Accurate refrigerant charge is critical for proper operation of the air conditioning system. Checking the charge with accurate instrumentation helps ensure customers get the efficiency they are paying for. (Photo courtesy of Ivan Kanewski.)
Technicians are constantly making measurements. What do we do with them? Making them without knowledge of how to use them is more dangerous than not making them at all. If we don't know what they should be, why even make them?

Imagine if your doctor took your temperature but had no idea that it was supposed to be 98.6 degrees F. You can bet some lawyer could find out through his team of technical experts that the reason you almost died was you were at 106 degrees.

Day after day, your technicians are leaving a legal document (a work order) with a customer that contains information that the technician may or may not understand, writing things like verified correct airflow, checked charge, and verified temperature drop across coil.

How many times have you seen the problem right on the equipment check sheet? The measurements were made but the technician had no idea that there was a problem. Realize that when you say operation is OK, it had better be or your company is on the line. It happens more often than you might think. It's only a matter of time before it bites you on the backside.

Measuring airflow in the return air duct helps ensure proper airflow across the evaporator coil. Note that a dry-erase marker was used to write down readings, then erased. (Photo courtesy of Ivan Kanewski.).

The Problem With Ranges

Often we work in ranges - temperature, superheat, or cfm - and we shouldn't. Part of the reason is that the analog instrumentation we are using is about accurate. We guess at the airflow, we guess at the refrigerant charge, and we guess that since there is cool air or heat coming from the registers that the air conditioner or furnace is working as it was designed to work.

Nothing could be further from the truth. Air conditioning and heating is an exacting science that deserves exact tools and instrumentation. I am not saying the instrumentation being used is a complete crapshoot, but sometimes the instrumentation we are using has so much internal error or error inherent with the measurement procedure that it may be really of little to no value.

At the next service meeting have all your techs bring in their manometers, thermometers, and sling psychrometers or digital hydrometers. First, make sure they have the tools to do the job. With a test station in the shop, maybe even using a piece of shop equipment, have your techs each make readings of cfm, temperature drops and rises, and return air wet bulb and dry bulb. Measure the suction and discharge pressures and calculate superheat and subcooling. Have them write down the information as they go.

If their instrumentation does not all read the same, how would they ever consistently set up equipment?

This might be the single most valuable thing your company can do on a quarterly basis. Use simple math to calculate the percentage of error.

While doing a refrigeration seminar in Michigan recently, one company performed this test and the results were staggering. Several of their techs most likely had been following behind each other thinking they were the only one in the company who ever checked anything because everything they checked was off.

Measurement Procedures

When it comes to measurement procedures and calibration standards, stop and think for a minute; could you tell me a way to accurately measure airflow across a coil? How about multiple ways? Do you know how and why to charge by subcooling and superheat? Can you verify the real-time superheat or subcooling? Do you have a repeatable procedure in place to set up equipment to the same standard consistently every time?

Do you trust your test instruments? If so, what standard do you use to verify their accuracy? Can you verify that the equipment you work on is working to its designed capacity? How do you know the equipment is operating as designed?

These are all questions digital instruments and a prescribed testing procedure can help you answer - not just any instruments, but National Instrument Standard Testing (NIST)-traceable, accurate instruments. If you are not using instruments that meet these standards, how do you know they are accurate?

Digital refrigeration technology allows us to do what we have not been able to do before, or allows us to do it in a time frame that has not been possible. Digital instrumentation is bridging the gap between the laboratory and the field, allowing technicians to set up equipment to a higher standard than ever before possible. With quality instrumentation, your techs can significantly reduce the error in measurement, reducing callbacks, increasing customer satisfaction, and taking your company to a new level of professionalism.

With digital instruments, you can spend your time using measurements instead of making and remaking them. The procedures are easier, and with high-quality instrumentation, they will be more accurate.

No More Babysitting

How many times have you spent your time babysitting a refrigeration system, waiting for the problem to happen again? When it does, you still see the symptoms, but not the cause. You make a repair or an adjustment and wait all over again. Wouldn't it be simpler to data log the problem while you're working on the next unit?

Quality digital instrumentation can solve these problems, allowing the technician to work smarter and not harder. It is now possible to be as accurate in the field with measurements as they are in the laboratory. Techs can have confidence in their tools. Their tools can make them more productive, making your company more profitable and professional.

With all residential air conditioning systems, there are only two things that can be adjusted, charge and evaporator airflow. We can't adjust the voltage, amperage, condenser fan speed, temperature drop, or temperature rise across the coils; all of these things are a function of charge and airflow. There is nothing else that requires anything more than inspection on a residential air conditioner.

Other factors that can affect operation of an air conditioner include improper line sizing, air bypassing the evaporator, incorrect wire sizes, or a loose expansion valve bulb, but for the remainder of the discussion, we will assume that a proper installation was done.

Evaporator pressure drop is compared against the manufacturer’s literature. Airflow must be set across the evaporator for proper operation. (Photo courtesy of Ivan Kanewski.)

Airflow Measurement

When new equipment is installed, a technician should go through a prestart checklist. An inspection and pre- and post-performance measurements should be made. Technicians should verify that wire sizes are correct, the proper fuses have been installed, the lines are the correct size, equipment placement is proper, and a proper evacuation has been performed.

The installation directions should be checked to verify that the installation was made according to the manufacturer's instructions.

Before any air conditioner can be properly charged, the airflow must be properly set. This means airflow across the evaporator must be set to the manufacturer's specifications (usually 400 cfm/ton for air conditioning, and 450 cfm/ton for heat pumps).

Airflow should always be set prior to system startup. A good time to set the airflow is usually while the system is being evacuated. Airflow cannot and must not be set by measuring the temperature drop across the evaporator coil. It must be set using a method that measures the actual cfm across the coil.

Using a capture hood and setting the airflow to meet register requirements will not do the job. The capture method does not verify airflow across the coil. It does not take into account leakage that is present in all duct systems.

In order to verify proper total system operation, airflow at the coil and the registers must be verified. If the registers do not have the required airflow as measured with the capture hood after airflow is set across the coil and the system is balanced, the duct system must be evaluated and/or properly sealed.

The most accurate way to verify airflow across an evaporator coil is using the pressure drop method and a manufacturer's chart. The manufacturer has spent a considerable amount of money to document these measurements. If other methods are used, care must be taken to ensure that air is not leaked in or out ahead of the point at which the measurements are being taken.

It is imperative that the airflow across the coil is correct. If the airflow is too high or too low, it will adversely affect the system operation. If information on pressure drop vs. cfm is not available, use an alternate method.

Refrigerant Measurement

After airflow has been set, the system's refrigerant charge must be verified. A standard condenser comes with enough refrigerant to operate with a 25-foot refrigerant line set and a matched evaporator coil. If the installation requires anything different, the charge will need to be adjusted. Follow the manufacturer's prescribed charging procedure.

In order to properly charge the refrigeration system, the type of metering device must be verified and accurate entering wet bulb and dry bulb temperatures must be made along the air temperature entering the condenser.

Different types of metering devices require different measurements. Some manufacturers have special charging requirements that should be followed. If none are available, use a charging calculator. There are many different types available by different manufacturers. A close examination will reveal that they are almost all identical.

In general, physics is physics and technicians will find that almost all air conditioning systems operate with similar characteristics. The laws that govern science do not change. Temperature transfer is a function of time, temperature difference, and turbulence. Since time and turbulence are a factor of the airflow set at a nominal 400 cfm/ton (450 cfm/ton for heat pumps), the operating characteristics will be almost identical across the board.

A common problem among service technicians is charging refrigeration equipment during low ambient conditions. With a digital manifold set (refrigeration analyzer) like the Testo 523, and an accurate wet bulb thermometer/hygrometer like the Testo 605-H2 and a charging calculator, it is possible and easy to accurately charge air conditioning systems at ambient temperatures as low as 55 degrees outdoor air, with indoor wet bulb temperatures as low as 50 degrees. Below 70 degrees dry bulb indoor air temperature, the wet bulb temperature must be used.

With low indoor ambients, wet bulb temperature is required because wet bulb takes into account the total heat in the air. There must be enough heat (latent and sensible) in the air to evaporate the refrigerant in the evaporator coil at a rate equal to the rate it is being fed into the evaporator coil or the evaporator will become flooded (overfilled with liquid refrigerant).

It is imperative that cap-tube systems are properly charged. A few ounces of refrigerant can drastically affect the operational characteristics of an evaporator using a capillary cap tube or other fixed-type metering device.

Digital leaves no room for interpretation; it is what it is. With digital, you will find yourself setting up the equipment exactly to the manufacturer’s specifications because you can. (Photo courtesy of Ivan Kanewski.)

Digital Data Benefits

Let's look at a recording digital gauge set. Many service technicians are reluctant to use digital instruments; there is a certain comfort using what we always have used. The truth is, digital instruments are faster, more accurate, more reliable, and have a higher repeatability than most analog tools.

Digital instruments stay in calibration, allow trending, allow more complex functions, and save time. Digital instruments allow data to be recorded and reported without human error, and provide reliable and accurate results for contractors and customers. Data can be recorded much faster than any technician could ever do the calculations, and data can also be recorded whether or not the technician is there to see it.

In most cases, the data record cannot be edited, so what you see is what was measured at the jobsite. System trends and symptoms recorded with the function of time allows the user to track cycles and determine if other factors (like automation or shift changes) are the cause of the problem. Permanent records allow the user to track system changes and determine if the system is operating within its design parameters or if changes have taken place.

Using a tool like the Testokool 523/560 refrigeration analyzer is really no different than using a conventional manifold gauge set. However, the information available to the user about the system's operation is far superior.

The high- and low-side connections are attached to their respective sides, and the readout of the refrigerant pressures and saturation temperatures are displayed. The analyzer reads pressure and temperature only, so it is important that the refrigerant is known before verifying the saturation temperature as the 523/560 calculates the refrigeration saturation temperature. The refrigerant selection can be changed anytime during the analyzer use.

A temperature sensor attached to the temperature probe/data cable connector port allows the analyzer to calculate refrigerant superheat or subcooling and/or measure line, fluid, or air temperature with an auxiliary probe attached.

Going digital may feel awkward at first. From experience, you know approximately where your gauge pressures should be. Sometimes, unless the pressures are outside of the normal operating range, you may not even be paying attention to the actual system pressures. A large part of the problem is if analog gauges are interpreted by the user. They are only an indicator of the approximate pressure.

If 10 users were to attach their gauges to an operating refrigeration system, even if all were calibrated, there would be a range of pressures and saturation temperatures interpreted by the users.

Digital leaves no room for interpretation; it is what it is. With digital, you will find yourself setting up the equipment exactly to the manufacturer's specifications because you can. If the manufacturer calls for 8 degrees of subcooling, you can charge the system to exactly 8 degrees. There is no learning curve outside of navigation of the menus of the refrigeration system analyzer. Real-time measurements allow the user to charge and evaluate the system outside of the range a normal gauge set would be useful.

When using the software, it is important not to let the amount of information obtained by the system analyzer overwhelm you. Analyzers can measure and store snapshots of system operation that include high- and low-side pressures, corresponding saturation temperatures, actual measured temperatures, and the calculated superheat or subcooling at any given instant during system operation.

The information allows you to see the big picture, and notice things like TEV hunting, pressures rising or falling, cycling, and when the system has reached steady-state operating efficiency.

Verify Performance

After the charge has been checked, verify that the system is performing to capacity. If you are performing routine maintenance, you will also want to perform this procedure before you service the equipment.

Using a wet bulb and dry bulb thermometer, measure the supply- and return-air conditions. It is imperative that you use accurate instruments for these measurements as even a 1 degree error in wet bulb measurement can produce significant resulting error.

Using a digital manometer and the manufacturer's instructions, measure the pressure drop across the evaporator coil to verify the cfm. If that's not available, use a pitot tube and a digital manometer to measure the velocity in the return air duct. Remember, air in should equal air out.

Verify that there are no air leaks between the appliance and the ducts to ensure accuracy when using a pitot tube. Using the gross total heat formula and a psychrometric chart or a digital equivalent, calculate the pre- and post-Btuh capacity, proving that the system is working as designed or providing you with the information you need to take it to the next step.

Check your techs' instruments; it's your reputation on the line. We make measurements for a living. Making an investment in digital instrumentation will reap benefits for your company for years to come.

If you don't want to make a big jump, make a small investment in the technology and see how it works for you. Try it; I did, and I will never go back.

Contact Jim Bergmann, HVAC/R Technical Specialist, at

Publication date: 08/22/2005