[Stoves] 'BBM' and the accuracy of RTD's (Way off topic)

Frank Shields franke at cruzio.com
Sun Jun 7 12:01:23 CDT 2015


Dear Crispin, Stovers,

<snip>

> 
> I am in Yogyakarta helping the YDD lab recover from the devastating fire they had last year. Today we were calibrating the thermocouples and testing RTD's - Resistance Temperature Detectors. They are installed on the heat exchanger that sits inside the pot on the stove. 
> 
> This exchanger allows the fire to be operated on any desired cycle and the water heated in the pot‎ never boils. By monitoring pot and the inlet and outlet temperatures the heat gained can be calculated.  This means the heat gained at high and low power can be determined accurately in real time. It obviates the need to swap pots (like the Indian Protocol and others) and avoids boiling which complicates real time mass of fuel measurements. 

Seems to have several advantages. One being continue mixing of the pot water  as the water is returned, no need to decide on a simmer temperature and a boiling temperature (based on elevation) and then needing a true accurate temperature reading that can be hard to get. All it depends is a change in temperature from leaving the pot and entering the pot and flow rate of water - if I understand the process. Do you have pictures or can explain in more detail the process?
> 
> The test today was to see what the drift of K band thermocouples was compared with four wire RTD's. ‎The K's wander up and down about 0.4 degrees while the RTD's are dead constant. Offsets are entered to get them to agree as best we can. The RTD's are within a range of about 0.004 degrees and the K's wander around the correct number as much as 0.6 total range, sometimes more. Different manufacturers have different 'wanders'. The offset for some is 5 degrees. Generally K's report high. 
> 
Using K thermocouples? I would think J would be more to measuring the range around boiling. 


> We are reading them on an Agilent 34972A with a 34901 Temperature armature with lots ‎of channels. The RTD with four wires occupies two channels. One is a reference and the other reads the temperature. The Agilent can read 0.01 degrees and does a pretty good job at 0.003. It can discriminate 0.6 microvolt changes. 
> 
> Using a K band thermocouple, what is the error on any single measurement of coolant? That is the money number. 
> 
> Assume that a single thermocouple in a pot can tell us the bulk temperature (which is definitely not correct) and that the flow through the heat exchanger is 35 cc per second. There is five litres of water in the pot. 
> 
The flow is constant? And the temperature is kept the same in the pot? What am I missing? Its the same 5 liters of water just being circulated?  The only thing left is the change in the energy required to keep the cooling water / heating water in the heat exchanger to maintain the temperature of choice. 

I need a picture.

Thanks

Frank

  




> Assume the heat exchanger takes all heat gained away and the pot water temp is constant. The error on the measurement is 0.25 degrees plus or minus even though it is constant. The error on the heat exchanger is the same, and the delta T is 3.4 degrees, constantly. 
> 
> So the error in the pot is 5000x4.186x0.25 Joules from the true central value. The error over ten seconds with the heat exchanger is 35x10x4.186x0.25 Joules if one is spot on and the other is not. (We are not calculating the heat transfer, just the error.)‎. The sum of the two calculations is the total error, assuming one was correct, and we are only looking for the difference, not an absolute temperature. Assume we know the flow rate perfectly. 
> 
> The total drift is about 5600 Joules in ten seconds or 560 Watts! Using RTDs cuts this to 22 Watts (but still assumes the pot temperature is known - we do what we can.)
> 
> In our favour, we low tech testers, is that the error over a longer period is the same, if the thermocouples are linear and correct. After an hour the error has a very different makeup. The pot error remains the first number: plus-minus 5270 Joules. ‎The heat exchanger however now dominates. The total flow is 35x3600 cc = 126 litres. An error of 0.25 deg is possible for both readings. Might be one up and one down. The error might be as large as:
> 
> 126,000x4.186x0.25x2= 263,718 Joules which is about the energy of 16 g of wood. Not much, except this is pot heat not fire heat. At 33% heat transfer efficiency it is equal to 48 g of fuel. That's a big error if you are only burning 4-500 g. 
> 
> Fortunately it is only an error of 73 Watts, right? But what if the stove is only putting out 1500 W and the heat transfer efficiency is ‎33%. 73 is 14.6% of 500 (14.6% of value). Hmmm... so is the heat transfer efficiency 33% or 37.8% or 28.2%?? Wow!
> 
>  Remember my 0.25 error was modest. It can be double that. The max error based on what we measured today would give a range is 23.4% to 42.6% heat transfer efficiency just for the thermocouple errors. 
> 
> Using RTDs reduces the potential error to 0.2% in this example. Much better. ‎For short tests it would be better to have an array of them in the pot too!
> 
> ‎If anyone is interested in this and other fascinating testing topics come to the Ontario Stove Testing Camp, 9-10 July at Burt's Greenhouses near Kingston. $100 little Canadian dollars. Cheap! Includes fuel. 
> 
> Show, share, learn and spread the lessons around. We will call it Burt's Greenhouse effect. That's a catchy name. It heats up the discussion. 
> 
> Regards 
> Crispin in Central Java
> 
> 
> [Default] On Sat, 6 Jun 2015 19:07:19 +0700,Crispin Pemberton-Pigott
> <crispinpigott at outlook.com <mailto:crispinpigott at outlook.com>> wrote:
> 
> >
> >
> >BBM 'Crispin'
> 
> BBM?
> 
> AJH
> 
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