A Tale of Two Sling Psychrometers

Although there are two Sling Psychrometers above, these are not the two this post is about. The Left one looks like similar to one used in the Chemical Lab I used to work at in the Steel Mill, it first interested me in buying a Sling Psychrometer. The picture is blurry though, so I found something similar, the Top and Bottom Pictures on the right. It was on Ebay for $39, that’s more than reasonable. You’d need to use Online Charts or a Cell Phone App to transpose the Temperature Readings into Relative Humidity for either of these two models. And these 2 item appears to have Mercury Thermometers, the newer ones I discuss below are Spirit or Alcohol Based Thermometers.

“A Sling Psychrometer is an instrument that measures the relative humidity and dew point in an area. A sling psychrometer has two thermometers: a wet bulb and a dry bulb. The wet bulb has a cotton wick over the bulb of the thermometer, which is moistened with room temperature water. The dry bulb is simply a thermometer. Both are attached to an assembly so that they may be spun through the air. A sling psychrometer works on the premise that evaporation is a cooling process. The drier the air, the more evaporation takes place off of the wet bulb, dropping the temperature on the thermometer. The Dry Bulb Temperature is then compared with the Wet Bulb Temperature (One the Scales on the models below, or using an App on a Cellphone, Program on a Computer, or use of a Chart – DA).”

The greater the difference between the Dry Bulb and Wet Bulb Temperatures, the Lower the Relative Humidity. If the Wet and Dry Bulb Temperatures are the same:

  • The user either forgot to check to see if the Wick on the Wet Bulb Thermometer is wet, or the wick has dried fully while in use.

  • The Relative Humidity is 100%.

Psychrometric Charts: Psychrometer

The above Video, he uses one of the models I want to discuss. Start Watching at 1:50 to see it in use.

More Terms:

Humidity may be absolute or relative.

Absolute Humidity is the amount of water vapor in a unit volume of air which is expressed in kilograms per cubic meter. It does not change according to the temperature of the air. When there is a high amount of water vapor in the air, absolute humidity will also be high.

Relative Humidity is the percentage or ratio of the amount of water vapor in a volume of air at a given temperature and the amount that it can hold at that given temperature. An amount of water vapor in warm air will result to a lower relative humidity than in cool air.

Dew Point is the atmospheric temperature (varying according to pressure and humidity) below which water droplets begin to condense and dew can form. Dew point is the temperature at which the air is saturated (100 percent relative humidity).

Some time ago, wanting to experiment in measuring the Relative Humidity, my wife bought me the Baker 6012 Sling Psychrometer in Fahrenheit and is available as the Baker B6015 Sling Psychrometer for the Celsius model.

Recently, due to some minor cracks evolving in the Baker 6015, I bought the Bacharach 0012-7012 Sling Psychrometer for Fahrenheit model or Bacharach 0012-7043 Sling Psychrometer for the Celsius model.

I use these things all the time. The Baker 6012 or 6015, an honest price for them with non-calibrated Thermometers is under $50 USD. If you want Calibrated Thermometers, that’s an additional $136 USD for each, and you need 2. Since most of us don’t have a real need for high accuracy, for under $50 USD, you’ll be in business. The links to the Baker 6012 and 6015 are at Amazon, the manufacturing company is in Shanghai China.

The Bacharach 0012-7012 or 7043 are at a company that sells instruments, and I paid about $85 plus shipping. This is a U.S. Made Instrument, and the quality is significantly better than the Baker. Nevertheless, they both are more than serviceable, and worth the money, if you consider a few things about them. 

I use mine to assess the conditions in and around the house at various times during the year. When the furnace is running, the humidity drops, and outside is often lower that time of the year anyway. We use a humidifier in the room, a Heated Mist one, not Ultrasonic. I like the room at 50% to 60% Relative Humidity.

Those summer days, hot during the day, and as the air cools, guess what, the Relative Humidity Increases with the cooling air, not unlike those glasses or cups with ice cold, or well refrigerated liquids inside that causes such a nuisance in the summer by sweating profusely and leaving wet spots wherever you put them down. I can view the Weather Forecast in text, or I can view it on a graph with many points plotted by the National Weather Service (NWS). Looking at the NWS graph, for the large part, the Relative Humidity Decreases as the Air is Heated, and Increases as the Air Cools, forming a plotted line that decreases in the day, and increases during the night, over and over from day to day. But if it feels especially oppressive, it’s nice to pull out the Sling Psychrometer and see what the Relative Humidity actually is.

I use it all year round, to compare how I feel with actual conditions. Imagine a fan blowing air on to you. With High Relative Humidity air, at 60° F, you may feel fairly comfortable, but with Low Relative Humidity air, at 60° F, you may feel fairly cold.

Same with exercising, as we get hot, we sweat, and ideally the sweat evaporates and causes cooling. The closer to 100% Relative Humidity it becomes, the less that cooling effect works. So, in drier weather, exertion may be much better tolerated than more humid weather.

There are Evaporative Air Conditioning Systems that Cascade Water through porous filters, and the filters allow air to pass through. The Air Cools as a result, and can be used to provide cooling at an economical cost, more so in drier climates.

So, this Blog Entry is the Baker 6012, made in China, vs the Bacharach 0012-7012, Made in the U.S., near Pittsburgh I believe. They both have 2 Red Colored Spirit Thermometers (I think the Celsius Version in at least one of these 2 Sling Psychrometers uses Blue Colored Spirit Thermometers). 

See: Wiki Article on Alcohol (Spirit) Thermometers

Predictably the U.S. version seems to be better designed and built, but the cost is nearly twice as much. The Baker is Flat, the Bacharach Tubular.


Bacharach vs Baker (1)


Bacharach vs Baker (9)

They both have scales; though the scales are not linear, i.e. the spaces between the numbers decreases as the values increase: on Voltmeters, this is referred to as Logarithmic Scales, and it may be so on these Sling Psychrometers as well; for the Dry Bulb and Wet Bulb Temperatures, and you slide the scale until the two readings are aligned with each other, and the pointer indicates the Relative Humidity. Comparing the two, as an example, using 60° for the Wet Bulb Temperature, and 75° for the Dry Bulb Temperature, the Baker Reads 39.5% Relative Humidity (from now on RH) and the Bacharach Reads 40% RH. The App on the Computer Tablet says that those Temperatures represent 38% RH. On this point, the Baker is closer. But look at the Scales on the Sling Psychrometers above, they are both inserted fully into their casings, on the Baker, the pointer should be at 100% RH, but it’s only 90% on the top one, and 95% on the bottom. With the Temperatures at 70° and 70°, the reading should be 100% RH, but it’s impossible on the Baker. 


Bacharach vs Baker (2)


Bacharach vs Baker (3)

Both of these Sling Psychrometers have removable caps with a reservoir to hold water to take successive RH readings without needing to re-wet the wick each time. The Baker Reservoir slides off, and that is an issue, since the retaining tabs are relatively frail. There are two plastic tabs on the Thermometer Tray that retain the reservoir, and on mine, one tab is partially cracked, and the other tab had bent away from the Reservoir, I had to heat the tab and bent it back in, and it deformed a little, but still works fine. In the Image Below, the Left Image Shows the Poor Mating, the Top Right, the Cracked Tab, and Bottom Right, the one I deformed from Heating to push it back in place, as it was nearly no longer holding the reservoir and was impeding the Thermometer Tray from easily sliding into the case or being withdrawn from it.

These are the 2 things (Reservoir Tabs and Scales not Pointing to 100%) that are really a problem with the Baker.

In Use:

The Tray containing the Thermometers, is pulled out of the case

The Wet Bulb must be assured not only to be wet, but wet enough that it won’t dry during use. I just use a slight pour of water from a drinking bottle, or briefly let a cold water stream from the faucet, soak the Wick, then briefly hold it vertically to encourage water on the Wick to run into the Reservoir (on both models). If you saturate the wick in the reservoir, and fill the reservoir with water, it’s going to leak out. So wetting the wick on either model is sufficient, and the excess can run into the Reservoir, and that will keep it wet or well dampened for awhile.

Be sure the Dry Bulb Thermometer is Dry.

  • Pull the Thermometer Tray out of the Handle.
  • On the Baker, I hold the Sling Psychrometer near the Top of the handle portion, and I place my finger into the slot on the rear and push up on the Pivot Point to keep the Pivot Point from flipping back and forth and to keep it from slipping back into the Handle while spinning the Thermometer Tray which may cause the Thermometer Tray to Impact the Handle.
  • On the Bacharach, I hold the Sling Psychrometer near the Top of the handle portion, this helps stabilize the Handle since holding at the bottom and spinning the Thermometer Tray will naturally cause the handle to tilt back and forth otherwise. At this point, the Pivot doesn’t slip back into the case in use, and the Tube Design is well suited for preventing the spinning action from abruptly stopping the Thermometer Tray.
  • I spin them about 50 Seconds before taking the reading, holding the handle vertically and letting the Thermometer Tray Spin Horizontally around the top of the Handle. Be sure while Spinning that the Thermometer Tray doesn’t contact anyone, including your face, or anything. Slow the Spin Down prior to stopping it, do not stop abruptly.
  • Read the Wet Bulb First, taking mental note of it, then read the Dry Bulb.
  • Slide the Thermometer Tray back into the Handle. Use the Appropriate Scale and line-up the Wet Bulb Reading and the Dry Bulb Reading, then look at the pointer to see what the Relative Humidity is. Or use a Graph, Program, Website, or App and plug the numbers in to see the Relative Humidity.

Bacharach vs Baker (7)

Close Up, Done in Direct Sunlight, not a smart thing to do on a Hot, Sunny Day. The Thermometers, when looked at from the correct angle, are easy to read on both models, the lower Thermometer on the Baker Sling Psychrometer shows what it really looks like.

Bacharach vs Baker (10)

Both the Thermometer Trays only go back into the Handle one way, this is necessary to have the Numbers on the other face of the Thermometer Tray aligned with the case, they go back in as shown below. Incidentally, the Baker shown above on the Top, at the Top of the Handle, see the Opening. I place a Finger in the opening and push up on the Pivot Point to keep the Pivot Point from flipping back and forth and to keep it from slipping back into the Handle while spinning the Thermometer Tray.

Bacharach vs Baker (13)

Close-Up of the Bacharach, Align the Arrow on the Handle with the Grove on the Thermometer Tray. 

Bacharach vs Baker (12)

Accuracy, in my case, Both Model’s Web Bulb Thermometers read the same, the Baker Dry Bulb reads 1.5° F Higher than the Bacharach Dry Bulb. With Equal Wet Bulb Temperatures, and a slightly Higher Dry Bulb Reading, the RH would seem to be a little less than it actually is. I did not presuppose that the Bacharach was right and the Baker wrong, I took readings with multiple Thermocouple Devices and came to the conclusion that the Bacharach is more correct. This may vary from Instrument to Instrument. When compared to the Android App where the Wet and Dry Bulb Temperatures are written in, the Baker reads a little low for the Temperatures, the Bacharach reads a little high, and this is around 50% RH and with 1° F Subtracted from the Baker Dry Bulb Reading to make it closer to the Bacharach Dry Bulb reading. In any case, around 50% RH, the two Sling Psychrometers are within 5% of each other. 

I have no need to have the perfect reading, though technique can have an influence on the readings. How long you spin it, if they Wet Bulb is sufficiently Wet, if the Dry Bulb is fully Dry, etc.

Pro’s and Con’s.

Baker – Pros:

Less Expensive.

Flat, easy to carry in shirt pocket. Lighter. Spinning it is more pleasant as the weight is so light. Thermometers seem to be made using less glass, which is an advantage, as less mass means that less time is needed for the Temperatures to stabilize while in use.

Baker – Cons:

The Scales on the Thermometer Tray used to determine the RH do not go to 100%. This will be a problem on higher humidity days and points to the likelihood that much of the Scale, especially above 50%, is also compromised.

The Pivot for Spinning the Thermometer Tray is loose insofar as rocking back and forth while in use, necessitating a finger inserted into the opening on the rear of the unit to hold it stable.

The Tabs, part of the Thermometer Tray, that hold the Reservoir Cap in place are poorly designed, they bend back from the Reservoir Cap, potentially letting it fly in use, and one spontaneously cracked after many hours of use.

Parts do not seem to be available, except for Replacement Thermometers.

Bacharach – Pros:

High quality for the price, Scales Align Properly, Reservoir Cap threads on, Spinning the Thermometer Tray works without needing to hold the Pivot Steady, the Pivot doesn’t slip back into the case, or tilt back and forth in use, and it seems parts are available, all parts, so the unit can be repaired.

Bacharach – Cons:

More Expensive.

Tubular Design makes it thicker, more difficult to stuff into a shirt pocket.

The Thermometers seem to have more mass, thicker, and it may take slightly longer to Spin the Thermometer Tray to a Stable Temperature.

The Entire Psychrometer is slightly heavier than the Entire Baker Psychrometer, so the process of Spinning the Thermometer Tray takes a little more effort.

Comments for Both:

The Scales for determining the Relative Humidity are nice, they are adequate for most uses in common ranges of humidity. But you’re actually buying 2 Thermometers on a Stick. Even without the Scales on the Case, Programs, Apps, Graphs, etc., can be consulted to determine the RH just by using the temperatures noted, see the 2 Sling Psychrometers at the beginning of this post.

Be sure both thermometers read the same. Let the Web Bulb go Completely Dry, or look at the temperatures, with the Thermometer Tray Open, on the new unit before wetting the wick. They should be the same, or very close (Close Enough for Home Use).

Author: Dr-Artaud

A Doctor that is not a Doctor, but named after a character in the movie "No Such Thing", as is the Avatar.

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