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Temperature Probe Testing

Discussion in 'Project Construction Technologies' started by enli, Nov 10, 2021.

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  1. enli

    enli

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    Jan 14, 2016
    Twice a year I test temperature probes on kilns for heat treating wood pallets. Would like to build a small, portable, constant temperature, water bath unit that can maintain a temperature of 140°F AND 170°F. The probes have set points of 130° or 160° and these are minimum test temperatures. Can fill water bath unit with hot tap water probably around 110° prior to turning unit on. Need to maintain test temperatures for at least 30 minutes (for testing 3 or 4 probes at each site).

    Need to explore the possibility of a battery powered unit and, if that is not plausible, a unit utilizing 120V AC. Have looked at a couple of web sites on induction heaters but those temperatures are way too high for this application. Thinking the water bath would be a 12 to 20oz container, but this is just one of the variables that can be changed for maintaining accurate temperatures. Is this a plausible project? Are there similar projects you can refer me to? How to get started?

    Thanks in advance for any advice you care to share,

    Doug
     
  2. Nanren888

    Nanren888

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    Nov 8, 2015
    Out of interest, what would you use to measure/check the bath temperature?
    .
    Quite plausible. Should be able to find such on the market. What accuracy do you need? What traceability do you need?
     
  3. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    Depending on the size and number of temperature probes you need to test at one time, a large glass beaker and a temperature-controlled hot-plate with a motorized magnetic stirrer should do the job. Maybe use two of these rigs, so you don't have to wait for the water to warm to two different temperatures. "Play" with the stirrer speed until all the probes reach the same temperature.
     
  4. crutschow

    crutschow

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    May 7, 2021
    Don't know if it would work or be accurate enough but you might look at an electric sous vide machine used to heat water in a pot to a controlled temperature for cooking.
     
  5. enli

    enli

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    Jan 14, 2016
    Thanks for the quick answers to my questions!
    nanren888: Currently using a digital scientific thermometer for reading water temperature and probes need to test within 4° of thermometer reading to pass inspection. Not sure what traceability aspect you are asking about, but form for testing is filled out onsite and signed off by kiln operator at each location.
    hevans1944: Looked at these and seems it would require a lot of adjustment (tinkering) to maintain a certain temperature for each setup and portability of all pieces might be an issue. Ambient temperatures will vary from around 32°F to 95°F so each setup might require close monitoring (adjusting) to get desired results.
    crutschow: Had never heard of electric sous vide machine until now, but sure looks promising. Could rework an insulated (gallon?) container to accept machine, probes and thermometer, plug it in and let it reach desired temperature then check probe readings. Now do I spend $50 or $200 for a machine? Could also use at home to explore sous vide cooking techniques so longevity may be the most important feature of the machine.

    Post Update Info: Kilns vary in size from 40’x40’ to 50’x 100’ and can usually test two probes at same time, but still requires moving testing apparatus at least once in each kiln sometimes having to move twice in one kiln, thus a battery operated setup would be the most desirable (not having to find 110 outlets and carrying an extension cord around). Once test equipment is at desired temperature I have to walk to control room were computer is showing probe readings and record findings.
     
  6. crutschow

    crutschow

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    May 7, 2021
    Water takes a lot of energy to heat, so a battery powered system would likely require a healthy sized battery heating a minimum amount of water in an insulated container.

    For example, it takes 55 watt-hours to heat one quart of water from room temperature to 170°F.
    This would require 10A from a 12V battery for nearly 1/2 hour.
    Alternately you could heat the water from the mains and then maintain the temperature with a battery.

    What do you think would be the minimum amount of water for your test?
     
    hevans1944 likes this.
  7. enli

    enli

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    Jan 14, 2016
    crutschow: Currently use a 20oz insulated plastic container that's filled with hot (110°±) water and adjusted with boiling water to desired test temperatures. So could do same with this setup and all it would have to do is maintain the temperature of test water.for about 30 minutes. Heat loss in current setup is substantial (10° to 15°) over 30 minute span, so have to keep adjusting test water with hotter water to get to desired levels. So what size battery unit would be required for maintaining 40oz of water at 170° for 30 minutes if heat loss is 15°? The size of water container is variable but probably no smaller than 20oz. Need a battery powered sous vide machine!
     
  8. crutschow

    crutschow

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    May 7, 2021
    My calculations show that it would take 10WH to keep the water at a steady temperature, so the battery would have to supply 10WH of energy over a 30 minute period.
    So a 12V battery would need to supply 20W or about 1.7A continuously.
    The size battery will be determined by how long you need to keep the water at temperature.

    A better insulated container should reduce the amount of needed power.

    Something like this may work for you as a temperature controlled heater.
     
  9. enli

    enli

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    Jan 14, 2016
    Crutschow: Think this will work? Own a portable power supply (Black & Decker) with the following ratings:
    power pack.JPG

    Buy smallest wattage sous vide (700W) build insulated gallon paint can, fill with 110° water, then let it get to desired temperature and take probe readings. Can dose water bath to get close to desired test temperature if you think it would take too long for 500w supply to heat it up with sous vide device. What would happen if I use a 1000W device would the 500W supply still heat up or maintain the water temperature? Thanks Doug
     
  10. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    @crutschow: I like your reasoned responses in this thread. Clearly, @enli does need some sort of battery-powered heater to maintain a constant "calibration" water temperature of either 140°F ±4°F or 170°F ±4°F, although not both temperatures at the same time. You could use two water baths, with one maintained at 140°F and the other at 170°F, to save time. I would seriously consider this alternative. However, there must be some way to avoid an excessive loss of water temperature, without expending an excessive amount of battery power, while moving the calibration rig from kiln to kiln. The OP, @enli hasn't said how much "time in the field" is expended to move the calibration rig from kiln-to-kiln and to record the results, although the following statement provides a "clue":

    This may be inclusive of the time needed to move from the calibration rig, located at a particular kiln, to the "control room" where presumably the permanently-installed kiln thermocouples are read and their value recorded. Then whoever is certifying the calibration moves the calibration rig to the next kiln, makes sure the water baths are at the correct temperature and then returns to the "control room" to record and document the calibration results.

    This sounds like a two-person job for a team equipped with walkie-talkies. One person stays in the control room to record readings, the other stays in the field to pull and insert kiln thermocouples into the calibration rig. Sure, one person could do both jobs, running back and forth as necessary to beat the thirty-minute "cool down" period of the calibration rig, but it would be better to have the calibration rig not cool down at all between individual kiln calibrations. Hence the need for a heater and some good thermal insulation.

    The two main ways to minimize heat loss, and hence temperature degradation, are either to use a massive amount of thermal mass (larger water bath), or better thermal insulation, or both.

    All of this is measurable, and to some extent controllable, especially on a small scale. The science of it is called thermodynamics. And as @(*steve*) says in his tag line: Thermodynamics... it's not just a good idea, it's the Law!

    Having had some experience with temperature control, I must say that I would not suggest increasing the mass (volume) of water for this type of calibration measurement. Water has a large thermal heat coefficient (4.184 J/g °C), meaning it takes a relatively large amount of energy to change its temperature, so it is better to heat a smaller volume of water (quickly) and to increase the thermal insulation to minimize heat loss. Even so, a good PID (Proportional, Integral, Derivative) control algorithm is necessary to heat the water quickly to the desired temperature with zero overshoot, i.e. critical dampening of the temperature control algorithm.

    Why critical dampening? Because your (hopefully) excellent thermal insulation will slow down any recovery from overshooting the calibration temperature. This is a typical problem in trying to control the heated temperature of a well-insulated environment: the environment heats up a lot quicker than it cools down. If you have lots of power available for heating the water, you could get a faster response by increasing the heat losses, for example with less thermal insulation or by means of a thermoelectric (Peltier device) cooler that can either extract or provide heat. I wouldn't go there with a portable battery-powered water bath, unless you don't mind pulling around a large cart loaded with batteries. Peltier coolers are not very efficient.

    My "gut" feeling (after looking at a 20 oz Gatorade bottle) is 12 oz looks about right but 20 oz is waaay oversized for @enli's application. Your really need a vacuum-insulated container, such as this over-priced 12 oz Starbucks vacuum insulated coffee tumbler: Buy two of them so you can have both temperature references handy.

    From here on it gets a lot more complicated, depending on how accurate your results must be. You stated that the "gold standard" for your calibration test was a digital scientific thermometer, which presumably would be inserted in the water bath to verify its temperature, and perhaps to actually control the water bath temperature by means of a negative feedback control of an electrical resistance heater immersed in the water bath. Sounds like it's getting a little crowded inside the water bath chamber!

    Each of the thermocouples you remove from the kiln has a thermal mass and a specific heat capacity that will literally begin to suck heat out of your water bath as soon as the probe is inserted. it is up to your immersion heater to resolve this problem by adding back the amount of heat that was initially lost because of probe insertion. Unfortunately this is NOT a simple problem with a simple solution. Still water is an effective insulator. Just because the heater is on does not mean the entire volume of water will be at the same temperature.

    There are two ways to accomplish a homogeneous water temperature profile: time and stirring. With time, any quantity of heat, added at a point to a volume of anything, will eventually disperse evenly throughout the volume, provided there are no thermal losses or gains at the boundaries. This process can take hours, or even weeks, with good thermal insulation. To speed up the temperature homogenization process, the next step is to stir the liquid. This is very effective. Even more effective is to circulate the liquid past or through an electric immersion heater.

    So there you have it: the outline of a design for your kiln thermocouple calibration rig. If you decide to go this route, I recommend using a peristaltic external aquarium-type pump to circulate water bath water around an immersion heater. This does not have to be a commercial immersion heater like the kind sold for preparing a cup of tea or instant coffee. Any resistor of about ten watts dissipation capacity with suitable resistance will work. Use silcone sealer to coat and insulate the under-water soldered wire connections.

    I would also consider designing my own microprocesser-based heater control to minimize the "warm up' time of the water bath. Try to obtain a stead-state electrical consumption of around ten to twenty watts during warm-up and perhaps a tenth that much after equilibrium temperature has been reached. These are just rectal extractions from my experienced arse, so it would pay to see if they can be made practical for you. I would start by inserting thermocouples inside my vacuum-insulated container, adding hot water, and recording the temperature drop as a function of time, say every minute for at least thirty minutes. Record either the weight or volume of water used. After sealing the container, shake it once every five minutes to evenly distribute the heat losses. This data will give you some idea of how much energy you need to add to the water to maintain the set-point temperature for thirty minutes.

    Always remember to cap and insulate the mouth of your vaccuum-insulated container to avoid excessive heat loss by this mechanism.

    And if you want some really good professional information and advice on all things related to temperature measurement, control, and calibration, my advice is to seek help from Omega Engineering Inc. I have used this company for more than fifty years and trust their advice and products. As mentioned before, it helps to know the rudiments of thermodynamic engineering before attempting to engage them in conversation, but their online catalog is a big help in that area too.
     
  11. crutschow

    crutschow

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    36
    May 7, 2021
    Hard to say.
    I see no mention of how long it can provide power.

    I suggest a wide mount vacuum, bottle such as this, to store the water.
    It should have a minimum heat loss to ambient.

    You can heat it to temperature from the mains, and then use a battery powered heater to maintain the temperature.
     
    Last edited: Nov 19, 2021
    hevans1944 likes this.
  12. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    Good point! Eliminate the mess of trying to add boiling water to already heated tap water, while at the same time trying to maintain an even temperature distribution while maneuvering the "digital scientific thermometer" to obtain a stable reading.

    I mentioned using a resistance heater rated for about 10 watts, but this is typically the rating for use in still air. When surrounded by water, which is never allowed to be heated to boiling temperature or 212 °F, the wattage that can be dissipated is much larger. So whether you choose to use a commercial immersion heater, or simply a fixed power resistor, you are not likely to "burn up" your resistance heater while heating the water from ambient to an operating temperature of 140 °F or 170 °F using 120/240 vac mains power. Once the operating temperature is reached (as verified by the "digital scientific thermometer"), you can switch over to battery power. The actual power you need during the "warm up" period will depend on how fast you want to heat the water from ambient to operating temperature. A convenient "rule-of-thumb" is to use about ten times as much power for initial heating as is required to maintain the container(s) at their final operating temperature. This amount of power will depend on how much water is heated, as well as the rate at which the heated water loses heat energy through the highly effective vacuum-insulated walls of the container.

    My wife and I were shopping at T.J. Maxx discount store earlier last week where I saw some heated coffee mugs... from China, of course. I didn't examine these very closely, so I don't know what sort of "reverse engineering" would be required to get a pair of them to work as temperature-controlled water baths. I don't think the heated mugs were vacuum-insulated, since these cups were made to operate from mains voltage and did not have an adjustable temperature. Might be something for @enli to look into before spending big bux on name-brand vacuum-insulated containers. BTW, it has been my sad experience that these imported vacuum-insulated bottles lose their "hard" vacuum sooner rather than later, so caveat emptor on that.
     
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