Help me make sense of these 2 hygrometers

[Nickster]

I recently did the salt calibration for two, one white and one black. I left them in the bag overnight. The next day, the white one came in at 68° and the black one came out to 73°. However, today as well as every time I’ve had them, the white one always shows a higher humidity than the black one. Shouldn’t it be the opposite?

[abn556]

Watch out Shelia Jackson Lee will start calling you names….

[Bowie]

They aren't accurate across the range, which is why I don't recommend using regular salt. There is another thread on the first page you should read, potassium carbonate is discussed.

[printer2]

The amount of humidity the air can hold is related to the temperature, the higher the temperature, the more humidity it can hold. So if the one reads 73 at 50% RH the other at 68 F will read 60% RH.

A calculator. Set the temp and humidity then (solving for Dew Point) then click on the one you want to solve for, either temp or humidity. Then adjust to the 'new' parameter, the other will then be displayed.

http://www.dpcalc.org/

One of them are out in temperature (duh) and the electronics uses the measured temperature to calculate the % humidity given the temperature reading and the voltage off the humidity sensor.

[Mandobart]

Reposting this from a similar reborn zombie thread yesterday:

Just a reminder from your former USN instrumentation tech that a salt test is NOT a calibration! It's a simple one-point cal check.

Unless you're testing the hygrometer's reading at 5 cardinal test points AND adjusting the response to be within a specific tolerance at these points, you're NOT calibrating anything.

It's just a go/no-go check of what your hygrometer reads at a single relative humidity value. Without knowing the range, span, linearity, etc. you can't know with any certainty if it reads X% off the actual value at more than the one point you tested.

BTW people who perform calibration on instruments used to monitor and control a nuclear power plant don't sweat non-identical readings in a given instrument loop, so long as they are all within their tolerance. And they know about hysteresis - an instrument will read one way on the way up from say 100 F to 500 F, but the response will be different from 500 F back down to 100 F.

So don't get all wadded up over a 2% - 6% difference between two or more hygrometers reading different values even when they're sitting right next to each other.

Most laypeople expect instrumentation to behave exactly linearly, repeatedly, identically and digitally (even when measuring an analog parameter). There are micro spatial and temporal relationships at work. It takes a few years of training and experience to learn this stuff. A little knowledge, mfr hype and OCD misunderstanding can cause you stress you really don't need.

[printer2]

Originally Posted by Mandobart

Reposting this from a similar reborn zombie thread yesterday:

Just a reminder from your former USN instrumentation tech that a salt test is NOT a calibration! It's a simple one-point cal check.

Unless you're testing the hygrometer's reading at 5 cardinal test points AND adjusting the response to be within a specific tolerance at these points, you're NOT calibrating anything.

It's just a go/no-go check of what your hygrometer reads at a single relative humidity value. Without knowing the range, span, linearity, etc. you can't know with any certainty if it reads X% off the actual value at more than the one point you tested.

BTW people who perform calibration on instruments used to monitor and control a nuclear power plant don't sweat non-identical readings in a given instrument loop, so long as they are all within their tolerance. And they know about hysteresis - an instrument will read one way on the way up from say 100 F to 500 F, but the response will be different from 500 F back down to 100 F.

So don't get all wadded up over a 2% - 6% difference between two or more hygrometers reading different values even when they're sitting right next to each other.

Most laypeople expect instrumentation to behave exactly linearly, repeatedly, identically and digitally (even when measuring an analog parameter). There are micro spatial and temporal relationships at work. It takes a few years of training and experience to learn this stuff. A little knowledge, mfr hype and OCD misunderstanding can cause you stress you really don't need.

Single point verification is good enough as a check to see if the meter is off. They are usually linear enough that we do not have to check outside our 35-65% RH band of interest. If they are reading outside this band you may have some problems and if the meter is out 5% is not a great concern. And no I was never in the navy but took instrumentation in college.

[Nickster]

But shouldn’t the one that registered at 68° be showing a lower humidity than the other one? That’s the part I can’t figure out. It consistently shows a higher measurement than the other one.

[ljguitar]

Originally Posted by Nickster

I recently did the salt calibration for two, one white and one black. I left them in the bag overnight. The next day, the white one came in at 68° and the black one came out to 73°. However, today as well as every time I’ve had them, the white one always shows a higher humidity than the black one. Shouldn’t it be the opposite?

Hi Nickster
The salt test is not a calibration test, nor even that accurate.

Fire them both up over night, place them side by side and see what they read.

And a point or two difference doesn't cause guitars to explode (not even 5% difference).

The goal is to keep the guitar around 40%. Mine sound and play better at 35% and have lived that way happily for about 47 years in our current home…without cracks, deformation or anything other than a tweak of the truss rod going into and out of winter.

I have 4 of them around the house - and if side by side they read within a percentage or two of each other. Move them across the room and it may vary by 3-5 points.

That's because the air in our homes, buildings, auditoriums etc is not 'uniformly' humid.

Most of our homes/apartments are not sealed up like a museum cabinet.

Hope this adds to the discussion…

[Bowie]

Originally Posted by Nickster

But shouldn’t the one that registered at 68° be showing a lower humidity than the other one? That’s the part I can’t figure out. It consistently shows a higher measurement than the other one.

I answered that for you. Their accuracy is not linear and they aren't going to be consistent across the entire range. If you've ever looked at the performance of anything on a graph, you've probably seen the line can peak and dip. These are not high precision instruments so don't expect performance to be consistent at all humidity levels.

[phavriluk]

Besides which, the calendar says 'April'. We're running a fraction of the heat we did in January. No idea where 'Nickster' is located.

[Bluenose]

If house is not currently being heated or if the temperature outside does not go below freezing at night you can safely put your hygrometers away. Trust me

[Bowie]

Originally Posted by Bluenose

If house is not currently being heated or if the temperature outside does not go below freezing at night you can safely put your hygrometers away. Trust me

No thanks. I live in one of the largest cities in the US and if I took your advice I'd risk damage. The humidity is dipping below 30% in rooms with no humidification (and the AC has not run in more than a month). It will get much worse in the summer and would certainly cause damage if I didn't humidify.

[printer2]

Originally Posted by Bowie

I answered that for you. Their accuracy is not linear and they aren't going to be consistent across the entire range. If you've ever looked at the performance of anything on a graph, you've probably seen the line can peak and dip. These are not high precision instruments so don't expect performance to be consistent at all humidity levels.

What range? They were in the same temperature and humidity.

Originally Posted by Nickster

But shouldn’t the one that registered at 68° be showing a lower humidity than the other one? That’s the part I can’t figure out. It consistently shows a higher measurement than the other one.

Hot air holds more humidity, right? Say at 73 F the air can hold 1 lb of water (nowhere close but good enough for us). At lower temperatures the air will be able to hold less, at 68 F the air may only hold 0.9 lbs. We call it relative humidity as the amount of water vapor the air can hold is relative to the temperature. So say at 73 F the air is holding 0.5 lbs of water and the gauge is reading 50%. At the same time the other gauge is reading 68 F and 60% RH. You can think of the RH as a bucket, the 73 F air has a 1 lb bucket and the 68 F air has a 0.9 lb bucket. The humidity sensor (not display) reads the same in both cases as the amount of vapor pressure is the same. The meter then multiplies the voltage signal from the sensor by how much the air could hold.

So the meter at 73 F has the sensor reading 0.25 volts, the microprocessor multiplies that by 2. So the 0.25V x 2 / 1.0 lbs = 55% RH.

The other meter has the sensor also reading 0.25V, but rather than multiplying by 2 the microprocessor knows at 68 F the air can only hold 0.9 lbs. So the 0.25V x 2 / 0.9 lbs = 55% RH.

You can have cold outside air that is 40 F @ 40% RH but when you bring it inside and heat the air to 70 F, the RH drops as the air can hold more moisture than it did at 40F.

If the meter is wrong in detecting the temperature the microprocessor gives the wrong reading in humidity as it uses the temperature to figure out how much moisture the air can hold. If the temperature is reading low, the humidity will be reading high.

Manufacturing has become pretty good at building electronics, here are couple of my meters, $4 at the Dollar Store. Took them to work and in the midrange they were only off by a percentage. I do have one meter reading the temperature wrong and am seeing the same type of change as the OP. I toss it and get another one (I always compare at least three to each other).

[LAPlayer]

Originally Posted by printer2

.....Hot air holds more humidity, right?

Only if there is humidity to hold. A lot of people live in very-dry climates - no matter the temperature. At 100 degrees in Phoenix, the RH may be +/- 10%. To me, the best thing to do is have a nice, proper hygrometer in your home, then you can compare your small inexpensive hygrometers to that known level.

[gfspencer]

I used to know exactly what time it was until I bought a second watch. Now I'm not sure. Same with hygrometers.