Air condition, precision measurements and temperature oscillations  - Page 1
Forum section: electronics / general

author: aronake
author_rank: Frequent Contributor
timestamp: March 11, 2025, 08:40:26 am
content: For you living in hot climate how do you deal with air condition and temperature oscillations they usually produce when doing precision measurements?When I use AC I see swings of typically around 2 degrees with a frequency that can vary quite a bit, from 10 minutes to 1 hour.I have done some quite some experimentation, like lower and higher fan speed on the AC, using fans to circulate the air, get the temperature sensor out of the AC and connect it to wires and keep the sensor closer to where i have my equipment.I have seen various results but still above 1 degree swings in temperature which is very observable on measurements.I guess getting inverter air conditioner would solve the problem, but its a rented place I live in, so not an option.Anyone have any solutions you have applied? What results did you achieve?I have ideas like making an arduino based temperature meter that control the AC with IR, or a electrically variable resistor on the sensor wires to more frequently turn AC cooling on and off.I think if on and off can be done every 5th minute I would get stable temperature enough. Could too frequent on and off of AC damage the AC? Say down to 1 minute minimum between on and off and off and on? I kind of got indications that it may be the case, but no real proof.

author: ap
author_rank: Frequent Contributor
timestamp: March 11, 2025, 09:19:31 am
content: One thing you could try (and may not be feasible easily, but worth giving it a try, depends on your setup and so) is to keep your sensitive equipment in a tower or the like that isolates the gear from the oscillating ambient temperature.

author: EC8010
author_rank: Frequent Contributor
timestamp: March 11, 2025, 10:16:56 am
content: I measured temperature in our environmental chamber and found a 1C saw tooth, and I would expect the same for air conditioning. It's the hysteresis built into the system. Ideally, you would like a proportional control rather than bang-bang (on or off). You can do that with a heater, but since cooling is done by pumping refrigerant through a nozzle to vapourise it, I suspect you can't proportionately control that and still achieve cooling. You could probably achieve smaller variation by simultaneously heating and cooling, allowing the cooling to be bang-bang but using proportional control on the heating to smooth out the variations. But you would still have the thermal inertia of the heating to contend with. And simultaneous heating and cooling would be extremely wasteful of energy. But I know it's done where tight temperature control is needed.In the end, putting your sensitive stuff in a tower (as suggested) to isolate it from local changes is likely to be the cheapest and quickest solution.

author: Grandchuck
author_rank: Super Contributor
timestamp: March 11, 2025, 11:53:33 am
content: Ian Johnston has shared an outstanding software tool (WinGPIB) that supports data collection.https://www.eevblog.com/forum/metrology/3458a-logging-via-windows-app-revisited/

author: aronake
author_rank: Frequent Contributor
timestamp: March 11, 2025, 03:07:37 pm
content: Quote from: EC8010 on March 11, 2025, 10:16:56 amMaybe use crypto mining for the heating to recover some of the heating and AC costInverter based AC units seems to work proportionally, rather than just on and off, so that would probably solve much of the problem. Not an option though, but for anyone putting a new AC unit where they do measurements this is probably a good idea. I dont know how well they in reality are at keeping temperatures stable.

author: Addicted2AnalogTek
author_rank: Frequent Contributor
timestamp: March 11, 2025, 03:17:37 pm
content: Quote from: aronake on March 11, 2025, 08:40:26 amHVACR Technician here. What you are describing with frequent on-off cycles is called "short cycling" and it will definitely damage your HVAC equipment, specifically the compressor.Your thermostat is also designed to prevent such short cycling. In more advanced units, you can access what will be titled "Differential Temperature Setting", which is the number of degrees below and above your desired temperature set-point that your equipment will turn on and off at. There will also be settings for "minimum compressor off time" and "minimum blower on time", also for protecting the HVAC equipment.That said, I have no experience trying to keep temperature deviation to a minimum in a room and I have no data center HVACR experience, either. I suspect you will need to isolate that room from the rest of the HVAC system and install a much more precise, purpose-designed unit for your lab.

author: Simmed
author_rank: Frequent Contributor
timestamp: March 11, 2025, 04:05:11 pm
content: https://www.eevblog.com/forum/metrology/diy-large-thermal-chamber-for-metrology-tm-testing/to further clarifyi would attempt to use the least amt of powerto produce the most stable "sandbox" temperatureto contain what is needed to use (1 DMM ? 2 DMM ?)

author: Kleinstein
author_rank: Super Contributor
timestamp: March 11, 2025, 07:08:10 pm
content: I have used a sensitive amplifier inside a food cooler. Initially even with ice water to stabilize at 0 C, but later just the thermal delay was good enough and the experiment was only about 1 hour at a time anyway.Some extra cover over the critical section / meters can already dampen the speed of the thermal response quite a bit. One still has to balance with the temperature rise inside. If the power consumption is low and things don't mind a slightly higher temerature this can be an easy way.The point is getting the thermal response significant slower than the AC cycle.If needed one could add a temperature controlled fan to the box. So regulate with the fan and power consumption of the experiment.

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: March 12, 2025, 12:53:31 am
content: Hello aronake,Modern AC equipement use inverter (=VFD) andEEV(an electronic metering device) for fine adjusting the instaneous amount of cooling. But the main reason of this sophistication is minimizing power consumption.What you actually need is is very precise regulation as for a cal lab orclean room. A regular AC appliance is not designed for that purpose.The suggestions made by Simmed and Kleinstein to regulate the temperature in a medium sized well isolated box or chamber is probably the best path to follow.

author: aronake
author_rank: Frequent Contributor
timestamp: March 12, 2025, 05:09:01 am
content: Quote from: Addicted2AnalogTek on March 11, 2025, 03:17:37 pmGreat! someone who actually know something about ACs and not only internet rumors!Would you have any guestimate on shortest on to off, and off to on cycles for a typical home split AC, before this become too much of a problem?

author: aronake
author_rank: Frequent Contributor
timestamp: March 12, 2025, 05:21:58 am
content: Thanks for input on TEC boxes.I use these for more critical measurements to put the DUT in. And to do temp sweeps.Two full height racks and desk full of multimeters, calibrators and other measurement equipment is the bigger problem. Temperature does not have to be exactly stable, but 2 degrees swing with maybe 20 min period gets a bit too much. Sure, could get a temperature controlled rack... but thats also quite impractical.Looks like some hack on the AC may be a way forward. And not turn it on and off or adjust temperature too frequently on it. I have put a potentiometer on the temperature sensor cable to add some finer control than 1 degree steps. If I turn the potentiometer i can instantly get the compressor to stop or start, but the fan continues to run. So seems the short cycle protection is based on temperature hysteresis rather than time. So something like an arduino with good temp sensor and a variable resistor to replace the AC thermistor, and some suitable not too short on-off time may be a solution.

author: aronake
author_rank: Frequent Contributor
timestamp: March 12, 2025, 05:25:49 am
content: Quote from: timeandfrequency on March 12, 2025, 12:53:31 amMy main problem is the 2 degree around 20 min oscillations. I would think an inverter AC may not have too exact temperature, but at least reduce oscillations quite a bit. Do you have any experience of this? And if needed more precise temperature, that could be done with arduino+temp sensor+electrically controlled potentiometer to replace the ACs own thermistor. So same as i plan to do with my non-inverter AC, but the inverter AC, would have the benefit of providing more of a proportional control rather than just on and off.

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: March 12, 2025, 11:02:56 am
content: Hello aronake,I've no specific experience in precision HVAC design but just having a VFD upstream the compressor is probably not sufficient for your goal.It is the metering device that defines how much cold (or heat, in case of a heat pump) is provided.It is just^^ thermodynamics, so everything depends also on temperature difference, humidity and the speed of the wind through the coils.So trying to replace the existing thermistor with a additional system, as you suggest, will not significantly change the overall control law of the internal HVAC controller. Give it a try if you want, but I'm not conviced that this will lead to a blatant success.Most advanced AC systems combine variable compressor speed, stepper metering device and variable fan speed for both coils (evaporator and condenser), to achieve minimal power consumption and always scrape the highest possible COP even if ambient conditions change significantly.These units are fitted with many sensors : at least for pressure, temperature and humidity.Sure that an AC system with all these bells and whistles can be optimized for temperature precision and constancy, but that means that you throw off the vanilla HVAC controller and replace it with a dedicated device designed(*) for that particular purpose.I would say that it is the reason why climatic chambers that include the 'cold' feature are so expensive.(*) Fuzzy logic control might be a valuable alternative to interwinded PIDs.

author: Addicted2AnalogTek
author_rank: Frequent Contributor
timestamp: March 12, 2025, 01:01:33 pm
content: Quote from: aronake on March 12, 2025, 05:09:01 amWhen I install an HVAC system in a home and set up the thermostat settings, I set the "compressor lockout timer" for 5 minutes. That means no matter what the user changes the set temperature to and when, if the compressor has not been off for 5 minutes, it will not turn back on for 5 full minutes. The blower will still run, but the refrigerant will not move through the system and cool or heat. This allows the pressures on the high side and low side to equalize so that the compressor does not try to start again while the high side refrigerant line is under pressure (R410 will be at about 350-370psi while the system is running). Five minutes is somewhat of a common standard to be sure pressures equalize, and measurement would be required to know the exact time for your specific system.You would be better off to run your air conditioning constantly than you would be to allow your air conditioning to short cycle. Some systems have the compressor lockout timer built into the compressor circuitry. Some systems have pressure transducers that will tell the system when the lines are equalized. Those types cannot be bypassed, only the systems that rely on the thermostat for this setting can be bypassed. Before you change your thermostat to shorten the duration of the compressor lockout timer, I highly suggest you get quotes for a replacement system and/or a replacement compressor. If you gently pull your thermostat off of the mounting plate, you should be able to find a model number and find a .pdf manual for it online. Same with your HVAC equipment - Outside unit should have a data placard on the removable cover or directly inside of it. The inside unit should have a data placard on one side or the other, or inside of the first door that is removable.

author: EC8010
author_rank: Frequent Contributor
timestamp: March 12, 2025, 03:28:59 pm
content: Quote from: Addicted2AnalogTek on March 12, 2025, 01:01:33 pmWhich comes back to what I mentioned about simultaneously heating and cooling. I know that's done at Britain's National Physical Laboratory (NPL), now I know why.

author: tszaboo
author_rank: Super Contributor
timestamp: March 12, 2025, 03:36:57 pm
content: In one of my earlier jobs I was evaluating a voltage reference circuit with a 3458A. I could definitely tell when the last person left the office and turned off the airco. The thermal introduced noise was orders of magnitude higher then otherwise.I also learned to do thermal measurements without external heating or cooling. So take a heated up board for example, place it in a thermally isolated container together with some water bottles and let it slowly reach room temperature.

author: aronake
author_rank: Frequent Contributor
timestamp: March 12, 2025, 06:40:29 pm
content: Quote from: Addicted2AnalogTek on March 12, 2025, 01:01:33 pmThanks! That was some good and useful technical explanations.And thanks for the warnings! The AC itself is not particularly expensive. Kind of entry level split AC. Something like the cost of 5 sealed vishay resistors. But i live on 6th floor, so scaffolding will be required to have it changed, and that would add another 5 resistors or so to the cost. And then around 4 more to have someone install it.

author: TimFox
author_rank: Super Contributor
timestamp: March 12, 2025, 07:25:05 pm
content: Before the Energy Crisis of 1973/1974 (due to an oil embargo), it was common in US to run air conditioning continuously and stabilize temperature by regulating heating (since energy was cheap then).I remember the new library building opening at my university in 1970, where this was done to improve storage conditions for the collection.With the increase in power cost, the contractor spent a long time to switch to a more efficient system, during which time there were serious imbalances in climate throughout the building.

author: aronake
author_rank: Frequent Contributor
timestamp: March 13, 2025, 03:35:02 am
content: Quote from: TimFox on March 12, 2025, 07:25:05 pmFirst cooling the air with AC then heating it is also a way to remove humidity. Cold air can hold less water so it condenses, but it would be too cold to be comfortable so then it is heated again. As a library with books that probably should be kept dry that makes sense. I think this technique is still used when there is a need. Its probably quite a bit fine tuned since the early 1970ies though.

author: CalibrationGuy
author_rank: Unknown
timestamp: March 13, 2025, 04:34:12 pm
content: If your measurements are really affected, by a single degree in temperature change, then your only option is an environmental chamber, like the one that TiN built. To keep an entire space uniform to less than a degree difference would require an impressive air conditioning system, and it would also require a decent airflow, which in and of itself, is disruptive to sensitive measurements. Ebay has some relatively inexpensive heating/cooling chambers, which you can rip the guts out of and use a bigger homemade enclosure like TiN did.TomG.

author: ap
author_rank: Frequent Contributor
timestamp: March 14, 2025, 06:32:59 am
content: Quote from: aronake on March 12, 2025, 06:40:29 pmExcept you won't be able to buy 14 of these as the MOQ has gone up to 25.Or you find them on stock somewhere.

author: aronake
author_rank: Frequent Contributor
timestamp: March 14, 2025, 12:25:53 pm
content: Quote from: CalibrationGuy on March 13, 2025, 04:34:12 pmMy performance objectives are very unprecise. The typically somewhat high frequent 2 degrees swings are clearly showing up to measurements. If i just could get that down a bit that would be a step forward. For instance, i have noticed that slower fan speeds give less temperature swings, which quite make sense. Even partly blocking the airflow with a cardboard box a bit away from the AC give some further improvement. So maybe a mix of less airflow and some smarter control that somewhat more frequent turn cooling on and off would bring something that is good enough. Evenness of temperature in the room also not too critical.

author: Simmed
author_rank: Frequent Contributor
timestamp: March 14, 2025, 01:25:57 pm
content: now i am very sureit is not a smarter control you needit will simply be a waste of timeQuote from: aronake on March 14, 2025, 12:25:53 pm

author: CalibrationGuy
author_rank: Unknown
timestamp: March 14, 2025, 04:57:42 pm
content: I agree. I think that cold air blowing on your instruments is producing a larger, very localized temperature swing. Isolating the instruments from airflow. Will probably be enough to solve your problem.TomG.

author: Dr. Frank
author_rank: Super Contributor
timestamp: March 14, 2025, 06:15:31 pm
content: Quote from: aronake on March 11, 2025, 08:40:26 amHi aronake,varying temperature and air draught might be a problem in a lab, but one needs to know, which kind of measurements (freq., Temp., DCV, ACV, AC/DCI, Ohm, ...) you are doing, and - maybe - having problems. You did not mention, what and how you are doing your measurements, and how your typical setup looks like, and which effects you really see, and to which extent you want to mitigate these.I guess, that there might be better ideas for stabilization of your measurements , than only trying to change something on your HVAC.Frank

author: CalibrationGuy
author_rank: Unknown
timestamp: March 14, 2025, 07:03:02 pm
content: +1 for what Dr. Frank said. Also, we have emi/rf field strength meters around the lab to make sure that other influences are not present. Certain types of circuits are very much influenced by hands and your body being near them. Examples include rf circuits, tc measuring circuits, etc.TomG.

author: aronake
author_rank: Frequent Contributor
timestamp: March 15, 2025, 02:09:09 pm
content: Quote from: Dr. Frank on March 14, 2025, 06:15:31 pmThats some relevant questions! I do the "usual stuff" a using a bank of 3458a to track voltage references (including 6 world famous Dr Frank design LTZ1000), calibrate equipment, build and track resistance standards (resistance on 3458a generally not good enough so typically use current source and ratio toward SR104), Fluke 742a, Fluke 5450), develop some precision electronics (DACs, ADC,s PWM voltage amplifiers, etc) and measure capabilities and stability of these. Typically I keep DUT in a TEC box. The 3458a i have have TC without ACAL of around 0.01, 0.03, 0.08, 0.14 and 0.38. The first three are quite immune toward AC temp swings, the last two its clearly visible. Then I use a DMM7510 for more casual logging measurements, which is clearly sensitive against AC temp swings.So I do not need 23C +/- 0.1 degrees, more just try to reduce the +/- 2 degrees swings a bit in the room, so something that have less impact. So the general pursuit of making things better. During the "winter" here when its been around 20 degrees i have had windows open. Not a great solution, but much slower temperature swings making different kinds of measurements maybe not more accurate but more stable, which in many cases is more important.