System
Suction line (low side)
Liquid line (high side)
TXV target
Use the subcool listed on the outdoor unit's data plate. 8–12 °F is typical when not listed.
Results
Approach method
Liquid line − outdoor air, no gauges
Approach method
Liquid line − outdoor air, no gaugesThe liquid line temperature minus whatever the condenser is rejecting heat into. Lennox prints an approach target on the charging label, but the check works on any equipment — it just needs the right reference and a target, and the buttons below cover both. Its advantage: temperatures only, so the charge can be checked without hooking up gauges and losing refrigerant every visit.
Lennox charging labels give the approach target for that model — often a single number per size, held to ±1 °F, so tighten the tolerance above when you're working to a real label. Read it off the unit in front of you rather than carrying one over from the last job.
Any brand, in cooling: the target is the condenser's design split minus the subcool the maker already specs. Approach = split − subcool, so if the condenser rejects heat the way it was designed to, the two targets are the same target.
Design split comes from the equipment's efficiency: more condenser surface rejects the same heat at a smaller temperature difference. It's an estimate, so keep the tolerance at ±3 or wider, and confirm the first reading against subcool with gauges on.
The most accurate target on any equipment: record the approach once, with the charge known good, and compare against it from then on. A benchmark is only valid near the conditions it was taken at — this flags a reading if the reference temperature has moved more than 10 °F.
Take the outdoor air in the shade at the coil inlet, not downwind of the fan. Approach rises as the charge falls, so a high reading is usually low charge — but a dirty condenser coil, a slow fan or recirculating hot air raise it too, which is what the condenser split is there to tell you apart.
Charge by weight on a new install, a line-set change or after a recovery — on every brand. Approach is for checking and trimming an existing charge, and it wants a system that has run 15 minutes on high stage with a clean filter and a clean condenser.
Charging curve
Required CFM from tonnage
Lower CFM/ton removes more moisture (humid climates); higher CFM/ton gives more sensible capacity (dry climates).
Nameplate
Clocking the gas meter
The small test dial on the meter face, not the odometer-style register. Pick the smallest one that still takes 15 seconds or more to come around.
1030 BTU/ft³ is a common default for natural gas; the local utility publishes the real number and it moves with the season. Propane is metered by the gallon, so clocking does not apply — set propane by manifold pressure and orifice size instead.
Indoor ambient
Outdoor ambient
Psychrometric chart
Indoor results
Outdoor results
Indoor vs outdoor
Enthalpy is total heat (sensible + latent). The moisture difference is the water the system has to deal with in any outside air that gets in.
Sends indoor wet bulb & outdoor dry bulb straight into the SH/SC tab's piston superheat target.
What these mean
- Relative humidity — how full the air is of water compared with the most it could hold at that temperature. Same water, colder air = higher RH.
- Moisture (humidity ratio) — the actual water in the air, in grains per pound of dry air (7,000 grains = 1 lb). It doesn't change when air is only heated or cooled.
- Absolute humidity — water per cubic foot of air (gr/ft³), the weight of water in a given space.
- Moisture capacity — the most water the air can hold at its temperature before it condenses (100% RH). Warm air holds far more than cool air — why a cold coil wrings water out.
- Dew point — cool the air to this and water starts condensing. A coil (or duct) colder than the dew point gets wet.
- Wet bulb — what a wetted thermometer reads; used for piston charging targets and capacity. Enthalpy — total heat per pound of air.
- On the chart: left-to-right is dry bulb, up is moisture. Curved lines are % RH (the top one is saturation); dashed diagonal lines are wet bulb. Heating or cooling without adding/removing water moves a point straight sideways.
Pressures in psig. Values below 0 psig shown in inHg vacuum. Blends: use bubble (liquid) for subcool, dew (vapor) for superheat.
Plot a system
Cycle results
Reading a P-h chart
- Up is pressure (absolute, log scale). Right is enthalpy — heat content in Btu per pound of refrigerant.
- The dome: left edge = saturated liquid, right edge = saturated vapor, top = critical point. Left of the dome is subcooled liquid, inside is boiling/condensing liquid + vapor, right is superheated vapor.
- 1 → 2 compressor (adds the heat of compression) · 2 → 3 condenser (desuperheat, condense, subcool) · 3 → 4 metering device (straight down: no heat added, some liquid flashes) · 4 → 1 evaporator (picks up the refrigeration effect).
- Temperature lines run straight up in the liquid, flat across the dome (sloped for blends with glide) and curve down in the vapor. Quality lines show % vapor; entropy lines show ideal compression.
- Enthalpy uses the ASHRAE reference (0 Btu/lb for saturated liquid at −40°F), like most English-unit charts. Charts with another reference show different numbers, but the differences (refrigeration effect, heat of compression) are the same.
🚨 Emergencies
- Smell gas: no switches, phones or flames inside. Get everyone out, shut the gas at the meter if safe, call the gas utility / 911 from outside.
- CO 70 ppm or more: everyone out and call 911. 36–69 ppm: ventilate and shut off sources. 9–35 ppm: tell the customer and ventilate.
- Electric shock: don't touch the person until the power is off. Call 911; start CPR / use an AED if trained.
- Refrigerant on skin or eyes: get to fresh air; warm (not hot) water on skin — don't rub; flush eyes 15 minutes; get medical help.
- Heat stroke (confusion, hot dry skin, collapse): call 911 and cool them down right away.
- Burns: cool running water for 10+ minutes; get help for large or deep burns.
Before every job
Checks clear by themselves each new day.
General field safety guide. Always follow OSHA rules, your company's safety program, the equipment manufacturer's instructions and local codes.
Pressure diagnosis chart
Typical readings vs. normal for each fault. SH on piston systems; TXVs hold superheat until they run out of range.
▲ high ▼ low ● normal two symbols = either
Appearance
Theme
Motion
Turns off menu slides, fades and button animations. Everything appears instantly.
Units
Temperature units
Changes every temperature in the app, including the P-T chart. Pressures stay in psig; airflow stays in CFM and BTU/h.
Ask Gary
Ask Gary voice
Gary speaks each answer using your phone's text-to-speech. Tap the 🎤 in Ask Gary to ask by voice.
Gary uses a male English voice if your phone has one, otherwise a deeper pitch. On iPhone you can add voices in Settings → Accessibility → Spoken Content → Voices → English.
The 🎤 button uses your phone's speech recognition and may need a signal. If it doesn't work, tap the microphone on your keyboard instead.
Technician
Your name
Learning
Student Mode
Adds HVAC Basics, 608 Prep, Faults and How To to the Menu: 31 beginner lessons with diagrams and quizzes, 40 practice troubleshooting faults in 4 tiers (Beginner to Expert Technician), and step-by-step guides for the basic skills every tech needs.
App
Developer
Safety & accuracy
The short version: this app helps you think, it doesn't decide for you. Check anything it tells you against the equipment in front of you.
Read the full thing
What this is
A field reference and a set of calculators for qualified HVAC/R technicians. It is not training, it is not a substitute for the manufacturer's instructions or service manual, and it does not make you qualified to do work you aren't qualified to do. Where this app and the manufacturer disagree, the manufacturer wins.
Readings taken from a photo
Gary can read a digital manifold screen, needle gauges or a data tag from a photo. It gets a lot of them right and it gets some of them wrong — glare, angle, focus and dirt all cost accuracy, and it cannot tell you when it has misread something. Every number it fills in for you is a suggestion to check, not a measurement. Look at your gauges and correct anything that's off before you act on it. A wrong pressure leads to a wrong diagnosis, and refrigerant added or recovered on a wrong diagnosis costs real money and can damage equipment.
Numbers that were worked out rather than read
When Gary can't read a value, he sometimes works it back from the ones either side of it through the refrigerant chart — a head pressure from its saturation temperature, say. He labels these worked out. They're usually within a psi, but they are arithmetic from other readings, so an error in one carries into the other.
Charts and calculations
Pressure-temperature data, target superheat and subcool, airflow, psychrometrics, electrical tables and gas figures are from published references and standard formulas. They're rules of thumb and reference values — the manufacturer's charging chart and service manual come first on every job, and local code beats a rule of thumb every time.
Online Gary (beta)
Switched on, your typed questions and the chat they belong to are sent to Anthropic and answered by Claude. Photos stay on the phone unless you also turn on the camera switch at the top of the chat — with that on, a copy of each photo goes too, with its location and timestamp stripped out first. Answers are generated and can be confidently wrong. Refrigerant figures are checked against this app's own charts and flagged when they disagree, but anything outside that — procedures, electrical advice, diagnosis, code — is not checked by anything. With the camera switch on it also comments on your photos: that is an opinion formed from a picture, by something that cannot touch the equipment, and it can be wrong about a part it can only half see. The charge verdict you get from Check these readings comes from the charts in here, not from Claude. With Let Gary look things up switched on, questions about a model or serial also go out as a web search and the answer carries the pages it came from — check them, because a spec sheet for a similar model is an easy mistake to make. Treat it as a knowledgeable mate on the phone, not the manual. Don't type customer names, addresses or anything else you wouldn't want leaving the phone. You need your own API key and you pay for your own usage. Offline Gary is the default and always works without a signal.
Your data
Everything — settings, job notes, chat history, readings — is stored on this phone only. There is no account and no server. Uninstalling the app deletes the lot, so export anything you want to keep.
Photos are read on this phone. The camera reader runs here and always does, in every mode, and nothing about it needs a signal. No photo you take is sent anywhere.
Work you shouldn't be doing from a phone app
Refrigerant under pressure, live 240V, gas and brazing all injure and kill people. Isolate and lock out before you work on anything. Venting refrigerant is illegal under EPA Section 608. Gas piping, service changes and anything else requiring a licence need a licensed professional and usually a permit and inspection. Never defeat a safety or limit control.
No warranty
This app is provided as it is, with no warranty of any kind. You are responsible for the work you do and for checking anything you rely on. To the fullest extent the law allows, the author is not liable for any loss, damage or injury arising from its use.
About
Thermal Gauge v2.32 (web) · built for working technicians.
Gary's photo reading uses Tesseract OCR (Apache-2.0) and runs on this phone, with no signal needed.
🛠️ Developer Settings
Unlocked for this visit. Leaving this page locks it again.
App info
Feature switches
Developer Themes
Only available here. Picking a normal theme in Settings (or the Northern Lights easter egg) turns the developer theme off.
Online Gary Beta
Offline Gary answers from the charts built into this app and always will. Switched on, you also get a second Gary who can take an open question — "customer says it's not cooling, where do I start" — and talk it through. You pick which one you're using from the chat screen.
It needs an API key from Anthropic, and it costs you per question — about a third of a cent for ten of them. Your questions go to Anthropic to be answered. Photos are read on this phone either way — and stay here unless you turn on the camera switch in the chat, which sends a copy so Claude can see what you see.
How to set it up (5 minutes, once)
- Make an account. Tap the button below to open the Claude Console and sign in, or sign up if you haven't got an account. It's free to make.
- Put some credit on it. In the Console, go to Billing and add a card. $5 is about 15,000 questions — most techs will never spend it.
Already pay for Claude Max, Ultra or a Team plan? You get monthly credit included — in claude.ai go to Settings → Billing → API credits → Link organization and you won't need a card at all. - Make a key. In the Console go to Settings → API keys → Create key. Name it something like "Thermal Gauge". If it asks for a workspace, pick one.
- Copy it straight away. It starts
sk-ant-and the Console shows it once only. Lose it and you just make another. - Paste it below and tap Test the key. You want "Working".
- Go to Ask Gary and tap the Offline badge at the top of the chat. It turns amber and says Online. Tap it again any time to go back.
The key is kept on this phone and sent to nobody but Anthropic. It's yours — don't hand it to anyone, because they'd be spending your credit. Anyone else wanting Online Gary makes their own.
For model numbers, serials and nameplates only — capacity, refrigerant, what superseded a discontinued unit. Answers come back with the pages they came from so you can check them. Without this, anything Gary says a model number means is recalled from memory and can be confidently wrong.
A search costs about a penny — roughly thirty times an ordinary question — so it only runs when you ask about a specific piece of kit.
Diagnostics
Error log
Home
House
Design conditions
Use your area's 1% summer / 99% winter design temperatures (local code or ACCA tables), not record highs and lows.
Construction
Windows
Leave all four blank to assume glass equal to 15% of the floor area, spread evenly. A typical window is about 15 sq ft.
Results
Estimate only. For equipment selection, confirm with a full ACCA Manual J, and size equipment with Manual S.
Tap an item to mark it: ✓ OK → ! Needs attention → N/A → clear. Only marked items (and any readings you type) go into the note.
Checklist notes
Adds to the note you're working on, or starts a new note marked "Maintenance / tune-up". The checklist clears once it's added.
A guide, not a substitute for the manufacturer's instructions. Lock out power before touching wiring; line-voltage tests are for qualified techs.
Charge check
Is it charged right? Take readings with the box near setpoint, the coil clear of frost and the condenser clean.
Low side
High side
From the maker's chart optional
⚠️ Walk-ins must open from the inside — check the interior release. R-290 and R-600a units are flammable: no open flame or brazing until the system is recovered and purged per the maker. Follow EPA Section 608 and the manufacturer's instructions.
⚠️ Safety rules — read before setting any fault
Shows how to set each fault, its safety limits and the answer. Turns off when you leave this page.
Practice only, on equipment an instructor has approved. Always follow the manufacturer's instructions, lockout/tagout, and your school's safety rules.
Learning reference for students. Practice under an instructor, and always follow the manufacturer's instructions, local codes, EPA Section 608 and OSHA rules.
Backup
Notes are saved on this phone only. Removing Thermal Gauge from your Home Screen or clearing website data deletes them. Copy them out now and then to keep a backup.
Paste notes back in
Moving to a new phone or a new install? Copy the notes out of the old app, paste the whole lot here and they come back as service notes. Photos stay on the old phone.
New service note
Photos & videos
Take a photo or video, or pick one from your photo library.
Field reference only — not training, and no substitute for the manufacturer's instructions, the equipment's own charging chart, or your judgement at the unit. A reading taken off a photo, or a value worked back through a chart, is a suggestion to check against your gauges — not a measurement. Online Gary, and anything it says about a photo, is generated and can be confidently wrong: refrigerant figures are checked against the charts in here, nothing else is. Your settings, notes and chat stay on this phone, and photos never leave it. Follow EPA and OSHA standards and local codes — knowingly venting refrigerant is illegal, and electrical, gas and refrigerant work is for qualified technicians only. Provided as is, with no warranty: you are responsible for the work you do and for checking anything you rely on.