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Temp RH Discrepancy

08/01/2017 12:15 PM

i have a climatic chamber which has no cooling system, temp is generated by air heaters and RH by boiler. They r controlled independently. The control system is PLC based auto PID. Conditions 65 deg cel and 80%RH.

yesterday there was a sudden drop in temp from 65 deg to 41 deg (data logging happens every 10 Sec) but no change in RH except normal variation between +/- 5%. after 10-15 mins temp came back to 65 deg. During the whole time RH didn't show any drastic change.

questions:

1. Why there was a sudden drop in temp?I m cancelling power surge here as I get stabilised and uninterrupted power.

2. Why did the RH not show any drastic change in spite of 20 deg change in temp?

explanations are eagerly awaited.:)

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#1

Re: Temp RH discrepancy

08/01/2017 12:26 PM

You're asking us to troubleshoot an unknown system from a remote location....but you did say the temp and humidity are independently controlled....

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Associate

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#2

Re: Temp RH discrepancy

08/01/2017 12:47 PM

Someone opened the chamber door, got a text message, had to reply immediately and left the door open all the while.

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#3
In reply to #2

Re: Temp RH discrepancy

08/01/2017 2:02 PM

That would be perfectly in sync with today's world. The all-important cell phone takes pre-eminence over everything else.

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#4

Re: Temp RH discrepancy

08/01/2017 2:52 PM

With that big of temperature change and no measurable humidity change, I would suspect measurement error. Maybe the temperature logger had a glitch. With temperature changing and a constant amount of moisture in the air the humidity would certainly have changed. Or maybe the temperature changed faster than the humidity sensor could respond.

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#6
In reply to #4

Re: Temp RH discrepancy

08/01/2017 4:42 PM

that would be my guess, as the humidity should attempt to go with everything else constant (semi-closed system), and temperature falling.

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#5

Re: Temp RH discrepancy

08/01/2017 3:26 PM

How quickly did the temperature drop?

Is this rate of change possible without opening the door?

Try replicating the situation by cutting power to the heaters, and, monitoring temperature and humidity.

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#7

Re: Temp RH discrepancy

08/01/2017 5:40 PM

If, as you said, "They r controlled independently" why would you expect the RH to change?

They [a] r [e] controlled independently.

Why the temperature changed is your problem to solve.

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#8

Re: Temp RH discrepancy

08/01/2017 7:10 PM

Maybe a bad temp sensor or bad connection? A 24 deg C drop in 10 seconds seems a bit extreme. Did the temp come up suddenly or creep back?

The RH and temperature control loops are not truly separate. If you change the temperature, you change the RH. (That's where clouds come from.) The fact that RH did not change, to me, points to a temperature sensor error.

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#9

Re: Temp RH discrepancy

08/01/2017 10:07 PM

What is the ambient temp and humidity just outside you climatic chamber?

Knowing this could give you strong indication of instrument error or open door causes.

There is a pretty narrow range of RH and temp combinatioms external to your climatic chamber for which an open door could result in your data without instrument error.

As an example, if the ambient relative humidity external to your climatic chamber was >=80% and the temp was 28 C, it would be very difficult to get the readings you describe without a measurement error.

.

The previous assumes RH and temp are measured at close to the same location. With distance between the measurements the readings could occur from a small openning in the chamber near the temperature measurement but remote from the RH measurement without measurement error.

.

Can you explain how you are controlling RH in more detail?

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#10

Re: Temp RH discrepancy

08/01/2017 10:13 PM

In order to fully understand the issue and what to do, you MUST provide us here the same data that you have seen!

There are many vital details that are absent from your post!

Plot the data in an Excel chart and take a screenshot of the chart.

Before you do that, make sure that you label the axes and then, when you use the Snipping Tool, include them in the breadth and width of the Snipping Tool selection.

In separate snips, show the charts for:

  • Timestamp vs Chamber RH
  • Timestamp vs Chamber Temperature
  • Timestamp vs Air Heater State (On or Off)

and

  • Timestamp vs Boiler State... and here we have another thing we need to know: EXACTLY How does the "boiler" provide moisture to the chamber? IN FACT, USING A BOILER TO CONTROL RH SEEMS LIKE A BAD IDEA. Maybe that's your whole problem

Providing these details may seem arduous to you, but believe me, they are necessary.

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Active Contributor

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#11

Re: Temp RH discrepancy

08/02/2017 2:05 AM

You are asking the same questions as me.

some pointers though

1) no one opened the chamber

2) The sensor was working fine when checked last, its a normal PT-100 and has been calibrated.

3) there is a very remote chance that a water droplet/condensation may have fallen on the sensor.

Question:

Is point three a valid argument?

I am attaching the zoomed pic. Please note that the complete protocol takes 10 days and this was the only glitch on 5th day, before things became smooth again.

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#12
In reply to #11

Re: Temp RH discrepancy

08/02/2017 3:22 AM

Yes, as the chance is not identically equal to zero.

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#16
In reply to #11

Re: Temp RH discrepancy

08/02/2017 12:04 PM

If the water shorted out the leads on the sensor either at one end of the other of cabling, then yes, it would lead to large negative systematic error, problem solved.

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#13

Re: Temp RH discrepancy

08/02/2017 4:41 AM

When a temp sensor is wet, it is a 'wet bulb', not a dry bulb; half of a wet bulb/dry bulb RH sensor.

A dry bulb that becomes wetted continues to report temperature, but it is wet bulb temperature, not dry bulb temperature.

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#14

Re: Temp RH Discrepancy

08/02/2017 11:29 AM

If you are reporting temperatures in centigrade, the dew point is 60 deg C for 65 degree air at 80% humidity. So a drip on the temps sensor should have gone to 60 deg C. The gradual recovery indicates that it was not likely a loose connection or electrical glitch.

Under the conditions described, you should have seen a great deal of condensation forming in the chamber and you should have seen the RH spike to 100%, since the boiler adds moisture to the air, but does not subtract it.

This depends on how the RH sensor actually measures the air moisture content.

I think there is more to the story.

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#15
In reply to #14

Re: Temp RH Discrepancy

08/02/2017 12:03 PM

Good answer.

The humidity control does seem problematic.

The chamber is probably poorly insulated, at least in a location such that subcooled condensate could build up then drop or pool onto the sensor. Given the step change in temperature followed by the gradual recovery, if the chamber was indeed not opened, movement of a mass of subcooled condensate onto the sensor, as an explanation is at least consistent with the changes.

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#17
In reply to #15

Re: Temp RH Discrepancy

08/02/2017 12:10 PM

Sensor, or sensor connections? IF connections, then he has the identical problem observed, most likely.

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#18
In reply to #17

Re: Temp RH Discrepancy

08/02/2017 12:57 PM

I tend to agree with Jpfalt on the connections. Seems improbably that a connection fault would gradually recover.

I am open to knowledge upgrades. Have you experienced connection faults that have gradually recovered?

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#20
In reply to #18

Re: Temp RH Discrepancy

08/02/2017 1:12 PM

Not so much, but I seem to recall at least once of having a Pt100 drop out due to wet connection, enough the indicated temperature dropped all the way to default value of 25 °C (more like sensor disconnect than a short).

I would expect sensor input shorted would show something way out of sync with reality, much lower than ambient.

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#19
In reply to #14

Re: Temp RH Discrepancy

08/02/2017 1:05 PM

Thanks for the reply. I was considering the drop as wet bulb temperature and not dew point. The RH sensor is rotronic capacitive type. RH is generated only by steam and a PLC controlled solenoid valve.

My question is, shouldn't the RH have shown a sharp drop once the temp started increasing?

I checked my PT-100 after the test, it was perfectly fine and giving correct resistance values.

The abnormality has created a lot of trouble in subsequent activities. Hence the persistence.

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#21
In reply to #19

Re: Temp RH Discrepancy

08/02/2017 1:16 PM

The system RH response should be thus: decrease--> time constant for valve to open --> transfer function of steam into chamber --> response time of capacitive RH sensor instrument --> time constant for valve to close. If that cycle time is faster than the system changes in RH, then all is good, and you have a "tight" control loop. If not, then RH can "be all over the place".

Temperature sensor drop out with RH steady probably means that your system did not change all that much, but that the indicated values were misleading.

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