What Is a Digital Manometer? Types and Selection Criteria
A digital manometer measures gas or liquid pressure with a sensor and shows it on a digital display; in refrigeration service it is used as a manifold, vacuum gauge or differential pressure meter.
A manometer is an instrument that measures the pressure of gases or liquids; a digital manometer does this with a pressure sensor and shows the value on a digital display. It is used in air-conditioning and refrigeration service to read system pressure, during evacuation to read vacuum, and in ventilation to read duct and filter differential pressure. Manometers usually measure relative to the atmosphere (gauge pressure): 0 bar is ambient pressure and -1 bar is roughly a full vacuum.
Types of manometer
| Type | What it measures | Model in the catalogue |
|---|---|---|
| Digital manifold gauge | Low- and high-side pressure of a refrigeration circuit, saturation temperature for the selected refrigerant | UNI-T UT336E, UT336E KIT |
| Wireless pressure gauge | Pressure at a single point; sends the value to a phone | UNI-T UT336P, UT336P KIT |
| Vacuum gauge | Vacuum at micron level during evacuation | UNI-T UT336V |
| Differential manometer | Small pressure difference between two points | UNI-T UT366A |
| Dial (Bourdon tube) gauge | Mechanical gauge fitted permanently to an installation | Not in the catalogue |
Selection criteria
- Measuring range: The highest expected pressure must stay within the range. The UT336E and UT336P measure -1–60 bar and the UT366A ±15 kPa (±0.15 bar); pressure above the range can damage the sensor.
- Accuracy: The manufacturer states ±0.5% of full scale for the UT336P, UT336E and UT366A. With full-scale accuracy, the relative error of small readings grows.
- Refrigerant list: A manifold converts pressure into the saturation temperature of the selected refrigerant, so the refrigerant you work with must be defined in the instrument. The UT336E has more than 162 refrigerants; the UT336P has 10 in the device and more than 90 in the app. The manufacturer states the series suits A1, A2L, A2, A3 and some B1 refrigerants and cannot be used with ammonia (R717).
- Temperature clamp: Superheat and subcooling calculations need the pipe temperature. The UT336E KIT comes with two wireless temperature clamps and the UT336P KIT with a temperature clamp; the UT336P has a 3.5 mm input for the clamp.
- Connection: The common service-port connection is 1/4" SAE, and the UT336V comes with it. R410A and R32 systems usually have 5/16" SAE service ports; check whether you need an adapter.
- Wireless link and logging: The UT336V connects wirelessly to the UT336E manifold and offers an IP54 housing and 8000-record memory.
How to read superheat and subcooling
Superheat: The difference between the pipe temperature measured on the suction line and the saturation temperature corresponding to the suction pressure. Subcooling: The difference between the saturation temperature at liquid-line pressure and the pipe temperature measured on the liquid line. The manifold calculates both for the selected refrigerant; if the wrong refrigerant is selected, both values are wrong.
Common mistakes
- Relying on the manifold's vacuum reading: A reading around -1 bar does not show that evacuation is sufficient; check moisture and air removal with a vacuum gauge that reads microns.
- Selecting the wrong refrigerant: Saturation temperature, superheat and subcooling are calculated wrongly.
- Not zeroing before measuring: If the instrument has a zero function, zero it with the hoses open to atmosphere.
- Connecting a low-pressure meter to high pressure: Differential meters with a kPa range such as the UT366A are not connected to compressed-air or refrigerant lines.
- Using it on an ammonia system: The UT336 series is not used with R717.
Models in the catalogue
The Manometers category has 6 active models from Unit: UT336E, UT336E KIT, UT336P, UT336P KIT, UT336V and UT366A. All models are sold on quotation; request a quote on the product page for price and delivery time. For air speed, light and sound measurement, see the Environmental Measuring Instruments category.
Frequently asked questions
What is the difference between a manifold and a vacuum gauge?
A manifold reads low- and high-side pressure on a running system and calculates superheat and subcooling. A vacuum gauge measures very low pressure in microns during evacuation; the UT336V covers 1–19,000 microns.
How do microns, Torr and mbar relate?
1 micron is 1/1000 Torr, and 1 Torr is about 1.33 mbar (133 Pa). So 1,000 microns is about 1.33 mbar.
Which refrigerants can a digital manometer be used with?
According to the manufacturer's UT336E page, it is used with A1, A2L, A2, A3 and some B1 refrigerants, not with ammonia. Check that your refrigerant is on the instrument's list.
What is the UT366A used for?
For measuring low pressure and differential pressure in ventilation ducts, filters and clean rooms; it measures ±15 kPa with 0.01 kPa resolution.