Frustrated with inconsistencies and poor documentation of the Raspberry Pi and Arduino boards. Radical decision time! Searched the internet for a better solution, industrial grade, tested and with a real manual. Found LabJack T7 with the ability to measure thermocouples directly.
The T7 family of devices has the resolution and amplification necessary to directly measure raw thermocouple signals. If you save 1 channel for an external cold junction temperature sensor a single T7 can measure up to 13 thermocouples itself .
The power-meting with an ACS712 goes also directly. In the picture below it’s hooked up to AIN0 with a reading of 2.526106 whit no current flowing and 5V power-supply.
It comes with a extensive library with functions to integrate in different programming environments like Python C++ etc.
After a couple of failed attempts to make a reliable power-meter to measure the electricity that is consumed by the infrared heaters I backtracked and went back to a earlier design. With an Olimexino_stm32 Maple clone board and an ACS712 the last piece of the computer-control is operational.
The ACS712 chip 20A has a linear curve of 100 Mv per ampere between -20 till 20 ampere from 0,5 volt till 4,5 volt by a supply-voltage of 5 volt
The Olimexino_stm32 works only with 3.3 volts. With a resistor divider (potentiometer between 5V and ground trim the output till 3.3V and you have the correct divider)
on the output of the ACS712 is fed into the ADC of the Olimexino-stm32, as long there is no current flowing the division is nearly linear so take at least 4,7K as value.
The Olimexino stm32 has a 12 bit DAC digital analog converter. 2^12 is 4096 the zero-crossing is at 2048
If the measured value is less than 2048 the relevant voltage = 2048 min the measured value (example 2048 – 490 = 1558)
If the measured value is more than 2048 the relevant voltage = the measured value min 2048 ( example 3567 – 2048 = 1519)
The result is only positive values. 3.3 Volts divided by 4096 = 0.00080566 volt per bit step. Multiplication of the measured value with 0.00080566 gives the voltage of the output from the ACS712 every 1 mV = 0.001V = 0.01 amp. So if we measure as value of 1346.5 Millivolt = 1.3465 Volt it’s 13,465 ampere (volts times 10 or millivolts divided by 100)
According to the Law of Ohm P=I*V The voltage of the net in Europe is 230Volt when we combine the previous step volts times 10 with this one we get 23 as multiplication factor
By preforming many measurements in rapid succession(1000)and store every measurement we get a clear picture of the real power that is being used.
after 1000 measurements we calculate the mean value and look is there is any interest in the value. If not we start a new cycle.
The last bits are coming together, piece by piece.
Not being able to extent the meter long lead from the thermocouples I used a plastic food-container as cheap box to mount the TC boards on the backside of the testfurnace.
Last step in the software development get a graphic user interface. The furnace has a micro-controller that preforms the measurements and controls the heaters. The micro controller is connected to the mirco-computer that gives the commands and logs the operation.
After trail and error it seems the furnace will finally get a real KWh meter. From Maxim we got two 78M6613 IC’s for free and Faber Electronics mounted them on the Adafruit breakout boards. It’s a real RMS single phase wattmeter where with a single command you get the used watt’s, precisely!
After the big Arduino one board and the mess that gave in the connection to the UEXT connector on the Olimex boards I ordered a Olimexino-nano
The calcium silicate isolation, before the final assembly start it’s wise to check if everything is made according to plan. Two pieces are the wrong size. Luckily we have got extra material to experiment with and we can easily correct the problem. The CS board is Promatec 1000L bought by Moors Ovenbouw. http://www.moorsovenbouw.nl/
It surprising difficult to find valves that are capable of handling low pressures by high temperatures. In order to pyrolyse material we have to prevent air to enter the pyrolysiscell. The pyrolysiscell has to be airtight but during the pyrolysis process volatile gases are formed, hydrogen and carbon based gasses like evaporating fat. In the first phase when we plan to dehydrate the corpse we must remove the water-vapor and bring fresh air in the cell. During the pyrolysis phase the produced gases are led to an afterburner that burns the hydrogen to water and volatile carbon gasses to CO2. All in all we need three valves that operate temperatures up to 500C and under 2 Bar of pressure.
A central heating valve demolished, a steel ball from a ball bearing as flow director and another problem is solved.
New choice for the embedded computer, the A20 from Olimex. The choice for their thermocouple boards made the switch from the Raspberry Pi as control computer easy. Now we have most of the electronics from one supplier.
How to measure reliable the amount of electricity that’s being used by the demonstration pyrolysis cell? With a couple of KW infra-red heaters not so simple especially when you want to integrate the measurements in the computerlog of the test. To test the concept I took an old transformer, replaced the old secondary winding by two windings of the 230 volt wire. Curious if I get a good potential difference on the old primary winding. The primary winding is been shorted with a 20 Ohm resistor. And the voltage over the resistor can be converted in a digital value and fed into the control computer.
Care must be taken that the secondary of a current transformer is not disconnected from its load while current is flowing in the primary, as the transformer secondary will attempt to continue driving current across the effectively infinite impedance up to its core saturation voltage. This may produce a high voltage across the open secondary into the range of several kilovolts, causing arcing, compromising operator and equipment safety, or permanently affect the accuracy of the transformer. source wikipedia
The practice is more complicated due to the load so the real RMS need to be calculated. A acs712 chip connected to an Arduino measures every cycle during one second the actual wattage that is being consumed.