Helpful Guide to the Best Digital Psychrometer

For many people, there will never be a need to get an ultra-accurate humidity reading. Sure, you might turn on a humidifier if the air is too dry, or run a dehumidifier in a damp basement. But very few people care whether their indoor humidity is 58 or 60 percent. That said, there are a number of reasons you might want a very accurate reading. For instance, a lot of electronics can be sensitive to changes in humidity. So if you’re running a server room, you may want to be alerted to even minor changes. Similarly, many commercial HVAC systems have a humidifier built in. If you’re a technician working on one of these systems, it’s important to have tools that can measure humidity.

But there are any number of tools for the job. For instance, you could use a wireless hygrometer. These track the current humidity, and can even monitor other weather conditions. However, they’re not quite as accurate as a proper psychrometer. They’re better-suited for continuous monitoring than they are for spot checking. If you want the maximum accuracy from a handheld device, the right tool is a digital psychrometer.

Today, we’re going to review three of the best digital psychrometers that money can buy. First off, we’re going to discuss the UEi Test Instruments DTH35 Digital Psychrometer. This is a powerful, accurate psychrometer with an easy-to-read triple display. Next, we’ll look at the Preciva Digital Psychrometer. This is an affordable, handheld psychrometer that sports an ergonomic rubber grip. Finally, we’ll examine the Extech RH350 Psychrometer. This psychrometer will store up to 99 data records, and also includes two remote temperature probes. To find out which is the best, we’ll have to take a close look at each of them. Let’s get started, and see how they perform!

What is a Psychrometer?

Most people are familiar with hygrometers. They might not be able to tell you how one works, but they know it measures humidity. But “psychrometer” is a less-familiar word, and might even be confusing. In fact, a psychrometer is just a particular type of hygrometer.

First developed in the 1600s, a psychrometer consists of two thermometers. Because old-school mercury thermometers have bulbs at the bottom, they’re referred to as bulbs in this case. One is called the dry bulb, and is exposed to the air just like any other ordinary thermometer. The other is called the wet bulb, and is wrapped in wet cloth or cotton wadding. You then allow the wet bulb to cool until the temperature stops dropping. Depending on the design, you may need to swing the wet bulb around during this time. Once the wet bulb has stopped cooling, you can measure the difference between it and the dry bulb. Then, combined with a measurement of the local atmospheric pressure, you can determine the humidity.

This works based on the principle of evaporative cooling. In dry air, a great deal of moisture will evaporate from the wet bulb. This in turn will lower the bulb’s temperature by a large margin. On the other hand, in humid air, there will be less evaporation, and therefore less cooling. At 100 percent humidity, you wouldn’t see any temperature differential at all.

Why Use a Digital Psychrometer?

As you might have guessed by now, using an old-school manual psychrometer was no picnic. First, you had to calibrate — and not break — a pair of accurate mercury thermometers. This process alone can take as long as 20 to 30 minutes. Then you have to wet the cloth. Then you have to swing it around, and use another tool to read the barometric pressure. And even after you’ve taken your measurements, you have to look everything up on a chart.

A psychometric chart contains a variety of information in addition to the humidity. You can also find a variety of other information. Among other data, you could find:

• The dew point, or the temperature where water vapor will begin to condense. If the temperature drops to this point, dew will start to form.
• The humidity ratio, or the ratio of moisture to air by weight.
• The specific volume, or how much space is occupied by a particular quantity of air.
• Enthalpy, or the number of BTUs of heat in each pound of air.

As you can imagine, this is a lot of data. Moreover, rather than a simple graph, all the data is contained in a chart with multiple columns. A 1915 chart from the US Department of Agriculture was 87 pages long. That’s an awful lot of trouble to go through just to get a few simple readings. But in the days before computers, it was the best method we had.

Nowadays, a digital psychrometer can do the same job in mere seconds. You literally press a button and your readings appear on the screen. In terms of simplicity, it really doesn’t get any easier. There’s no need to swing a thermometer around in the air or flip through a cumbersome, 90-page chart. You don’t have to be a laboratory scientist to use one of these devices. They can be operated by anybody with just a few minutes of training.

With a digital psychrometer, you can also take advantage of other modern features. For instance, many will allow you to store previous readings for later recall. You can also switch between different units of measurement. In most cases, there will be an option to switch between Fahrenheit and centigrade, for example. You can also use remote probes and other fancy gadgets. That’s not possible with a manual psychrometer.

UEi Test Instruments DTH35 Digital Psychrometer

The UEi Test Instruments DTH35 Digital Psychrometer is a handheld, wand-shaped device. Its housing is constructed from bright yellow plastic, which is both high-visibility and reasonably durable. The overall length is just under 10 inches but most of that is the probe. The screen is located towards the base of the wand, and measures just under two inches in height. At three inches in width, it’s also large enough to be easily readable. There’s even a probe cover that doubles as an extension handle that can be attached to the other end. The DTH35 comes individually by default. However, if you’re outfitting a larger crew, you can also buy it in a two, three, or four-pack.

The display can show you two readings simultaneously. By default, you’ll see the temperature and the relative humidity, but there are other options. You can use the UEi psychrometer to calculate the dew point or enthalpy as well. Temperatures can be displayed in centigrade or Fahrenheit, so it’s suitable for use in any country. And regardless of what you’re measuring, the screen is very easy to read. It has a backlit design, so you can take your readings even in the dark. There’s also a magnetic mount in the back, so the wand can be attached to a conductive surface. This means you can slap it on the side of an air duct while you write down your readings.

The DTH35 can operate between 32 and 122 degrees Fahrenheit. This covers most ordinary circumstances where you’ll actually care about the humidity. The lower limit for the wet bulb and dew point readings is even lower, at -69 and -90 degrees respectively. You can measure the full gamut of relative humidity, from 0 to 100 percent. Once you’ve taken your readings, you can save a min/max reading. This will allow you to recall your highest and lowest readings at the push of a button. If you want to start fresh, you can also reset these readings.

Preciva Digital Psychrometer

The Preciva Digital Psychrometer is a more affordable option, but that doesn’t mean it’s not worth considering. To begin with, it has a small form factor, much like a walkie-talkie. In fact, it sort of looks like a walkie, with the short probe in the center taking on the appearance of a fat antenna. The total length is less than seven inches, and the width is under three inches.

The display is located in the middle, with white backlighting and easy-to-read numbers. It shows the relative humidity on top, and the current temperature underneath. Humidity is displayed down to the hundredth of a percent, and temperature to the hundredth of a degree. Underneath, you’ll find a button for cycling between different modes, including changing from centigrade to Fahrenheit. There’s also a min/max memory button. The sides of the unit have small rubber grips, so it won’t slip out of a wet or sweaty hand.

The Preciva psychrometer can measure relative humidity between 32 and 104 degrees Fahrenheit. The wet bulb temperature can rise as high as 176 degrees. For the dewpoint, you have a range of -4 to 176 degrees. This is the same limit for the temperature reading. In total, humidity can be measured from 0 to 100 percent. So you can still use this psychrometer even in very wet or very dry conditions.

Extech RH350 Psychrometer

Extech is a well-known manufacturer of industrial testing equipment. Among other offerings, we’ve reviewed their digital light meters in the past. Their RH350 Psychrometer lives up to their tradition of quality. It’s long and green, with a very short probe. This might sound like a bad thing, but Extech includes a pair of remote probes in the package. You can take a quick reading without them, or slip them into a small space to get a more accurate reading.

The housing is olive green in color, and there are several buttons on the front. These buttons are necessary because of the psychrometer’s advanced functions. You need to switch between the built-in and remote probes, change temperature scale, and change readouts. You also need to be able to use the memory function. In all, the RH350 can store 99 different data records. This is enough to keep track of an absurd number of projects. You can also store a simple min/max reading with a single button.

Ordinary operating temperature is -4 to 122 degrees, which is quite impressive. The humidity can be read between 10 and 90 percent, and the dew point from -90 to 122 degrees. The wet bulb can even measure temperatures as low as -6.9 degrees. Combined, these features make it one of the more versatile psychrometers available.

Final Verdict

Each of these digital psychrometers has its own valuable features to bring to the table. Which one is best will largely depend on what you need. The UEi Test Instruments DTH35 Digital Psychrometer is a good all-around choice for most applications. Its wand-shaped design makes it easy to insert the probe into tight spaces and get a reading. The extension handle makes it easy to hold on to, and the backlit display is easy to read. You won’t find a ton of “extra” features here. There’s no remote probe, and the memory function is just a simple min/max. That said, it’s very sensitive, it’s easy to use, and it comes in multi-packs for crews.

Next on our list, we reviewed the Preciva Digital Psychrometer. This is our most affordable choice, and offers the fewest reading options. That said, it’s a great value for what you’re getting. You can use it in most normal temperatures, and it has a proper min/max function. It also has nifty rubber grips that make it very easy to hold on to. If you just need a simple humidity reading without any extras, you’re looking at a solid choice.

The Extech RH350 Psychrometer is what you buy when you’re willing to pay top dollar for top quality. With remote probes, you can measure the temperature and humidity in even the tightest of spaces. Moreover, you can use this psychrometer in virtually any conditions. Finally, you can store 99 different records, which makes it easy to track all your projects. If you need these features, the RH350 is an excellent option.