures sensors onto pipes, skip further down the page)

Introduction

The sensors used in OpenEnergyMonitor.org modules are DS18b20. These are Maxim 1-wire digital sensors.

They actually have 3 connections;   0v, 5v and signal.   Since each sensor has its own unique i.d. code, multiple sensors are simply connected into the same 3 terminals on the monitor device.

The great advantage of these digital sensors is that there are no errors due to cabling.  Phone extension cable can be good enough.  Thermistor sensors (variable resistance, as PT100) rely on a specific cable resistance and furthermore some sensors could ‘drift’ out of calibration.   A further advantage of digital sensors is that they do not in themselves generate heat. They can therefore be used in still air with good accuracy.

If several sensors are strapped together (with an elastic band) and tested in a thermos flask over a few hours, most seem to read within 0.2 degrees C of each other.  If you have say 6 sensors, you can chose those that agree most for the most important sensing (e.g. flow and return).   Anyhow, these sensors are perfectly accurate enough for this type of monitoring, and there is comfort in knowing that if you are seeing 35.2C (for example) on a graph, then that sensor tip must be at 35.2 +/-, with a small error margin of say 0.15 degrees.  This is because the digital signal is generated inside the sensor.

What to measure on a heat pump?

Typical temperature measurement could include the following:

Ground source
Water flow and water return from heat pump (hot side)
Ground collector glycol  inlet and outlet from heat pump (cold side) 
Outside (ambient) air temperature.  
Hot water cylinder temperature, and at least one room temperature.

Air source
Water flow and water return from heat pump (hot side)
Outside (ambient) air temperature and air-off (cooled air leaving heat pump) 
Hot water cylinder temperature, and at least one room temperature.

For either of the above there could be a buffer cylinder involved in the design, therefore the flow and return from buffer cylinder to emitter (e.g. underfloor or radiators) may need to be monitored.

There may also be mixing valves (sometimes unnecessary mixing valves) on underfloor manifolds. These may also need monitoring.

For more detailed analysis, the refrigerant internals of the heat pump can be monitored. These may include discharge, suction and liquid temperatures.   This is however a little intrusive, and could affect the warranty. It may be necessary to discuss this with the system installers.

Measuring air and liquids 

Air

Measuring internal air temperature is relatively straight forward, but sensors must be positioned away from any source of heat or radiation source.  Sunlight and close-by appliances and even lighting can greatly affect readings.
The general rule for outside air measurement is to keep the sensor well away from sunshine, or areas that get sunshine.  The sensor can also drop below the air temperature if it can ‘see’ a clear sky.  A small polystyrene roof is a good shield protection from positive or ‘negative’ radiation (i.e. when radiation from sensor is greater than its surroundings).  It is also necessary to keep the sensor dry, unless you purposefully wish to cover it with a wet ‘sock’ to record Wet Bulb Temperature.  There is a lot of information available about this relating to weather stations.

Liquids

Most heat pump monitoring involves the temperature measurement of water and liquids flowing in the pipes.

For very high accuracy results, as required by heat meter devices, an immersion pocket is required. (Heat Meters measure the difference between two sensors, so both must be very accurate)
The pocket usually involves a ‘tee’ fitting and a hollow pocket that is completely surrounded by the liquid. The sensor is inserted into the pocket.

  

An easier and cheaper method is to fix the sensor to the outside of a metal pipe. This method is used in all heat pumps for their on-board sensing.

Any sensor that is outside the liquid can be affected by the temperature of the surrounding air, but the error in the reading can be negligible if the sensor is mounted correctly.

There are various methods as follows-

1)  Sensor strapped to metal pipe or metal fitting
2) Sensor inserted in surface pocket (a pipe soldered to the pipe’s surface)

This shows 10mm pipes that have been soft-soldered to copper fittings. Due to the high conductivity of copper, the temperature of the inside of the small pipe is almost exactly the same as the temperature of the liquid inside the pipe. After insulating (lagging) this pipe, the accuracy of the sensor inserted inside the small pipe should be excellent.  Ideally the pocket internal diameter is close to the sensor diameter.  Conductive paste should be used before inserting the sensor

It is a good idea to fit several pockets like this in the right places during installation. Obviously the pipe must be empty.

Many heat pump manufacturers solder pipes on the outside of water or refrigerant pipes so that their sensor probes can easily be inserted to give accurate measurement

Strapping a sensor to a pipe (or a pipe fitting)

 Whilst not as good as surrounding the sensor with a conductive copper pocket, it is possible to simply strap the sensor to a pipe.   Strapping using a copper strap (as used in refrigeration) , or copper wire, will give better results.
The following shows one simple method.

Copper or brass conduct heat well.  The temperature difference between the inside of the pipe and the outside is tiny, so always fit sensors onto metals.  Plastic pipes are poor thermal conductors, so the effect of surrounding air can affect the sensor temperature. Furthermore, if the pipe is changing in temperature, the sensor may ‘lag’in time behind. i.e. it will respond very slowly to temperature changes.  This may or may not be a problem.

This is not the best position for a sensor since there is 3mm of plastic between the liquid inside and the probe, however, the copper wire strap is making the best of a bad job.  Heat will conduct along the copper. There is a lot of copper in contact with the plastic, and this will conduct and will transfer heat to/from the ‘surrounded’ probe.  If this is insulated well on the outside, it may give good-enough results for ground source comparisons.  Time response may be a little slow, which may not necessarily bad.   Fitting the probe onto a metal fitting is a far better bet.