Digital Manometers

January 04, 2011
January/February 2011
A version of this article appears in the January/February 2011 issue of Home Energy Magazine.
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One of the most frequently asked questions that I get when I teach building science courses is, “What equipment should I buy to get started in this business?” There is no question that getting started takes a big investment in tools, and there are more and better tools available every day. This magazine provides a unique resource into how to use the tools. The column is aimed at helping you select one.

Paul Raymer is chief investigator of Heyoka Solutions, a company he cofounded in 2006. He has been wandering through the mysteries of building science since 1977. He has multiple BPI certifications and is a HERS Rater.

Manometers are the most fundamental tools for building science, so that’s where we’ll start. Although manometers are commonly part of a blower door kit, they are capable of going way beyond the basics of blower door or duct testing, so this discussion is focused just on the features of manometers as stand-alone tools. These devices are very smart, doing many calculations automatically, but it is important to understand what they are measuring and how they are calculating the numbers they display (see “Manometer Basics”).

There are two primary competitors in the field: The Energy Conservatory with the DG-700 and Retrotec with the DM-2A. These are not the only digital manometers available, but many are not designed to provide the low-pressure readings that we need to measure in houses (see “Other Options”).



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Manometer Basics

The manometer is arguably the most versatile, fundamental tool for residential building energy analysis. Understanding comparative pressures—the difference in pressure between two points—can indicate whether a flue will be able to draw the combustion gases out of the house, calculate the airflow through a blower door, display the pressure in a duct, the airflow through a bath fan, or the airflow through an air handler. Getting familiar with the all the wonder of a manometer is worth the effort.

The key to using a manometer correctly is understanding what you are trying to measure and what it is that the pressure differences in a building can tell you. These manometers measure the difference in pressure between two points—an “input” and a “reference.” What is displayed on the manometer screen is the pressure at the input point with respect (or reference) to (WRT) the reference point. Sometimes knowing that pressure difference is all you need to know if, for example, you are determining the pressure in the CAZ (combustion appliance zone) with reference to the outside or the pressure in the flue with reference to the CAZ. Sometimes you want to know the airflow through a fan like the blower door or duct tester. These manometers calculate that airflow for you by knowing the size of the hole (the ring on the fan) and the pressure difference. They are not directly measuring airflow but deriving it from the pressure difference.

By the way, these manometers measure very, very small pressure differences. They are extremely sensitive. Although they can display in either inches of water gauge or Pascals (or lb/ft2 in the case of the Retrotec), the building science community has settled on Pascals. (One Pascal is equivalent to 0.00401 inches of water gauge or column. Going the other way, one inch of water gauge is equal to about 250 Pascals.) One Pascal is approximately equivalent to a dust mite’s sneeze!

The Energy Conservatory’s DG-700: Dual-Channel Digital Manometer

The DG-700 is a dual-channel manometer, which allows the pressure between the house and outside to be displayed on Channel A and the airflow through the fan to be displayed on Channel B. The manometer is measuring pressure on both channels, but, through the wonder of electronics, the manometer calculates and displays the airflow on Channel B using a variation of the formula CFM = 1.07 x A x √∆P where A is the area of the hole (blower door ring, for example) and ΔP is the pressure difference.

The DG-700 performs other internal calculations to make the user’s life easier. For example, when entering the baseline pressures, the DG-700 automatically subtracts the baseline pressure from the pressure displayed on Channel A, indicating that it is “Adj Pa.” (Note that when changing modes from Pressure/Pressure to Pressure/Flow, for example, the baseline measurement is erased and needs to be reset.) Other internal calculations are made depending on the device the manometer is attached to—the Model 3 Minneapolis Blower Door fans, the Model 4 Minneapolis Blower Door fans, the Series A and B Minneapolis Duct Blaster fans, the Exhaust Fan Flow Meter, or the TrueFlow Air Handler Flow Meter.

The DG-700 gauge measures 7.5 inches by 4 inches by 1.25 inches (19.5 cm x 10.16 cm x 3.175 cm) in a hard plastic case that is internally shielded. It weighs 16.5 ounces and can be handheld. It has Velcro strips on the back to mount it on a measurement bracket. The 12 membrane-covered control push buttons are protected from the field dust. There are four brass pressure-hose connectors, an “Input” and “Ref” connection for each channel, clearly marked as “A” and “B.” On the top there is an RS232 connector to attach to a computer for using the manometer in conjunction with TEC’s software. There is also a jack for connecting the DG-700 to TEC’s speed controller to operate the blower door or Duct Blaster fans in “Cruise” mode; this allows the fan to be automatically speed-controlled, maintaining a pressure difference by automatically adjusting the speed of the fan. The sound of the fan changing pressure can alert the operator to an unanticipated door or window adjustment (like the homeowner coming in from the garage). The DG-700 uses six AA, alkaline batteries, but on the side there is an AC adaptor input for an optional AC power adaptor. According to the manufacturer, battery life is over 100 hours of continuous use.

The DG-700 is a dependable device, working under pressure in a lot of houses for a lot of years. It is solidly built and reliable. The displays are clear and provide all the necessary information at a glance.

It would be nice if the baseline measurement number didn’t go away when the “Enter” key is depressed. The DG-700 knows what the baseline pressure is, but if you don’t write it down, it’s gone. For the most part that is just a minor annoyance, although it is worthwhile to check back at the office to note what the baseline number was because it clearly indicates that the step of establishing a baseline wasn’t missed.

Technical support and documentation are excellent. Because the DG-700 has been the workhorse, it has become the generic image in manuals, articles, and technical programs that require measuring pressure differentials. It is important to work with the gauge regularly and achieve a familiarity with the setup and functions of the controls, but even more important is understanding what information the user is looking, for and that leads to setting up and connecting the DG-700 to provide truly useful information.

Specifications
  • Pressure range:-1,250 to + 1,250 Pascals (-5 to +5 inch H20)
  • Display resolution: 0.1 Pa (0.0001 inch H2O)
  • Accuracy: 1% of pressure reading or 0.15 Pa, whichever is greater
Units of measure:
  • Channel A: Pascals, inch H20
  • Channel B: Pascals, inch H20, CFM, CFM@50, CFM@25, m3/h, m3/h@50, m3/h@25, l/s, l/s@50, in2@50, in2@25, cm2@50, cm2@25, fpm, m/s
  • Auto-Zero: On startup and then once every 10 seconds

The DG-700 kit includes the gauge, protective carrying case, 6-inch-long pressure probe, 10-foot (3m) red hose, 15-foot (5m) green hose, instruction manual, and a two year warranty.

It is available through The Energy Conservatory (www.energyconservatory.com). The retail price is $825.

Other Options

A manometer that reads in psi (pounds per square inch) does not have the resolution necessary for home pressure measurement (1 psi = 6,894 Pascals). Make sure to ask the salespeople if a product you are considering can read to 1.0 Pascal.

Infiltec offers a dual channel digital manometer, DM4, that is set up to work with the Infiltec E3 and Minneapolis Model 3 blower doors. It can also be used as a stand-alone building diagnostic tool. These units are commonly used in Europe as well as the United States.

Testo has a pocket-sized, single-channel manometer, the Testo 510, that is good for making quick spot checks. Its resolution is in whole Pascals (not tenths) and its accuracy is limited to ±2.5 Pa, which makes it suitable for use for zonal pressure diagnostic while performing a blower door test. Dwyer has some low-cost, fluid-filled, U-tube and inclined manometers that might answer budget restraints.

Retrotec’s DM-2A Mark II

Retrotec’s DM-2A is also a dual-channel digital manometer specifically designed for building diagnostics. The DM-2A is a flexible device that can be used with a wide range of products. This flexibility can make the initial setup a bit of chore, but once that is done, it simplifies daily operation.

The housing is a bright yellow impact-resistant plastic covered by bright red labeling. It has been shaped for handholding, with a narrow (3 inches wide [7.5 cm]) lower section and a wider, angled top section. It weighs 14 ounces (397g). The latest version (software version 3.01b) has a very brightly lit display area that shifts from the setup screen to simply the output from the two channels in 3/8-inch numerals and the “Range Config” or fan opening ring. It can be set to run on either four alkaline or rechargeable NiMH AA batteries. If rechargeable batteries are installed, the manometer can be plugged into the blower door fan and be charged during testing. The batteries do have a tendency to discharge, so it is a good idea to carry the recharger and spare batteries. Continuous operation from fully charged batteries with everything running and the display brightly lit is over 5 hours; this should get you through the majority of residential analysis chores.

The four connection ports are colored plastic (blue, red, yellow, and green) and recessed to reduce the chance of breakage. The location of these input ports is one of the primary differences between the DM-2A and the DG-700. The color coding can either be a blessing or a curse. If the operator clearly understands what he/she is measuring, the color of the hoses makes no difference. The ports are marked in small, yellow lettering on a red label. And since Retrotec uses red as its outside reference hose color while The Energy Conservatory uses green, it can be confusing for an operator who goes from one device to the other.

Along with the hose ports, there are also connections for AC charging, a USB connection for a PC, and the RS232 port for the speed control to fan.

The DM-2A can control a wide range of devices via the “Setup” control, including Retrotec, The Energy Conservatory, and the Infiltec E3 devices. The setup can be customized to disable various options so that if the user never uses an Infiltec E3, for example, he/she doesn’t have to cycle through that option during setup. This can limit the numerous options to just a few without limiting the general versatility of the DM-2A, greatly simplifying use. The units of the various measurements are also accessed through the setup menu. So if you want to measure the “in wc” of the air handler’s external static pressure and you are testing a house in Pascals, you will have to go into the Setup menu, go to Mode Setup, select Pressure and change the units.

The DM-2A can serve as both the measuring instrument and the speed controller for the test fan. A manual speed control can be used, but the DM-2A can vary the speed of the fan either manually with the “Jog/Hold” control or automatically by setting a specific pressure or percentage of fan speed for the system. Pressing the “Set Speed” button allows the user to set in a percentage of the fan system’s operating speed from 0% to 100%. Pressing the “Jog/Hold” button allows the user to press the up or down arrows to change the speed. The “Jog/Hold” procedure also works for changing pressure.

By entering the volume and/or surface area values, the DM-2A can calculate and display the air changes per hour at 50 Pascals or the Effective and/or Equivalent Leakage Areas or square inches of leakage per 100 square feet.

There are times when the desired pressure can’t be reached. Besides calculating the CFM at 50 or 25 Pascals, the DM- 2A can also display the pressure at any intermediate pressure levels. The manometer estimates where the pressure would be based on the pressure reached. This is based on a line with a slope of ‘n’ that can be entered into the DM-2A, or use the default value of 0.60. (This is continuously displayed on the home screen.) An actual slope could be determined by performing a multipoint pressure test that can be done with Retrotec’s software package, “FanTestic.” The closer the pressure is to the desired level, the more accurate the result. (Note that with the DM-2A, testing can begin in the Pressure/Flow mode and switched to Pressure/Flow@X any time during the testing. With the DG-700, switching to the Pressure/Flow@50 mode will delete the baseline setting.)

There are mixed reviews about the color scheme of the Retrotec equipment, but it is difficult to lose the manometer! It is an extremely versatile instrument, able to work just about any other piece of pressure test gear on the market. Although the chances are good that the DM2A will be used with Retrotec blower door or duct testing equipment most of the time, it can also be used with TEC’s True Flow grid and Exhaust Fan Flow meter. The new (last six months) display is much better than the previous version, very bright and clear.

Although the DM-2A is a terrific device once it is set up and familiar, Retrotec’s documentation can sometimes be bewildering. It does come with a CD with a lot of information. Its versatility adds to the difficulty of explaining and describing its capabilities. It is wonderful that the baseline number remains on the screen after “Enter” has been pressed, that the slope or ‘n’ value can be changed, and that the Auto-Zeroing function can be turned off to save battery life, but it takes a while to understand and make use of those assets. As with any sophisticated piece of equipment, familiarity and continuous use are the key to getting the most from the DM-2A.

Specifications
  • Pressure range: -1,250 to + 1,250 Pascals (-5 to +5 inch H20)
  • Display resolution: 0.01 Pa (0.0001 inch H2O)
  • Accuracy: 1% of pressure reading or 0.15 Pa, whichever is greater
Units of measure:
  • Channel A: Pascals, inch H20, lb/ft2
  • Channel B: Pascals, inch H20, lb/ft2
  • Air flow CFM @ any pressure
  • Leakage Area – EqLA (Canadian) and/or EfLA (US) in cm2, in2, ft2
  • Air changes per hour according to volume entered on keypad
  • Velocity in m/s, km/h, ft/s, ft/min, mph
  • Velocity-flow in cfm, l/s, m3/s, m3/h according to cross-section area entered on keypad

The DM-2A standard equipment is available with either an umbilical cable assembly or individual hoses, depending on the supplier. It comes with an instruction manual and two-year warranty, and is available through various suppliers and directly from Retrotec. The retail price is $1,310.

We Want to Hear From You

Perhaps this will take a little pressure off you in making a manometer selection. I would welcome comments and feedback on these tools as well as on other tools that we should review in the future. Contact the author at praymer@heysol.com.

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