How to Charge by Weight with Mastercool Scale: Step-by-Step Guide
Charging by weight is the most accurate and most defensible way to charge a refrigeration system. It removes the guesswork from the process, gives you a documented number to put on your commissioning paperwork, and protects both the system and your ARCtick licence obligations. This guide walks through the full process using a Mastercool scale, from final system prep through to post-charge verification, covering the decisions that matter along the way.
The Mastercool 98210-BL scale is used as the reference tool throughout this guide because it's the most common Mastercool model in professional use, but the technique applies equally to the 98201 manual scale and the 98315 wireless solenoid model. The process is the same. The hardware is what changes.
Written by Rica Francia Macaspac, HVAC Shop content writer, in consultation with Aussie HVAC tradies and industry experts. Published: June 2025 · Last reviewed: June 2025.
Pre-Charging System Preparation
Good charging starts well before you open the cylinder valve. The condition of the system going into the charge matters as much as the accuracy of the scale coming out of it. Skipping or rushing the preparation steps is where most charge-related callbacks start. Take the time to do it right before you commit refrigerant to the system.
Confirm Evacuation Is Complete
The system must reach and hold your target micron level before you introduce refrigerant. For most residential and light-commercial systems, a target of 500 microns or lower is the accepted benchmark, though some manufacturers specify tighter targets in their installation documentation. Connect your micron gauge and watch the reading after you isolate the vacuum pump. If the system holds below 500 microns and doesn't rise significantly over five minutes, the evacuation is done. A steady rise back up to several hundred microns indicates moisture or a leak that needs to be resolved before charging. Charging over a system that hasn't been properly evacuated is one of the fastest ways to damage a compressor and create a warranty dispute.
If you're reviewing your full vacuum pump setup alongside your charging kit, the vacuum pumps range at HVAC Shop covers the Mastercool dual-stage options suited to both residential and commercial evacuation work.
Hose and Coupler Preparation
Before connecting to the system, inspect your hoses for any signs of wear, cracking, or contamination. A hose that's been used on an R22 system without being properly purged can introduce contamination into a fresh R32 circuit. If you're working across multiple refrigerant types on the same day, the cleanest practice is dedicated hose sets per refrigerant type. At minimum, purge the hose set thoroughly before each different refrigerant application.
Cap and valve core cleanliness matters too. A blocked or sticky valve core creates a restriction that affects the flow rate during charging and can give you misleading pressure readings while the charge is building. Remove valve core caps, inspect the cores, and replace any that feel tight or show corrosion. On coastal jobs in Sydney or the Gold Coast, salt-affected valve cores are more common than people expect, and a sticky core on the low side will slow your charge and skew your gauge readings.
System Pressure Verification Before Charging
With the system evacuated and isolated, confirm that residual pressure is within the expected range for the system at ambient temperature. A system that shows unexpected positive pressure after evacuation and isolation has a source of non-condensable gas or a leak from an adjacent circuit that needs to be investigated before you proceed. This is a safety check as much as a quality check, particularly on systems where the previous refrigerant type is unknown or the system history isn't documented.
Under Safe Work Australia guidance, technicians working with A2L refrigerants like R32 have additional obligations around confirming the system is safe to charge before introducing the refrigerant. That includes confirming adequate ventilation around the work area and that no ignition sources are present within the exclusion zone for A2L gases.
Tradie Pro Tip: Set your scale up and let it stabilise before you need it. If the scale has been sitting in a hot ute in a Darwin warehouse carpark or on a cold Melbourne morning, allow five to ten minutes for the internal components to reach ambient temperature before zeroing. A scale that's temperature-stabilised gives you a more reliable zero reading and more consistent measurements through the charge.
Setting Target Charge Weight
The most important number in the charging process is the target charge weight, and it needs to be confirmed before you open the cylinder valve. Getting this number wrong at the start means the rest of the process is precise but inaccurate. It doesn't matter how carefully you watch the scale if you're targeting the wrong weight.
Reading the System Nameplate
The refrigerant charge weight is specified on the system nameplate, which is usually located on the outdoor unit. It's expressed in grams or kilograms and reflects the manufacturer's factory spec for the standard pipe run length stated in the installation manual. Find that number before you start and write it down. Don't rely on memory and don't assume the charge weight is the same as a similar model you've done before. Even systems from the same manufacturer at the same capacity can have different charge weights depending on the variant, the refrigerant circuit design, and the market the unit was built for.
Cross-reference the nameplate charge against the installation manual. If the nameplate and manual disagree, use the installation manual as the primary reference and note the discrepancy in your job records. On older units or units that have been in service for a while, the nameplate data plate can be faded, corroded, or partially obscured. If you can't read the nameplate clearly, get the model number off the unit and look up the specification directly from the manufacturer before you proceed. Do not estimate.
Pipe Run Length Adjustment
The nameplate charge weight is the factory specification for a standard pipe run, which varies by manufacturer but is often set at around five metres or seven metres depending on the model. If your installation has a longer pipe run than the standard spec, you need to add the manufacturer's additional charge per metre to the target weight. This is stated in the installation manual and is specific to the refrigerant type and pipe diameter. Calculate the top-up amount before you start charging, not partway through. Failing to account for pipe run length is one of the most common causes of a systematically undercharged installation, particularly on long runs in multi-storey residential or commercial buildings.
For accurate pressure monitoring during and after the charge, you'll need a calibrated manifold set. The refrigerant gauges collection at HVAC Shop includes both digital and analogue manifold options suited to R32, R410A, and other current refrigerant types.
Zeroing the Scale Correctly
With your target weight confirmed, set up the Mastercool scale on a stable, level surface. An uneven surface introduces a tilt error into the load cell reading that can be significant over a large charge. On a job site this often means finding a flat piece of concrete or using a levelling pad rather than sitting the scale on uneven ground or gravel. Place nothing on the platform and zero the scale. Wait for the display to stabilise completely before accepting the zero reading. On the 98210-BL, confirm the zero is registered in the Mastercool Connect app as well as on the unit display if you're using wireless monitoring.
Once zeroed, place the refrigerant cylinder on the scale platform and record the starting weight. The difference between the starting weight and the ending weight is the actual charge delivered. Programme your target delivered weight into the app if you're using the 98210-BL, so you have a live countdown to target rather than watching the absolute weight and doing mental arithmetic at the end of the charge.
Did You Know? Hose fill weight is a factor some techs forget to account for. When you connect your hose set to the cylinder and the system, the hose volume itself holds a small amount of refrigerant. On short residential hose sets this is a minor consideration, but on longer commercial hose sets or when using manifold gauges with extended hoses, the hose fill weight can be enough to create a measurable charge error. Check the hose manufacturer's data for the volume of your hose set and factor it into your target weight calculation if precision is critical.
Liquid vs Vapor Charging Sequence
How you introduce the refrigerant into the system matters as much as how much you introduce. Liquid charging and vapor charging each have appropriate applications, and using the wrong method for the refrigerant type or the system state can cause damage or an inaccurate final charge. Understanding when to use each method is a core part of professional charging technique.
When to Use Liquid Charging
Liquid charging is the preferred method for R32 and for zeotropic blended refrigerants. R32 must be charged as a liquid to preserve the refrigerant composition, because if R32 is introduced as a vapor into a partially charged system, the composition of the charge can vary from the cylinder specification. The cylinder should be inverted so that liquid exits the valve, or the cylinder should be used in its normal upright position with a dip tube valve specifically designed for liquid delivery. Confirm the valve type on your cylinder before assuming which orientation delivers liquid.
Liquid charging into a system that isn't running requires care. Introducing liquid refrigerant into a compressor that's off and has low suction pressure can cause liquid slugging if the refrigerant reaches the compressor suction port before it vapourises. For this reason, liquid charging on systems that can be operated is best done on the high side while the system is off, allowing the liquid to enter the condenser circuit where it can be metered through normally when the system starts. Always refer to the manufacturer's charging procedure in the installation manual to confirm the recommended method for the specific unit you're working on.
For the correct coupler connections for this process, the Mastercool high-side and low-side coupler set ensures secure, leak-free connections on both service ports during the charging sequence.
When to Use Vapor Charging
Vapor charging on the low side is the appropriate method for single-component refrigerants like R22 where composition preservation isn't a concern, and for topping up a system that's already partially charged and running. With the system operating, vapor charging through the low-side service port allows the running compressor to draw the refrigerant in gradually. This method gives you more control over the charge rate and reduces the risk of liquid reaching the compressor because the refrigerant is entering as a vapor and the system is actively managing suction pressure.
Vapor charging is generally slower than liquid charging on the high side, particularly on larger systems. For a full factory charge on a new installation, vapor charging alone can take significantly longer than liquid charging. Many techs use a combination approach: liquid charge the bulk of the system weight on the high side with the unit off, then top up the final amount as vapor on the low side with the system running. This approach is faster than all-vapor charging and safer than all-liquid charging on the low side. Confirm the approach against the manufacturer's documentation before you proceed.
Flow Control and Pressure Limits During Charging
Control the flow rate using the manifold or service valve rather than relying on the cylinder valve alone. A wide-open cylinder valve on a liquid charge will deliver refrigerant faster than the system can manage, particularly at the start of the charge when the system is cold and pressure differentials are high. Slow, controlled flow lets you monitor the scale reading accurately and gives you time to close the valve before you overshoot the target weight. The last 100 to 200 grams of a charge is where most overshoot happens. Slow down the flow rate well before you expect to reach target weight, not at the last moment.
Monitor the high-side pressure throughout the charge. If high-side pressure approaches the system's maximum operating pressure or the pressure rating of your manifold set, stop the charge immediately and investigate before continuing. High-side pressure rising faster than expected can indicate a restriction in the circuit, a non-condensable gas issue, or a system that's already closer to its design charge than the scale suggests. Don't override the pressure signal because the scale tells you there's still charge left to go in.
Tradie Pro Tip: On R32 systems, keep a fire extinguisher rated for flammable gas within reach during the charging process. This is a basic A2L safety precaution and is consistent with Safe Work Australia guidance on working with mildly flammable refrigerants. It's also the kind of visible safety practice that builds trust with facilities managers and commercial clients who are watching you work in their building.
Post-Charging Verification
Reaching the target weight on the scale is not the end of the process. It's the point where you confirm that the charge you've delivered is actually performing as expected in the system. Post-charge verification is where you cross-check the scale's weight-based result against the system's operational parameters and document the outcome.
Final Weight Confirmation
Close the cylinder valve and note the weight displayed on the scale. The difference between the starting weight and the current weight is the actual refrigerant delivered. Compare this to your target weight. If the delivered weight is within the acceptable tolerance, which is typically plus or minus 20 grams for residential systems and closer to plus or minus 10 grams for commercial systems where tighter specs are often required, the charge is confirmed as weight-accurate. Record the delivered weight in your job documentation against the target weight and note any deviation and the reason for it.
If you're using the Mastercool 98210-BL with the Mastercool Connect app, export or screenshot the session log at this point. That log includes the timestamped weight readings through the charge, which is a useful piece of commissioning evidence if the charge is ever queried. You can explore the full Mastercool range at HVAC Shop if you're looking to upgrade or add to your charging kit.
Superheat and Subcooling Check
With the system running after the charge is complete, verify the operating parameters against the design spec. Superheat and subcooling checks give you a functional confirmation that the refrigerant charge is working correctly in the system, not just that the right weight went in. Superheat is measured at the suction line and confirms that no liquid refrigerant is reaching the compressor. Subcooling is measured at the liquid line and confirms that the condenser is doing its job properly with the current charge.
Target superheat and subcooling values are specified in the installation manual and vary by system type, refrigerant, and ambient conditions. A system charged correctly by weight should hit the design superheat and subcooling range under stable operating conditions. If it doesn't, investigate the cause before signing off the job. Common reasons for a weight-correct charge that's outside the superheat or subcooling range include a restriction in the circuit, an expansion valve that needs adjustment, or an ambient temperature that's outside the design operating range for that system.
Pressure Range Verification
With the system running at steady state, confirm that the high-side and low-side pressures are within the expected ranges for the refrigerant type and ambient conditions. Use a pressure-temperature chart for the specific refrigerant to interpret the gauge readings at the current ambient temperature. A correctly charged system running at typical Australian summer or winter ambient temperatures should land within the manufacturer's specified operating pressure envelope. Significant deviation from the expected range warrants further investigation before commissioning is signed off.
| Verification Check | What It Confirms | What to Do If It's Off |
|---|---|---|
| Final scale weight vs target weight | Correct refrigerant mass delivered to system | Add or recover refrigerant to hit target. Document reason for deviation. |
| Suction line superheat | No liquid reaching compressor, system processing charge correctly | Check expansion valve operation, pipe run insulation, ambient conditions. |
| Liquid line subcooling | Condenser operating correctly, no flash gas in liquid line | Check for restriction in liquid line, condenser airflow, or condenser fouling. |
| High-side operating pressure | System not overcharged or restricted on discharge side | Check for non-condensables, overcharge, condenser restriction or ambient. |
| Low-side operating pressure | System not undercharged, evaporator operating at design conditions | Check for undercharge, restriction on suction side, or evaporator airflow issue. |
| Hose disconnection check | No refrigerant loss during hose removal | Use valve core tools to minimise loss. Replace valve core caps and torque correctly. |
Hose Disconnection and System Isolation
Close the manifold valves before disconnecting any hoses from the service ports. Disconnecting a hose with an open manifold valve releases refrigerant to atmosphere, which is a compliance breach under ARCtick licensing requirements and a direct refrigerant loss cost. With the manifold valves closed, purge the hose pressure through the manifold's centre port or a recovery connection before removing the hoses from the service ports. Replace valve core caps with the correct torque and confirm both service ports are capped before you consider the job complete.
Log the final system pressures, superheat, subcooling readings, and the scale's delivered weight in your job documentation. Under ARCtick record-keeping obligations, you're required to maintain records of refrigerant handling work including the type, quantity, and purpose of any refrigerant used. The charge weight from your scale log is the primary record for the quantity delivered. Keep this documentation for at least five years or per your licence conditions, whichever is longer.
Frequently Asked Questions: Charging by Weight with Mastercool Scale
If you're looking to upgrade your charging scale or add the Mastercool 98210-BL Bluetooth model to your kit, the HVAC Shop team can confirm current stock and trade pricing. Browse the full refrigerant scale collection online or get in touch for more information.